How to initialize static fields in C++?

Question

I need a class with static std::vector<int> simples (first N simple numbers). I created it in static method __init__, which is called before any instance of MyClass is created:

class MyClass
{
public:
    MyClass()
    {
        /* I need to use MyClass::simples here */
        printf("%d\n", (int)MyClass::simples.size());
        /* But I get the error here :( */
    }

    static void __init__(int N)
    {
        simples.push_back(2);
        /* ...
           here I compute first N simple numbers and fill 
           MyClass::simples with them
        */
    }

private:
    static std::vector<int> simples;
};

int main()
{
    MyClass::__init__(1000);
    MyClass var;

    return 0;
}

But when I tried to use this vector in construction, I get undefined reference to 'MyClass::simples' error. How to solve my problem?

Using double underscore in names [is reserved to the compiler/standard library](http://stackoverflow.com/a/228797/440558). — Some programmer dude, Jun 25 '15 at 09:41

score 1 · Accepted Answer · answered Jun 25 '15 at 09:41

When defining a static member in C++, you need to write it two times: first in class definition as you did:

static std::vector<int> simples;

And then outside (preferably in an external .cpp file):

std::vector<int> MyClass::simples;

If you know about C language, this can help you: static members in C++ are comparable from global variables in C: defined as a prototype in .h file included whenever you need it, and value initialized in one .c/.cpp file.

score 0 · Answer 2 · answered Jun 25 '15 at 09:40

0

You have to define the static data member outside the class

std::vector<int> MyClass::simples;

Within the class definition it is only declared.

answered Jun 25 '15 at 09:40

Vlad from Moscow

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How to initialize static fields in C++?

2 Answers2