How to initialize private static members in C++?

Question

What is the best way to initialize a private, static data member in C++? I tried this in my header file, but it gives me weird linker errors:

class foo
{
           


        

Answer 1

For a variable:

foo.h:

class foo
{
private:
    static int i;
};

foo.cpp:

int foo::i = 0;

This is because there can only be one instance of foo::i in your program. It's sort of the equivalent of extern int i in a header file and int i in a source file.

For a constant you can put the value straight in the class declaration:

class foo
{
private:
    static int i;
    const static int a = 42;
};