When you compile a single file, such as main.cpp, youonly have one translationunit. James E. King III (JIRA) Mon, 14 Jan 2019 07:04:35 -0800 [ External linkagerefers to things that exist beyond a particular. C++,C#,MFC,ACE Framework. will get multiple variables. Manage SettingsContinue with Recommended Cookies. But it is generally a good one. If you defined functions here, they would also be able to see and share the staticvariable. Static initialization happens first and usually at compile time. May 6, 2009 at 4:36am imgravity (3) can u explain it a bit.. i have declared the static variables abool and xyz in cpp May 6, 2009 at 4:42am helios (17339) std::ostream doesn't have a constructor that takes no parameters. What does it mean? How would you create a standalone widget from this widget tree. A third concept is "initialization". The convention that the C source is in a file named .c and public declarations are in files named .h is only a convention. Netscape FastTrack), and OSU DECnet-based scripting. Connect and share knowledge within a single location that is structured and easy to search. In a such case you always want to make a copy of the function so this is not a bad pattern. Is it possible to define variables in a header file only? C/C++: Static function in header file, what does it mean? be accessible only from that translation(compilation) unit (i.e. So it could actually use less memory, if the function is short enough. Technically Many, Practically One There is a very important property of inline functions and variables that we haven't discussed yet. Storage: 2048 TB Static variables are initialized only once. Perhaps the function contains static variables that preserve their value between calls (internal state), and thus each module gets "its own private copy" of the vars by having its own clone of the function? The consent submitted will only be used for data processing originating from this website. Why was USB 1.0 incredibly slow even for its time? The advantage over just using the intrinsic directly is of course that wrapping it in another layer of abstraction makes it possible to build the code on compilers lacking that particular intrinsic, by providing an alternate implementation and picking the right one depending on which compiler is being used. static int iMyInt = 0; This is dubious. Asking for help, clarification, or responding to other answers. Not the answer you're looking for? Now static variable is behaving like a extern variable. Past Company: Tech Mahindra, Logitech, Aspire System and Marlabs static means that the variable is only used within your compilation unit and will not be exposed to the linker, so if you have a static int in a header file and include it from two separate .c files, you will have two discrete copies of that int, which is most likely not at all what you want. Initializing Constant Static Array In Header File. That is, the variable is scoped to the file, and cannot be accessed by code outside of that file. Specifically, a header should have header guards and include all other headers it needs. Say I have two following files: I have declared static variable say static int Var1 in both the header files. Lets start with static variables declared in a file. A compilation unit is basically a .cpp file with the contents of the .h file inserted in place of each #include directive. There are no restrictions about headers files. Improve INSERT-per-second performance of SQLite. //Header file .. myNameSpace.h. rev2022.12.11.43106. It sees addTwo on b.obj not being referenced then it removes the definition from the obj? In the header file, it is included by 2 C files respectively, which is equivalent to defining 2 times in the 2 C files, thus when the obj files generated by the 2 C files are finally linked to an executable file, there will be duplicate definitions. I need to read a file and store the arguments into individual variables, let's say the file(readfile.txt) looks like the following: abc="g/h/I" jhk="l/m/n" opq="r/s/t", Cannot declare a variable of static type 'System.IO.File' in c# .net file upload. Variable declarations in header files - static or not? Storage: 0 TB. Since, at main.cpp there is Var1 declared twice at the same scope, multiple declaration error will arise. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Similarly, one header file (and only one header file) should declare the variable. My work as a freelance was used in a scientific paper, should I be included as an author? Zero runtime overhead, early problem diagnosis, and, as we will see later, safe. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Or, some people prefer a . not inside any other code), then you are creating a so-called "global" variable that will: be available for the entire duration of your program, and be accessible only from that translation (compilation) unit (i.e. Gajendra Kumar 898 score:1 const variables are by default static in C++, but extern C. So if you use C++ this no sense what construction to use. Now, in a compilation unit you can't have two global variables with the same name. When is a global not a global? In this particular case, the static keyword makes the symbol be private to the module, so there isn't a multiple-definition conflict waiting to cause trouble. Under that convention, the only things that should appear in .h files are declarations so that you generally avoid having the same symbol defined more than once in a single program. JDK9 Hotspot debug using gdb, causing SIGSEGV Segmentation fault in eclipse/Ubuntu terminal. People also askIs there a global variable with name I in header file?Is there a global variable with name I in header file?There is a header file foo.h that contains a global variable declaration int i;. When is a header file required for using a static library? The same applies to global variables. gSoap shared data types between interfaces, Manually converting a char to an int - Strange behaviour. +1 (416) 849-8900. Static variables declared in the header file can be initialized only once in the source files including the header file. But after compilation I found it is showing conflict. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14.The name follows the tradition of naming language versions by the publication year of the specification, though it was formerly named C++0x because it was expected to be published before 2010. This may increase the size of your executable, but this may be negligible if the function is small. 1) A static int variable remains in memory while the program is running. Header Files - C++ Tutorial For Beginners #14, Global variables in a multi-file project in C, My thiniking was there will be two compilation unit.Thanks for clearing by doubts, You have provided the best and simplest definition of a, TabBar and TabView without Scaffold and with fixed Widget. If you include the same variable in another unit, you will effectively have two variables with the same name. We and our partners use cookies to Store and/or access information on a device.We and our partners use data for Personalised ads and content, ad and content measurement, audience insights and product development.An example of data being processed may be a unique identifier stored in a cookie. ( i.e, one copied from file1.h and the other form file2.h by the pre-processor). What happens if you score more than 99 points in volleyball? Declaring static variables in a header is a bad concept, but possible. I did this since the static variable will have file scope so it won't conflict each other. All rights reserved. A third concept is "initialization". Next time well look at static variables declared inside functions. static is a guarantee that a variable gets internal linkage. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Assuming static variable static int Var1 is at global scope in both the headers and included both the headers in main.cpp. Is there any way to access everything except the last template parameter? You should declare your variable extern in the header and define it in the source file (without the static keywork: static in source file provides internal linkage). VS2010 bind implementation doesn't support move-only types? . For each program, one source file (and only one source file) defines the variable. I would rather encapsulate private data in file-global static vars (C) or class members (C++). Say I have two following files: file1.h file1.cpp file2.h file2.cpp I have declared static variable say static int Var1 in both the header files. Others have given good advice. This is nice and simple. If you define the function in a header file (not simply declare it), a copy of the function will be generated in each translation unit (basically in each cpp file which includes this header). Compared to having a single shared definition which is. When should i use streams vs just accessing the cloud firestore once in flutter? Both file1.h and file2.h are included in main.cpp file. UPDATE: In many cases, it's actually a good idea to do something like the above, and I realize my answer sounds very black-and-white about this which is kind of oversimplifying things a bit. Since, at main.cppthere is Var1declared twice at the same scope, multiple declaration error will arise. If you are trying to create global variables then you should declare them. I'd personally avoid it because it would provide too many chances for a clever optimizer (or code maintainer) to break it by "cleverly" eliminating the apparently redundant function bodies. Long version: This command, $ find mozilla-central -name '*.h' | xargs grep -n 'static inline' finds 1851 matches here. So that the actual code is given directly in the function, like this: static int addTwo (int x) { return x + 2; } Then that's just a way of providing a useful function to many different C files. In the United States, must state courts follow rulings by federal courts of appeals? namespace myNameSpace {. [jira] [Resolved] (THRIFT-1759) for generated Obj-C constants, move static variable declarations in implementation file to 'static const' declarations in header file. /*****/ /* CgiLib.c For C Language scripts these functions provide a number of common CGI activities as a simple object code module. 'const' objects have internal . Spark, Pyspark and pandas whenComplete() method not working as expected - Flutter Async, iOS app crashes when opening image gallery using image_picker. Does this principle apply to const variables as well or is it ok to place such definitions in source files? A static variable is only available to a single translationunit. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. How to change background color of Stepper widget to transparent color? Where does the idea of selling dragon parts come from? How do I use extern to share variables between source files? Yes it can. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, What are the pros and cons of using static function in header file, [C++]function template with static keyword, Storing C++ template function definitions in a .CPP file. Current Technology: C++11 introduced a standardized memory model. If the .h file is generated code and only included in a single .c file, then I would personally name the file something other than .h to emphasize that it isn't actually a public header at all. Now, in a compilation unit you can't have two global variables with the same name. Static variables are local to the compilation unit. But when you compile more than one .c or .cpp file, you have multiple translationunits. This header file is being included in one.c and two.c respectively (I am aware that that's not a very good life choice, but it's not mine). c++ condition_variable wait_for predicate in my class, std::thread error. appear in header files. Was the ZX Spectrum used for number crunching? An ordinary variable is limited to the scope in which it is defined, while the scope of the static variable is throughout the program. @quinmars Good point, I've edited. other .c or .cpp files).. static gives the variable internal linkage, hiding it from other translation units. There is not semantic difference in defining in source file or header file, basically both means the same in plain C when using static keyword that, you are limiting the scope. If the function is declared inside the header, and defined . Still bigger than (size of the function), but not as bad? Can any body explain how scope and linkage are working in this scenario. Both file1.h and file2.h are included in main.cpp file. If you would like to change your settings or withdraw consent at any time, the link to do so is in our privacy policy accessible from our home page. Why is the eastern United States green if the wind moves from west to east? Default argument v templates priority in overload resolution, STL containers and threads (concurrent writes) in Linux, cin.getline sets the begin of a string a '\0'. Why can templates only be implemented in the header file? So saying that the only difference will be performance and code size is wrong. use of constexpr in header file. email is in use. Posted 7-Nov-18 0:20am CPallini Solution 2 It means that if you include (say) a header that contains a static variable in two different source files, you will end up withtwoglobal variables with the same name. There is a strong semantic difference if the function contains a static local variable. How do I go about declaring my variables/methods? I will just elaborate (cause more . Inline variables, therefore, extend the same capabilities to general constants with static storage duration (i.e. They are local to the block. The. contents. home > topics > c / c++ > questions > using constant variables in header file Join Bytes to post your question to a community of 471,625 software developers and data experts. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? You can mark it as extern, if you want a variable to be shared among the source files. So you end up with a separate my_variable for each translation unit (".c file"). Chances are they have and don't get it. Is it possible to use an existing Makefile to build a project in Code::Blocks? The content must be between 30 and 50000 characters. They're available in any compilation unit that includes an appropriate declaration (usually brought from a header file). C++11 is a version of the ISO/IEC 14882 standard for the C++ programming language. Define image datatype and image_t in C language. If these are the same variable, move it into a separate header file, var1.h, and include var1.h from both file1.h and file2.h, not forgetting the #include guard in var1.h. So, compiler don't report an error. A normal or auto variable is destroyed when a function call where the variable was declared is over. Each source file is compiled individually. Making statements based on opinion; back them up with references or personal experience. The clean, reliable way to declare and define global variables is to use a header file to contain an extern declaration of the variable. Embedded C questions: 24. Static variables are local to the compilation unit. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. How to return a generic iterator (independent of particular container)? If logically these are two distinct variables, give them different names (or put them in different namespaces). The keyword static can be used in three major contexts: inside a function, inside a class definition, and in front of a global variable inside a file making up a multifile program. When defining a static variable in a header file, a new instance of the variable is created for each file including the header file. Editor's note: In C++, const objects with neither the static nor extern keywords in their declaration are implicitly static. The use of static inside a function is the simplest. @ranReloaded, That is a possibility. Declare the variable extern in the header file: extern int global_int;, then define it and optionally initialize it in one and only one source file: int global_int = 17;. Now, first the pre-processor copies the content of included files to the main.cpp. Lets take static from_string (const char *str) as an example. This variable is now a global that you can use in any source file by declaring it extern, for example, by including the header file. Now, first the pre-processor copies the content of included files to the main.cpp. Each C file that includes the header will get its own definition that it can call. c++17: header only: class static variable errors, c++ static class variable without cpp file, Crash when accessing static variable exported with a def file. This is called constant initialization. In C, for example, a structure can be declared in the public API via the header file for a set of functions that operate on an item of data containing data members that are not accessible to clients of the API with the extern keyword. The memory for that static is only going to be allocated if an address or reference to it is taken, and the address is going to be . Yeah, I don't agree with that 'design' either. Thus other translation will not be able to access it or declare extern variables referring to it. the file itself and any file that includes it). So that the actual code is given directly in the function, like this: Then that's just a way of providing a useful function to many different C files. From the return type (and hinted from the documentation), these seem to be functions that create instances of the address class. You're only allowed to do this once. If these are the same variable, move it into a separate header file, var1.h, and include var1.h from both file1.h and file2.h, not forgetting the #include guard in var1.h. If the function is declared inside the header, and defined in a source file, then the counter will be shared across your whole program. c++ template singleton static pointer initialization in header file. Static variables have translation unit scope (usually a .c or .cpp file), but an #include directive simply copies the text of a file verbatim, and does not create another translation unit. Can static variables be declared in a header file? This is often surprising as people often expect to have only one instance of the variable. in a header file, seriously consider doing "inline" instead. Do you need your, CodeProject, So yes, const variables defined in header files can be used in a way that is prone to the static initialization fiasco. Why doesn't Netbeans recognize `cbegin()`, `cend()`, `unordered_set`, among other C++ features? Is the function defined in the header file? An example will explain it more succinctly. Little Programming Guides | C, C++, Linux and GDB. confusion between a half wave and a centre tapped full wave rectifier. C++ Initialize const class member variable in header file or in constructor? 1) The #ifndef guard prevents multiple definitions in a single source file (thus the extern definitions do nothing). This is what's happening in your case: main.cpp includes file1.h and file.h, and each of the two headers defines its own Var1. Assuming static variable static int Var1is at global scope in both the headers and included both the headers in main.cpp. How to use function from static library if I don't have header file, const variables in header file and static initialization fiasco, Same Header File for both DLL and Static Library, Variable in header file not declared in scope, private static const member variable in header vs const variable in cpp. Ready to optimize your JavaScript with Rust? If a question is poorly phrased then either ask for clarification, ignore it, or. Moving inline methods from a header file to a .cpp files. Understand that English isn't everyone's first language so be lenient of bad What does this declaration mean in visual C++? Internal linkage with static keyword in C - Stack Overflow, C pre-processor help to substitute function declaration with two variables. Still, the above is cleaner than using a macro. TCP/IP,STUN,P2P,Bonjour,WiFI Technology. Now, when you declare seperately in their source files, each source file is unaware of existence of the other static variable present in the other source file bearing the same name. :) Thanks. Yes, it's perfectly OK. It can be used with both variables and functions, i.e., we can declare a static variable and static function as well. Is the function defined in the header file? Using flutter mobile packages in flutter web. Thanks for contributing an answer to Stack Overflow! The output of this program is as follows: Storage: 0 TB Static variables can be defined inside or outside the function. Find centralized, trusted content and collaborate around the technologies you use most. Determine CPU on which the calling thread is running? They're stored wherever the linker puts them, usually a place called the "BSS segment." Extern? See 3.5/3. However, there is a problem in writing this in header file, this is because every time you include the header in a source file you'll have a copy of the function with same implementation which is much similar to have a normal function defined in header file. The static and extern tags on file-scoped variables determine whether they are accessible in other translation units (i.e. Hi, I have a file structure where I need to declare some variables and then reference to them on other header files. Since, at main.cpp there is Var1 declared twice at the same scope, multiple declaration error will arise. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. The advantage is that the most compilers may inline the function, which may increase the code performance. What does it mean to declare static variable in header file? This is what's happening in your case: main.cpp includes file1.h and file.h, and each of the two headers defines its own Var1. If your function uses a static local variable such as: static int counter () { static int ctr = 0; return ctr++; } Rather than: //header int counter (); //source int counter () { static int ctr = 0; return ctr++; } Then each source file including this header will have its own counter. If you include this header in more than one source file you. This tells the compiler to actually allocate an instance (memory) for the variable. Solution 2. @jheriko Since each C file that includes the header will get its own local function, the code in the function will be repeated many times. Are static variables shared memory between threads? And how is it going to affect C++ programming? On the other hand if I declare the static variable in both .cpp files, it compiles well. They are made static in order to be able to be used without the need to instantiate an object. Each C file that includes the header will get its own definition that it can call. the file itself and any file that includes it). If possible, initial values for static variables are evaluated during compilation and burned into the data section of the executable. So now we have twostatic variables in our program, both called storage, one in each translation unit. spelling and grammar. c++ private member declared in header vs static variable declared in cpp file, No linker error when global variable declared static in the header file, Const static variable defined in header file has same address in different translation unit. declared static) that are either not known at compile time or are not of a literal type. Multiple definition error on variable that is declared and defined in header file and used only in its cpp file, Why linker is giving error for global variable in header file, make/cc not finding header file even though its directory is present in PATH variable, template behavior for static templatized member function in the header file only. Books that explain fundamental chess concepts. We increment it in the code, and then we output that variable to see that it has changed accordingly. One, and only one, of those .c files can then declare the variable again, leaving off the extern keyword and/or including an initializer. you need to declare all your static variables at the top of .cpp files where you use them. So, compiler don't report an error. I am reading some code, found that static function in header files could be invoke in other files. If you change the value of the variable in one. Now, when you declare seperately in their source files, each source file is unaware of existence of the other static variable present in the other source file bearing the same name. Is MethodChannel buffering messages until the other side is "connected"? Now, first the pre-processor copies the content of included files to the main.cpp. Adding this instance variable to the header of a C++11 file drives the compiler up the wall. Header file inclusion static analysis tools? Don't tell someone to read the manual. Should v initialize like this. The default value of static variables is zero. A compilation unit is basically a .cpp file with the contents of the .h file inserted in place of each #include directive. However, this gives you an easy way to insert separate interface and implementation parts in the single header file: Apple vector math library in GLK framework uses such constuction (e.g. Defining an extern variable in the same header file. This tells the compiler to actually allocate an instance (memory) for the variable. Add a new light switch in line with another switch? There's a typo in your code: extern int varArray[]; should be extern int vararray[]; This When its a static variable. GLKMatrix4.h). As others are saying, it has exactly the same meaning as a static function in the .c file itself. Is it possible to make a static library from a header only file through visual studio? But I would prepend an "inline", so you don't get compile warnings about unused static functions. Answer (1 of 6): Short answer: Only if the static variable is defined in a header file, which is included in the .c file that you are trying to access the variable from. its a source file (.c or .cpp), and all its includes. Therefore, I suggest you should have your implementation only in your source file and not in header. Remember that #include:ing a header basically just pastes the contents of the header (and any other headers included by it) into the C file as seen by the compiler. How to declare a variable in header file to be used in two .cpp? This aspect was inherited from C. C++ also allows static variables to be declared inside a class . You can mark it as extern, if you want a variable to be shared among the source files. What are static variables C++? But in the absence of a guarantee that the function would be inlined, you take the risk that the function would be instantiated in every module that happened to #include that header file which at best is a waste of memory in the code segment. This post, and the next three, will talk about static variables. Although the pro is that the private data is encapsulated, visible right where needed and nowhere else. on my header 1 I have: File 1.h #ifndef EEPROM_HANDLER_H #define EEPROM_HANDLER_H #include "ScaleStruct.h" #include "ChordStruct.h" static int ScaleEEPROM_Address; static int ScaleEEPROM_count; static const int ScaleEEPROM_size . [Kha hc lp trnh C++ C bn] - Bi 17: Bin tnh trong C++ (Static variables in C++) | HowKteam, Can we use static variables in Header files | Embedded C Interview Questions. Why is there an extra peak in the Lomb-Scargle periodogram? Static variable has file scope. Declaring a string array in class header file - compiler thinks string is variable name? To understand how that works you should be aware of three things. How many transistors at minimum do you need to build a general-purpose computer? But there may be a big difference in doing this which wasn't mentioned in any answer. Iterating through STL containers and removing/adding multiple items. If your function uses a static local variable such as: Then each source file including this header will have its own counter. Static variable in a Header File 42,608 Solution 1 Static variables are local to the compilation unit. constexpr implies const and const on global/namespace scope implies static (internal linkage), which means that every translation unit including this header gets its own copy of PI. What is the effect of the DT_NOFULLWIDTHCHARBREAK when calling DrawText? DLIB : Training Shape_predictor for 194 landmarks (helen dataset). The translation unit is the individual source file. Past Experience: 20 Bay Street, 11th Floor Toronto, Ontario, Canada M5J 2N8 Each source file is compiled individually. . Are the S&P 500 and Dow Jones Industrial Average securities? static means that the variable is only used within your compilation unit and will not be exposed to the linker, so if you have a static int in a header file and include it from two separate .c files, you will have two discrete copies of that int, which is most likely not at all what you want. By omitting the 'static' keyword, you're defining a variable with external linkage. This is because there is no semantic difference between .c and .h files; there is only the compilation unit made up of the file actually passed to the compiler (usually named .c) with the contents of any and all files named in #include lines (usually named .h) inserted into the stream as they are seen by the preprocessor. not inside any other code), then you are creating a so-called global variable that will: Number two is the important one here. In this way, the compiler will generate the same initialization for each time the static variables are accessed. When a header declares inline functions or templates that clients of the header will instantiate, the inline functions and templates must also have definitions in the header, either directly or in files it includes. This example has four files, main.cpp, Storage.h, DiskDrive.cpp and DiskDrive.h. Static variables in a file If you declare a static variable at file level (i.e. Now, in a compilation unit you can't have two global variables with the same name. Static variables have translation unit scope (usually a .c or .cpp file), but an #include directive simply copies the text of a file verbatim, and does not create another translation unit. int A::x; // definition The definition could be in the header, but others have given reasons why it is probably best to put the definition in the .cpp file. Since myclass.cpp has its own copy of the const variables, these might not be initialized when MyClass::MyClass () is called. Due to the One Definition Rule you can only define (a non inline) variable once and that's exactly what you correctly do when you define it in the .cpp file. Should a const static variable be initialized in a c++ header file? One objective is the reasonably transparent, core support for WASD CGI and CGIplus, VMS Apache (CSWS), Purveyor, "vanilla" CGI (e.g. The general practice is to put the definition of variable into a C file. See, for instance: Internal linkage with static keyword in C - Stack Overflow [ ^ ]. If logically these are two distinct variables, give them different names (or put them in different namespaces). By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. ( i.e, one copied from file1.h and the other form file2.h by the pre-processor). How to make voltage plus/minus signs bolder? A compilation unitis basically a .cppfile with the contents of the .hfile inserted in place of each #includedirective. To learn more, see our tips on writing great answers. 2) Declaring a static variable in a header means that each source file that includes it will have its own version of that variable rather than a single shared variable. Getting Started With C Programming Hello World Tutorial, be available for the entire duration of your program, and. So in that one sense, it is safe to do. How can you know the sky Rose saw when the Titanic sunk? Static in C. Static is a keyword used in C programming language. This of course wastes memory, and is (in my opinion) a quite ugly thing to be doing, since having executable code in a header is generally not a good idea. After preprocessing, this: Assuming static variable static int Var1 is at global scope in both the headers and included both the headers in main.cpp. After preprocessing, this: Copyright 2022 www.appsloveworld.com. int A::x; // definition The definition could be in the header, but others have given reasons why it is probably best to put the definition in the .cpp file. For example below program prints "1 2" If so, the overhead is just the (size of the function)*(number of different obj files that reference it). For example, a utility that converts a binary file into an initialized variable definition might write a file that is intended to be used via #include and could very well contain a static declaration of the variable, and possibly even static definitions of accessor or other related utility functions. For example, we can use static int to count a number of times a function is called, but an auto variable can't be used for this purpose. In one case, the static variable will be shared, in the other case there will be multiple different static variables for each compilation unit. Static Variables: Static variables can be defined anywhere in the program. (Assuming you are referring to global declarations and definitions) Explanation: Let us f. Is it possible to link libstdc++ statically in Mac OSX 10.6? As you can see, the storage total output by the DiskDrive object is zero (output line 3). You can access this variable fromanywherein this file. This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL), You should not be defining them in the header file if they are static, just in the .cpp file. Static variable has file scope. Encapsulation is also possible in non-object-oriented languages. It simply means that once the variable has been initialized, it remains in memory until the end of the program. A static member variable is "defined" outside the class definition. What happens if you declare a static variable in a header file is that more than one translation unit will get a separate variable with that name. A static member variable is "defined" outside the class definition. What does the exclamation mark do before the function? When making accessible a nonconst variable in a header file I would use the extern keyword and define it in a corresponding source file. This is essentially "global." Static: . i.e. I write handy little guides to GDB, C and C++, and occasionally some Linux stuff for fun. Treat them the same, whether they're const or non-const. How to check if widget is visible using FlutterDriver. Well, the compiler will probably inline short functions. The static, in this context, means the scope of my_variable is limited to the translation unit. A question related to deriving standard exception classes, c++11 union contains data member with virtual function. To understand how that works you should be aware of three things. Each translation unit including your header will "see" a static const int. For instance, code that models (or just uses) intrinsic functions can be expressed like the above, and with an explicit inline keyword even: Here, the __add_two_superquickly() function is a fictional intrinsic, and since we want the entire function to basically compile down to a single instruction, we really want it to be inlined. I have a header file called myNameSpace.h which as the following. Current Company: Staff Software Engineer at GE Healthcare Python(App and Backend),AWS(Glue, EMR, Dynamodb),Kubernetes, Elastic search, Parquet,ansible As far as I can see this does only apply to variables not requiring static initialization: Why? It has a value of zero because DiskDrive.cpp creates a new translation unit that includes the static variable. By adding the definition in header you are not achieving the what the static function is meant for. It will effectively create a separate static function with the same name inside every cpp file it is included into. Not across .c files and otherwise. Remediation Can static variables be declared in a header file? A static variable can be defined in a header file, but this would cause each source file that included the header file to have its own private copy of the variable, which is probably not what was intended. I did this since the static variable will have file scope so it won't conflict each other. Therefore the function behavior may be totally different. You can see weve declared and initialised the static variable at the top of the file. Answer: Globals have application-scope. This leads to errors that are very difficult to track/understand. They can be defined in header files. The compiler never knows that the one particular function definition came from a header file. Well, its roughly the collection of code that is passed to the compiler after preprocessing. Second, static and extern specifiers are mutually exclusive therefore decl. I am not certain of what use cases would justify doing this at all in a generally available public header. If you declare a static variable at file level (i.e. Static duration means that the object or variable is allocated when the program starts and is deallocated when the program ends. Better late, I hope. Provide an answer or move on to the next question. Also, it provides exactly one set of internal state per translation unit. It would be less confusing and less fragile to put all of the state in an explicit state variable, passed in to each call. First, static specifier when used on global variables limits the variable's scope to the source file in which it is defined. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? When you put std::string header::value in your .cpp file you are not redeclaring the static variable, you are defining it (and default constructing its value). Create a header file to hold these constants Inside this header file, define a namespace (discussed in lesson 6.2 -- User-defined namespaces and the scope resolution operator ) Add all your constants inside the namespace (make sure they're constexpr ) #include the header file wherever you need it For example: constants.h: The static variables are alive till the execution of the program. I wonder if the linker will optimize that out. I know what it means when static function is declared in source file. The compiler persists with the variable till the end of the program. It is usefull in some "header-only" libraries with small inline functions. If you are declaring such a variable in a header filewhich is likely to be included from many different .c filesuse extern to make that declaration "just a declaration". 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