Inline variables

An inline variable is allowed to be defined in multiple translation units without violating the One Definition Rule (opens new window). If it is multiply defined, the linker will merge all definitions into a single object in the final program.

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;