c initialize static member variable in header file

How can I shave a sheet of plywood into a wedge shim? Static member functions can be used to work with static member variables in the class. Albiet still one definition per translation unit, but each translation unit uses a different definition. For this reason, the declaration approach for static class members is left perfectly "traditional": you only declare it in the header file (i.e. Codesearch uses this as a heuristic for finding C++ symbols in comments and generating cross-references for that symbol. Since the include guards are only affecting the compilation of one translation unit, they won't help, either. If you have definitions in header file and you use it across different files in the project. For external-linkage objects (as well as external-linkage functions) ODR is further restricted in 3.2/3 to one and only definition. Re "You can't define a static member variable more than once", well I can. C++ introduces two more uses for the static keyword when applied to classes: static member variables, and static member functions. Re "therefore", there are two counter examples (each as its own answer) here. For more information, see Using Const Correctly. Note that constructors and destructors can be more expensive than they appear and should also generally not be inlined. The nice thing about this solution is that all of the initialization code is kept hidden inside the original class with the static member. @martona is right. If you put variable definitions into a header, it is going to be defined in each translation unit where the header is included. Use std::make_unique() instead. This is something that a modern compiler/linker combination could easily resolve, and not a good enough reason for such a cumbersome limitation. c++ initialization of static object in class declaration, Class declaration in a header file and static variables. Second, in the lesson on global variables, you learned that global variables are dangerous because any piece of code can change the value of the global variable and end up breaking another piece of seemingly unrelated code. Initialization. Why can't initialize the static member in a class in the body or in the header file? A static member variable (but not a namespace-scope variable) declared constexpr is implicitly an inline variable., https://en.cppreference.com/w/cpp/language/inline. Asking for help, clarification, or responding to other answers. In this way, the compiler will generate the same initialization for each time the static variables are accessed. First, because all static members are instantiated only once, there is no way to have multiple copies of a pure static class (without cloning the class and renaming it). // Even better (likely inlined at compile time). // to use checks for DCHECK_IS_ON() anywhere. if you can dig up that macro I'll be very grateful. Section 9.4.2, Static data members, of the C++ standard states: If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a const-initializer which shall be an integral constant expression. Const . If your static variable can be directly initialized, no constructor is needed: you can initialize the static member variable at the point of definition (even if it is private). For example if you see the static const member. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. - Alf and Dietmar are more kind of a "hack", exploiting that definitions of, static data member of a class template (14.5.1.3), inline function with external linkage (7.1.2). Example: Declaring static variables in the header files, 2. @Daniel: It's not a problem in the case of multiple definition for member functions because those member functions get multiple definitions. Whether the OP can is quite another matter. Note that because all the data and functions in this class are static, we dont need to instantiate an object of the class to make use of its functionality! Header files can be used for global declarations or can be included in multiple source files. They are for providing interface declarations. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. // Until C++17, decltype(x) is std::initializer_list, not int! are allowed in multiple TU ( FYI: static functions defined inside a class definition have external linkage and are implicitly defined as inline ) . The reason why the values of static data members of other types cannot be specified within the class declaration is that non-trivial initialization is likely required (that is, a constructor needs to run). Any code written inside a DCHECK() macro, or the various DCHECK_EQ() and similar macros, will be compiled out in builds where DCHECKs are disabled. You also have the option to opt-out of these cookies. Why is the passive "are described" not grammatically correct in this sentence? Static variables can be declared in the header files or any other source file. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. First, because static member functions are not attached to an object, they have no this pointer! Any cookies that may not be particularly necessary for the website to function and is used specifically to collect user personal data via analytics, ads, other embedded contents are termed as non-necessary cookies. This is called the Static Initialization Order Fiasco. Any logical reasoning/constraint for this? if you can dig up that macro I'll be very grateful. Everything I've come up with requires the DerivedClass to do something (macro/template/etc) in the header AND the cpp file. The same issue is the main reason for having class member function implementations in a .cpp file instead of putting everything in your main class' .h. String has obviously to be default-initialized outside of the class. One exception from this rule was made for const static class members of integral or enum types, because such entries can for Integral Constant Expressions (ICEs). Thanks for contributing an answer to Stack Overflow! Is there a legal reason that organizations often refuse to comment on an issue citing "ongoing litigation"? Style, also known as readability, is what we call the conventions that govern our C++ code. Static variables can be used in the source file while non-static variables can only be used in the header file. Isn't there a way to define [the static data member] in the header? This reduces the chance of uninitialized variables, documents default values in the declaration, and increases the number of constructors that can use =default (see below). Checking with g++ 4.9.2, it needs either a, thanks! Just stick it in one of the .cpp files and be done with it. Normally we access private members through public member functions. @Cheersandhth.-Alf Technically, I'm not sure if any of the two is a counter example. A word of warning about classes with all static members This code will probably do expensive things that are not needed in official release builds, which is bad. In the previous lesson on 13.13 -- Static member variables, you learned that static member variables are member variables that belong to the class rather than objects of the class. is a structure and has to be defined in a .cpp file, but the values Others have given good advice. // BAD: bare call to new; for refcounted types, not compatible with one-based. How to fix this loose spoke (and why/how is it broken)? It would be like asking the linker to remove all of the code to initialize str in the compilation of the following: EDIT It is instructive to look at the assembler output of g++ for the following code: The assembly code can be obtained by executing: Looking through the SO4547660.s file that g++ generates, you can see that there is a lot of code for such a small source file. The term Style is a bit of a misnomer, since these conventions cover far more than just source file formatting. At least with member functions you do have the option to implement them in the header. Instead of initializing individual members the whole static structure is initialized: Note that this solution still suffers from the problem of the order of What the true purpose of information hiding? I think having initialization in the class is more "intuitive" and less confusing.It also gives the sense of both static and global-ness of the variable. If you wanted to intialize a::var with b::some_static_fn() you'd need to make sure that every .cpp file that includes a.h includes b.h first. The following code presents a method that behaves more like a regular constructor. // `FooImpl` implements the `FooBase` class. @Cheersandhth.-Alf Technically, I'm not sure if any of the two is a counter example. http://msdn.microsoft.com/en-us/library/s1sb61xd.aspx. This category only includes cookies that ensures basic functionalities and security features of the website. The initial value may be provided in the initializer section of a declarator or a new expression. Static members obey the class member access rules (private, protected, public). // Use =default here for consistency, even though the implementation is {}. Is it possible for rockets to exist in a world that is only in the early stages of developing jet aircraft? Globals variables are Evil. Can you expand on what the consequences are of declaring a variable inline? Within that constraint, mark methods as const where possible. As others have shown, This tells the compiler to actually allocate an instance (memory) for the variable. This website uses cookies to improve your experience while you navigate through the website. Obviously definitions of static data members of class type are not considered to appear in multiple translations units. the last time I tried to have multiple definitions of a method was years ago and the link failed.) However, if your DCHECK() relies on work that is done outside of the DCHECK() macro, that work may not be eliminated in official release builds. you need to declare all your static variables at the top of .cpp files where you use them. C++ supports the mechanism of. C++ does allow you to define integral static members within the declaration, but you still have to include a definition within a single translation unit, but this is just a shortcut, or syntactic sugar. Since the include guards are only affecting the compilation of one translation unit, they won't help, either. Thus, according to the standard, it is not allowed. It's wrong and wrongheaded. This is done by using the equal sign (or alternatively the constructor syntax (or initializer list)). This proposal looks interesting. Defining a class method in the declaration is an implicit request to inline it. I will just elaborate (cause more confusion) on some terms. In C++17 you can use inline variables, which you can use even outside classes. are allowed in multiple TU ( FYI: static functions defined inside a class definition have external linkage and are implicitly defined as inline ) . Would it be possible to build a powerless holographic projector? Thanks for helping to make the site better for everyone. Avoid this in header files except for cheap non-virtual getters and setters. Note that {} are allowed on the right side of the = here (e.g. You can always deviate from something on this page, if the relevant author/reviewer/OWNERS agree that another course is better. They can be defined in header files. For example, you can't just write x = 42 without first declaring 'x'. Initialization of a multidimensional arrays in C/C++. Prefer the following general rules: Use assignment syntax when performing simple initialization with one or more literal values which will simply be composed into the object: Using = here is no less efficient than () (the compiler wont generate a temp + copy), and ensures that only implicit constructors are called, so readers seeing this syntax can assume nothing complex or subtle is happening. The sample taken from Static variables are declared in the header file and non-static variables are declared in the source file. It is mandatory to procure user consent prior to running these cookies on your website. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The One Definition Rule is: "No translation unit shall contain more than one definition of any variable, function, class type, enumeration type or template". The linker can easily merge all occurrences of __ZN8my_class3strE, but it cannot possibly isolate the code that calls __ZNSsC1EPKcRKSaIcE within the object file's implementation of __static_initialization_and_destruction_0(int, int). But anyway, note that "For this to happen, it has to appear in a single object file" is disproved by both those answers. This means that you may need to guard the DCHECK() macro if it depends on a variable or function that is also guarded by a check for DCHECK_IS_ON(). initialize another static variable, the first may not be initialized, A static member variable (but not a namespace-scope variable) declared constexpr is implicitly an inline variable., https://en.cppreference.com/w/cpp/language/inline. What is the best way to initialize a private, static data member in C++? For static member variables in C++ class - the initialization is done outside the class. Anyway, I updated my answer, which was written a few minutes after the question and intended to explain why this cannot be done as suggested. This is done to avoid unused variable and unused function warnings when DCHECKs are turned off. In Return of the King has there been any explanation for the role of the third eagle? int x; // declaration x = 42; // use x. Use constructor syntax when construction performs significant logic, uses an explicit constructor, or in some other way is not intuitively simple to the reader: Use C++11 uniform init syntax ({} without =) only when neither of the above work: Never mix uniform init syntax with auto, since what it deduces is unlikely to be what was intended: For more reading, please see abseil's Tip of the Week #88: Initialization: =, (), and {}. So now struct can have virtual function and support inheritance ? Anyway, think twice about the design here. The full ODR has a more detailed set of requirements for each kind of entity. What one-octave set of notes is most comfortable for an SATB choir to sing in unison/octaves? Since this answer was posted we've got the inline object proposal, which I think is accepted for C++17. of a class template (14.5.1.3), member function of a class template (14.5.1.1), or template specialization for Regulations regarding taking off across the runway. @Elazar If I have to provide multiple definition files just to initialize single members in multiple classes it's counterproductive, and if I provided a single definition file for multiple headers its counterintuitive. // Creates 500 copies of the provided initializer, // Cannot use '=' since C() is explicit (and "()" is invalid syntax here). Downvoted because of the inaccuracies and wild claims. How to print and connect to printer using flutter desktop via usb? Instead, any code outside of a DCHECK() macro, that is only needed when DCHECKs are enabled, should be explicitly eliminated by checking DCHECK_IS_ON() as this code does. However, you can define static member functions! To keep the definition of a static value with the declaration in C++11 How to initialize a static member object? Can this be extended to define the contents of the bstring as a template argument somehow? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Non-static variables have the dynamic scope and are defined and used each time the variable is needed. Previous. a nested static structure can be used. UPDATE: My answer below explains why this cannot be done in the way suggested by the question. If we are declaring a static variable then the scope of the variable is restricted to a particular file only. Static member functions do not work on an object, so the this pointer is not needed. Checking with g++ 4.9.2, it needs either a, thanks! Instead of initializing individual members the whole static structure is initialized: Note that this solution still suffers from the problem of the order of This is fine. We can do better. No need. Isn't there a way to define [the static data member] in the header? Not the answer you're looking for? How to write your own header file in C? Is Spider-Man the only Marvel character that has been represented as multiple non-human characters? To learn more, see our tips on writing great answers. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Declaring static variables in the source files. // "()" intentionally omitted to avoid unnecessary zero-initialization. In this case the static variable headers will contain either { "" } Allowing in-class initializers for static objects would immediately go against this convention: the initializers would go into header files (where class definitions usually reside) and thus generate multiple definitions of the same static object (one for each translation unit that includes the header file). The main idea of ICEs is that they are evaluated at compile time and thus do no depend on definitions of the objects involved. How does a government that uses undead labor avoid perverse incentives? In this way, the compiler will generate the same initialization for each time the static variables are accessed. // BAD: same as the above, plus mentions type names twice. So, this is allowed: So long as a) the expression is const integral or enumeration type, b) the expression can be evaluated at compile-time, and c) there is still a definition somewhere that doesn't violate the one definition rule: In C++ since the beginning of times the presence of an initializer was an exclusive attribute of object definition, i.e. - Alf and Dietmar are more kind of a "hack", exploiting that definitions of, static data member of a class template (14.5.1.3), inline function with external linkage (7.1.2). base::WrapUnique(new Foo) and base::WrapUnique(new Foo()) mean something different if Foo does not have a user-defined constructor. Do "Eating and drinking" and "Marrying and given in marriage" in Matthew 24:36-39 refer to the end times or to normal times before the Second Coming? The suggested answers from Cheers and hth. How to keep the value of "a parameter of a class" CONSTANT so that it can be accessed from other classes? Of course, if you use this function to initialize other global objects it may also make sure that the object is constructed in time. If your first, @Daniel Trebbien: That's not the whole ODR. How much of the power drawn by a chip turns into heat? You can't define a static member variable more than once. Anonymous downvoter, please explain your downvote. This will make the program more memory efficient. How does a government that uses undead labor avoid perverse incentives? When possible, avoid bare new by using std::make_unique() and base::MakeRefCounted(): Never friend std::make_unique to work around constructor access restrictions. Classes can be created with all static member variables and static functions. rev2023.6.2.43474. Or is it purely legacy implementation - which the standard does not want to correct? Instead, specify the pointer part outside of auto: For safety and simplicity, don't return pointers or references to non-const objects from const methods. Static variables have the static scope and are defined once and used multiple times. Before 2017, this was thread-unsafe, and base::LazyInstance was widely used. Code works in Python IDE but not in QGIS Python editor. The declaration tells the compiler whether the element is an . Globals variables are Evil. Yeah except that it's a pain in the ass to create a cpp file for an "interface" class that would otherwise only require a single header which doesn't even need to be added to a project since it doesn't require compilation. Avoid const_cast to remove const, except when implementing non-const getters in terms of const getters. Because obvious is subjective and even the examples in the style guide take some thought to figure out, being explicit is clear, simple, and avoids any risk of accidental copying. __ZN8my_class3strE for sizeof(std::string) bytes as well as assembly code to call __ZNSsC1EPKcRKSaIcE within its implementation of the __static_initialization_and_destruction_0(int, int) function. This makes sense when you think about it -- the this pointer always points to the object that the member function is working on. How does the damage from Artificer Armorer's Lightning Launcher work? And while some modern languages do support static constructors for precisely this purpose, C++ is unfortunately not one of them. Declarations go into header file, definitions into cpp file. [class.static.data] 3 allows giving the initializer (. Flutter change focus color and icon color but not works. std::string BaseClass::bstring {"."}; Connect and share knowledge within a single location that is structured and easy to search. Can this be extended to define the contents of the bstring as a template argument somehow? class BaseClass { public: static std::string bstring; }; String has obviously to be default-initialized outside of the class. // GOOD; make_unique<>/MakeRefCounted<> are clear enough indicators of the, // For explanatory purposes. Be careful when defaulting move operations. The same holds true for pure static classes. // Using "()" here triggers "most vexing parse"; // "{}" is arguably more readable than "(())". The main purpose of the header files is to provide information to the compiler for the compilation process. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. Does the policy change for AI-generated content affect users who (want to) Static const getter file giving me an error. Is there a place where adultery is a crime? This class utilizes a static member variable to hold the value of the next ID to be assigned, and provides a static member function to return that ID and increment it. Sending signal from static class method in Qt, C++ Initialization of static variables (Once again), static member variables initialization c++, c++ initialization of static object in class declaration, When the initialization of static member variable take place, static const member variable initialization, Static variable initialization with non static member variable. Re "therefore", there are two counter examples (each as its own answer) here. An alternative is to use a function, as Dietmar suggested. But isn't that what the include guards or pragma once do, ensure that it is only included once in the compilation process? It's because of the way the code is compiled. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. There are myriad ways to initialize variables in C++. When called, they have no this pointer.. Static member functions cannot be virtual, const, volatile, or ref-qualified.. I tried this in my header file, but it gives me weird linker errors: class foo { private: static int i; }; int foo::i = 0; I'm guessing this is because I can't initialize a private member from outside the class. Background can be found in this thread and this thread. Necessary cookies are absolutely essential for the website to function properly. These variables are used to change while running the program. The inline specifier, when used in a decl-specifier-seq of a variable with static storage duration (static class member or namespace-scope variable), declares the variable to be an inline variable. Can const member variable of class be initialized in a method instead of constructor? The DCHECK() and friends macros still require the variables and functions they use to be declared at compile time, even though they will not be used at runtime. A third concept is "initialization". Why recover database request archived log from the future. Edit: Also, since this answer was posted we've got the inline object proposal, which I think is accepted for C++17. Tip of the Week #88: Initialization: =, (), and {}. Static variables are more preferred than non-static variables.

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