c++ static global variable in header

large table!! static renders variable local to the file which is generally a good thing, see for example this Wikipedia entry. The global variables get defined outside any function- usually at the very beginning/top of a program. Declaration and definition confusion in C, How to initialize a struct in accordance with C programming language standards, How to correctly use the extern keyword in C. What REALLY happens when you don't free after malloc before program termination? I don't think that's just "potentially" - it's for Content Discovery initiative April 13 update: Related questions using a Review our technical responses for the 2023 Developer Survey. gcc file1.c, everything works fine. Pre-calculated object representations are stored as part of the program image. except if the program starts a thread before a variable is initialized, in which case its initialization is unsequenced, // dynamically initialized to 0.0 if d1 is dynamically initialized, or, // dynamically initialized to 1.0 if d1 is statically initialized, or, // statically initialized to 0.0 (because that would be its value, // if both variables were dynamically initialized), // may be initialized statically or dynamically to 1.0, // If a is initialized before main is entered, b may still be uninitialized, // at the point where A::A() uses it (because dynamic initialization is, // indeterminately sequenced across translation units), // If a is initialized at some point after the first statement of main (which odr-uses. There wouldnt be a violation of the ODR, because there would be as many xas compiled files that #includethe header, but each one would only have its own definition. large header file. What risks are you taking when "signing in with Google"? Not the best worded answer, but if you want to know what he means by definition / declaration, here's a well-formed answer of what you. 2nd Cannon Place If you want something that is local to your file, you should use an anonymous namespace rather than the static modifier. C++ and C++ write that part, though). What are some best practices for using static? Note that for this to work, there needs to be exactly one definition of x. Fort Marcy Park, VA. P.S. So now we have twostatic variables in our program, both called storage, one in each translation unit. identifier with no linkage denotes a unique entity. To understand how to declare global constants in C++, you need to have some understanding of how a C++ program in built: preprocessing, compiling, linking. This is a problem for several reasons: Strictly speaking, the undefined behaviour makes the last two reasons rather theoretical, because in undefined behaviour anything can happen. With this change our program now correctly outputs: Constants inside of a class, declared static, have the same scope as global constants, and inlinesimplified their definition in C++17 too. that because you (ANDY) are an excellent embedded guy, and therefore 6.2 -- User-defined namespaces and the scope resolution operator, Create a header file to hold these constants, Inside this header file, define a namespace (discussed in lesson, Add all your constants inside the namespace (make sure theyre, #include the header file wherever you need it. Because global symbolic constants should be namespaced (to avoid naming conflicts with other identifiers in the global namespace), the use of a g_ naming prefix is not necessary. Constant values are an everyday tool to make code more expressive, by putting names over values. Since OP seems to be beginner, I simply gave the most basic rule about defining global variables in C. As you have noticed yourself--you usually cannot do yourself harm using global _const_s in header file (in C, it's not so simple in C++). As you can see, the storage total output by the DiskDrive object is zero (output line 3). Global constants as internal variables Prior to C++17, the following is the easiest and most common solution: 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 ) Always use static in .c files unless you need to reference the object from a different .c module. Is "I didn't think it was serious" usually a good defence against "duty to rescue"? This shows when the constructor of Xcan accept values: Note how the declaration in the header file doesnt take constructor arguments, while the definition in the .cppfile does. This code compiles, but doesnt define a global constant! But most people would just use a #define to a literal. forces/restricts the identifier to be internal. Thus outside of constants.cpp, these variables cant be used anywhere that requires a compile-time constant. make the table 'global', so only the files that include the header i.e. We and our partners use cookies to Store and/or access information on a device. My focus is on how to write expressive code. If you need global constants and your compiler is C++17 capable, prefer defining inline constexpr global variables in a header file. files would be a useful thing to do. Why does Acts not mention the deaths of Peter and Paul? This declaration informs all the #includeing files of the existence and type of x. Initialization of global and static variables in C, Difference between Static variables and Register variables in C. How to Access Global Variable if there is a Local Variable with Same Name in C/ C++? It is also potentially a waste of memory - every inclusion of the Constants aren't visible to linkers at all, they just affect generated code during compilation. files?? With inline, it was a definition. Storage: 2048 TB Now, when I initialize variable 'i' in the header file to say 0 and compile again I get a linker error: If I just compile file1.c (removing call to foo()) with the initialization in the header file i.e. An example will explain it more succinctly. rev2023.4.21.43403. @Bruce: Because in this case, it is only initialized once. i.e. Thus, an inline variable is one that is allowed to be defined in multiple files without violating the one definition rule. By using our site, you In most cases, because these are const, the compiler will simply optimize the variables away. It means that once you execute a program, its global variable will be available for use throughout the running of the entire program. The above method has a few potential downsides. This is nice and simple. With extern, the above code is a declaration, and not a definition. Question was: "Can someone explain when you're supposed to use the static keyword before global variables or constants defined in header files?" 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 are players required to record the moves in World Championship Classical games? 6.7 External linkage and variable forward declarations. This could make some sense if each copy of the table had to be The linker will consolidate all inline definitions of a variable into a single variable definition (thus meeting the one definition rule). @alex A very good question. scope more than once can be made to refer to the same object or Hello, my name is Jonathan Boccara, I'm your host on Fluent C++. The output of this program is as follows: Storage: 0 TB 3 If the declaration of a file scope identifier for an object or a Lets make a simple test to observe it with our own eyes: lets add a side effect in the constructor of X: With this addition, here is what our program with the two .cpp files outputs: Wow. Thanks for helping to make the site better for everyone! Global static variables shared in multiple source files, What is the exact reason for the keyword static working differently for variables and functions, C: Why static variables can't be linked externally like globals, Declaring and using a static list in order to print objects (in c++), Defining an array globally but its parameters will available later. We can take an example by assuming that we have a chair at our house and one in our school/college then we can say that the chair at our home can only be accessed by the people living inside the home but the chair in our college can be used by any student or faculty. Instead you should declare it extern in header file included by all .c files that need it. In order for variables to be usable in compile-time contexts, such as array sizes, the compiler has to see the variables definition (not just a forward declaration). The solution in C++17 is to add the inlinekeyword in the definition of x: This tells the compiler to not to define the object in every file, but rather to collaborate with the linker in order to place it in only one of the generated binary files. The currently-accepted answer to this question is wrong. imagine that you want to access a variable in another module: Now if you declare var to be static you can't access it from anywhere but the module where foo.c is compiled into. Given that writing X const xis such a natural thing to do (another hat tip to the const Westerners), you may doubt that such problems could appear. Prior to C++17, the following is the easiest and most common solution: Then use the scope resolution operator (::) with the namespace name to the left, and your variable name to the right in order to access your constants in .cpp files: When this header gets #included into a .cpp file, each of these variables defined in the header will be copied into that code file at the point of inclusion. How do you deal with initialisation? the reply. case 1 is undefined behaviour, the tentative definition causes an external definition to be generated for each translation unit it appears in . a header?! for global variables, it is undefined behaviour (objects must be defined only once in C++), for global constants, since they have internal linkage we're having several independent objects created. not inside any other code), then you are creating a so-called global variable that will: Number two is the important one here. With inline, the compiler picks 1 definition to be the canonical definition, so you only get 1 definition. When you change the type of x, it will change for everybody. The term optimizing away refers to any process where the compiler optimizes the performance of your program by removing things in a way that doesnt affect the output of your program. Without inline, you get 10 definitions. Constant initialization is usually applied at compile time. Which ability is most related to insanity: Wisdom, Charisma, Constitution, or Intelligence? Since this is a Semantic rule and not a Constraint, no diagnostic is required. All non-local variables with static storage duration are initialized as part of program startup, before the execution of the main function begins (unless deferred, see below). Content Discovery initiative April 13 update: Related questions using a Review our technical responses for the 2023 Developer Survey. The correct way to approach this is to have the header file say. Actually, if you are really aiming at defining a variable in a header, you can trick using some preprocessor directives: In this situation, i is only defined in the compilation unit where you defined DEFINE_I and is declared everywhere else. "FALSE" and 2. and put ALL the contents of every header file into one super redundant inclusions. rev2023.4.21.43403. Non-static data members can be initialized with member initializer list or with a default member initializer. Never use static in .h files, because you will create a different object every time it is included. 2) Otherwise, non-local static and thread-local variables are zero-initialized. However, as long as anything from a translation unit is odr-used, all non-local variables whose initialization or destruction has side effects will be initialized even if they are not used in the program. linkage denotes the same object or function. I have a method of #inclusion that works in a highly structured Embedded hyperlinks in a thesis or research paper. The "Includes.H" file contains and controls all included files Initialization - cppreference.com These variables will also retain their constexpr-ness in all files in which they are included, so they can be used anywhere a constexpr value is required. Although the use of static CAN be circumvented, as shown, it is After all, it's just compiler's enforcement. It also takes place during function calls: function parameters and the function return values are also initialized. I have seen this a couple of times before where an enum was declared in a header, and just below was a definition of a char** containing the corresponding labels. This means you save 9 constants worth of memory. The key is to keep the declarations of the variable in the header file and source file the same. There are two forms of static initialization: 1) If possible, constant initialization is applied. C++ : Variable declarations in header files - static or not?To Access My Live Chat Page, On Google, Search for "hows tech developer connect"As promised, I ha. How do I set my page numbers to the same size through the whole document? Wherever your code references variable x, the compiler can just replace x with 4 (since x is const, we know it wont ever change to a different value) and avoid having to create and initialize a variable altogether. Because these variables live outside of a function, theyre treated as global variables within the file they are included into, which is why you can use them anywhere in that file. An example of data being processed may be a unique identifier stored in a cookie. Initializer is not allowed in a block-scope declaration of a variable with external or internal linkage. something that was declared static! I have a 2 modules (.c files) and one .h header file: When I do gcc file1.c file2.c everything works fine and I get the expected output. How can I control PNP and NPN transistors together from one pin? The linker does not complain. is there such a thing as "right to be heard"? The correct mechanism for C++ in anonymous namespaces. @chrisharris - that is a limitation. Asking for help, clarification, or responding to other answers. That is because assigments are not valid on file level, only inside functions. Given the above downsides, prefer defining your constants in a header file (either per the prior section, or per the next section). Note that the OP isn't initializing the variable, it's a tentative definition. But their order of initialisation is undefined, so it's unspecified behaviour, it uses more memory, Initialization of a variable provides its initial value at the time of construction. still not conforming with the C spec: (1) An identifier declared in different scopes or in the same But if the two objectsare created, then they would consume more memory and two constructors (and destructors) would be called. C question: Why would one put 'static' variables in a header. The global variables get defined outside any function- usually at the very beginning/top of a program. Anyway, thats how it is, and its a good thing to master both anyway! All data allocation on a module basis should be kept in a header Do you define global variables in a C library? Can someone explain when you're supposed to use the static keyword before global variables or constants defined in header files? Don't you find it less cumbersome to have extern declaration in the header and definition in the C file? Find centralized, trusted content and collaborate around the technologies you use most. For example, lets say you have some const variable x thats initialized to value 4. Connect and share knowledge within a single location that is structured and easy to search. What is the purpose of the var keyword and when should I use it (or omit it)? Why this header file include create issue? I wrote the book The Legacy Code Programmer's Toolbox. Storage: 0 TB. Find centralized, trusted content and collaborate around the technologies you use most. Instead of redefining these constants in every file that needs them (a violation of the Dont Repeat Yourself rule), its better to declare them once in a central location and use them wherever needed. works of course, because you have only one .o file and so no possibility for collision. For both of these classes of variables, initialization occurs in two distinct stages: There are two forms of static initialization: After all static initialization is completed, dynamic initialization of non-local variables occurs in the following situations: If the initialization of a non-local variable with static or thread storage duration exits via an exception, std::terminate is called. I doubted that too. And after all and all, it's nothing but human's will Is including example.h necessary in foo.c? (Note: In C, int i; is a tentative definition, it allocates storage for the variable (= is a definition) if there is no other definition found for that variable in the translation unit.). The order of destruction of non-local variables is described in std::exit. Lets start with static variables declared in a file. As @caf commented, if the types don't match you get a warning (I do always include the header file in the corresponding c file anyway, since I require all functions to have a prototype). function contains the storage class specifier static, the identifier Rather, it definestwo global constants. In each scenario, imagine the contents of the header file inserted into the .c file and this .c file compiled into a .o file and then these linked together. file. Why xargs does not process the last argument? : in How a top-ranked engineering school reimagined CS curriculum (Ep. I *might* be wrong on the extern to a static. However, to use xwe need to define it somewhere. If we use a large number of global variables, then there is a high chance of error generation in the program. works fine because of the already mentioned "tentative definitions": every .o file contains one of them, so the linker says "ok". I don't think this has to do with "files", instead it has to do with "compilation modules". Such a declaration must appear with extern and cannot be a definition. Because the compiler compiles each source file individually, it can only see variable definitions that appear in the source file being compiled (which includes any included headers). Not the answer you're looking for? Everything in this article also applies to global variables as well as global constants, but global variables are a bad practice contrary to global constants, and we should avoid using them in the first place. First, these constants are now considered compile-time constants only within the file they are actually defined in (constants.cpp). We increment it in the code, and then we output that variable to see that it has changed accordingly. But I still don't see why having static definitions in header 1) The #ifndef guard prevents multiple definitions in a, Variable declaration in a header file [duplicate]. Within one Why did US v. Assange skip the court of appeal? Sample.c is only compiled once and it defines the variables. friction or gravity coefficients). Manually create gnu_unique_object symbols, Redefinition Error after moving code into another Header. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Using an Ohm Meter to test for bonding of a subpanel. I think you've missed the point. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. And what do you mean by file-scope? We had a guy here a while ago that took one of my projects and put You should not define global variables in header files. ", I asked, "So I only have to edit one file of course" came Instead you should declare it extern in header file included by all .c files that need it. Some kind of phobia of global variables. Inline global variables have external linkage by default. Correction-related comments will be deleted after processing to help reduce clutter. Even if C++ requires a unique definition of each object, it allows multiple declarations. Not the answer you're looking for? Indeed, if there is no definition we get an undefined external symbol error, and if there is more than one there is a duplicate external symbol. This works ok (assuming that Xhas a default constructor) when Xis defined and used only inside a .cppfile. rev2023.4.21.43403. pi or Avogadros number), or application-specific tuning values (e.g. Use std::string_view for constexpr strings. If the initialization of an inline variable is deferred, it happens before the first odr-use of that specific variable. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Initialization includes the evaluation of all subexpressions within the initializer and the creation of any temporary objects for function arguments or return values. You should not define global variables in header files. If global variable is to be used across multiple .c files, you should not declare it static. What is the Python equivalent of static variables inside a function? These can include physics or mathematical constants that dont change (e.g. Because const globals have internal linkage, each .cpp file gets an independent version of the global variable that the linker cant see. The value of a global variable can be changed accidentally as it can be used by any function in the program. // a function defined in File 1, forcing its dynamic initialization to run), // then b will be initialized prior to its use in A::A, https://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&oldid=146994, the order of initializing static data members, non-local references with static storage duration were, considered as static initialization, always, it was unclear whether evaluating function. the file itself and any file that includes it). Pre-calculated object representations are stored as part of the program image. When you compile a single file, such as main.cpp, youonly have one translationunit. (Note: In C, int i; is a tentative definition, it allocates storage for the variable (= is a definition) if there is no other definition found for that variable in the translation unit.) THen you can include your header file in as many places as you like. In case I have a variable that may be used in several sources - is it a good practice to declare it in a header? C++ : Declare and define static variable in C++ header?\rTo Access My Live Chat Page, \rOn Google, Search for \"hows tech developer connect\"\r\rAs promised, I have a secret feature that I want to reveal to you.\rThis is a YouTube's feature which works on Desktop.\rFirst, Make sure this video is playing.\rThen, type the letters 'awesome' on the keyboard.\rYour YouTube progress bar will transform into a flashing rainbow.\r\rA little intro about me,\rHi, my name is Delphi, nice to meet you.\rI can assist you in answering your queries.\rC++ : Declare and define static variable in C++ header?\rI am happy to answer more specific questions, so please feel free to comment or chat with me.\rWe encourage you to leave a comment below if you have an answer or insights on the answer.\rProviding an answer will be acknowledged and appreciated with a 'heart' from me.\rDeclare : define in static variable header? or is it better to declare it in a .c file and use extern in other files? If you declare a static variable at file level (i.e. You are welcome: I rarely give out such information. Does a password policy with a restriction of repeated characters increase security? can access it. Therefore, declaring static - by definition above - To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Inline variables have two primary restrictions that must be obeyed: With this, we can go back to defining our globals in a header file without the downside of duplicated variables: We can include constants.h into as many code files as we want, but these variables will only be instantiated once and shared across all code files. Why global array has a larger size than the local array? Another way to say this is: the entire point of. share stuff between source files; But the whole point of the 'static' keyword (at file scope) in 'C' "Why? To learn more, see our tips on writing great answers. You should define them in .c source file. The compilers are allowed to initialize dynamically-initialized variables as part of static initialization (essentially, at compile time), if the following conditions are both true: Because of the rule above, if initialization of some object o1 refers to a namespace-scope object o2, which potentially requires dynamic initialization, but is defined later in the same translation unit, it is unspecified whether the value of o2 used will be the value of the fully initialized o2 (because the compiler promoted initialization of o2 to compile time) or will be the value of o2 merely zero-initialized.
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