C++ Initializing static const structure variable

Question

I'm trying to add a static constant variable to my class, which is an instance of a structure. Since it's static, I must initialize it in class declaration. Trying this code

class Game {
    public:
        static const struct timespec UPDATE_TIMEOUT = { 10 , 10 };

    ...
};

Getting this error:

error: a brace-enclosed initializer is not allowed here before '{' token

error: invalid in-class initialization of static data member of non-integral type 'const timespec'

How do I initialize it? Thanks!

Solution

Initialize it in a separate definition outside the class, inside a source file:

// Header file
class Game {
    public:
        // Declaration:
        static const struct timespec UPDATE_TIMEOUT;
    ...
};

// Source file
const struct timespec Game::UPDATE_TIMEOUT = { 10 , 10 };  // Definition

If you include the definition in a header file, you'll likely get linker errors about multiply defined symbols if that header is included in more than one source file.

Comments

[Kolyunya]

I'm pretty noob in C++, I've heard, I should declare classes in classname.h file and define them in classname.c file. And so I will be able to include .h file into my programs as many times as I need, but when and how do I use .c file? I'm using a g++ compiler...

[Adam Rosenfield]

.c is for C source files, don't use it for C++. Use either .cc or .cpp for C++ source files (.cc is generally preferred on Linux, .cpp is generally preferred on Windows, but either will do). In general, a declaration says "here is the name of something, but that's all I know about it" (e.g. the name of a class or function). A definition says "here is the name of something and what it is", e.g. class members, function body, variable value, etc.

[Kolyunya]

yes, I get this, thank you! I declare my class in .h file, then I define it in .cpp file. Then I include .h to my program. Now the question: what should I do with my .cpp file? How do I use it? Should I write it somewhere here g++ main.cpp -o main? I'm using g++ on Linux.

[Aleks]

When compiling put all your .cpp files in the list. Do not put headers. ex. g++ main.cpp myclass.cpp -o main

[jave.web]

@Kolyunya if you want to keep the extension short you can use an UPPER-case .C extension... more on extensions in this answer: stackoverflow.com/a/3223792 and other answers on that page or just google "c++ source extensions"