10

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!

Kolyunya
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    Note that elaborated type specifiers (`struct timespec`) are pretty much not needed in C++. Just write `timespec`. – GManNickG Aug 22 '12 at 19:01

2 Answers2

23

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.

Adam Rosenfield
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    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... – Kolyunya Aug 22 '12 at 18:55
  • `.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. – Adam Rosenfield Aug 22 '12 at 19:13
  • 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. – Kolyunya Aug 22 '12 at 19:19
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    When compiling put all your .cpp files in the list. Do not put headers. ex. g++ main.cpp myclass.cpp -o main – Aleks Aug 23 '12 at 04:31
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    @Kolyunya if you want to keep the extension short you can use an ***UPPER**-case* `.C` extension... more on extensions in this answer: http://stackoverflow.com/a/3223792 and other answers on that page or just google "*c++ source extensions*" – jave.web Mar 04 '15 at 12:48
0

Declare the variable as a static variable inside a function and make that function return the reference to the variable.

corporateAbaper
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