Using the GNU Compiler Collection (GCC): Static Definitions

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#### 14.7.1 Declare and Define Static Members

When a class has static data members, it is not enough to declare the static member; you must also define it. For example:

    class Foo
    {
      …
      void method();
      static int bar;
    };

This declaration only establishes that the class `Foo` has an `int` named `Foo::bar`, and a member function named `Foo::method`. But you still need to define both `method` and `bar` elsewhere. According to the ISO standard, you must supply an initializer in one (and only one) source file, such as:

    int Foo::bar = 0;

Other C++ compilers may not correctly implement the standard behavior. As a result, when you switch to `g++` from one of these compilers, you may discover that a program that appeared to work correctly in fact does not conform to the standard: `g++` reports as undefined symbols any static data members that lack definitions.