Defining a static data member in the class definition

A static data member of the class may be fully defined within the class definition if it is declared inline. For example, the following class may be defined in a header. Prior to C++17, it would have been necessary to provide a .cpp file to contain the definition of Foo::num_instances so that it would be defined only once, but in C++17 the multiple definitions of the inline variable Foo::num_instances all refer to the same int object.

// warning: not thread-safe...
class Foo {
  public:
    Foo() { ++num_instances; }
    ~Foo() { --num_instances; }
    inline static int num_instances = 0;
};

As a special case, a constexpr static data member is implicitly inline.

class MyString {
  public:
    MyString() { /* ... */ }
    // ...
    static constexpr int max_size = INT_MAX / 2;
};
// in C++14, this definition was required in a single translation unit:
// constexpr int MyString::max_size;