What is the correct way to initialize static data members in C++ (98, 11 and 14)

The rules have always been as follows:

• A const static data member (SDM) of integral or enumeration type can be initialised in class with a constant expression.

• A constexpr SDM must be initialised in class with a constant expression.

  C++17 no longer requires an initializer when the default constructor initialises every member. Also, constexpr SDMs are implicitly inline variables, which makes their declaration a definition (external definitions are now deprecated).

• Other kinds of SDMs can have an initializer at their definition (which is in class if that SDM is declared inline).

Nothing has substantially changed between C++03 and C++11+ for code that is valid in both languages.

Note that for SDMs that are not inline, the in-class declaration is not a definition—regardless of whether an initializer is provided—and they must be defined if they are odr-used.

As of C++17, we can make your SDM inline, which makes its in-class declaration a definition:

class BufferedOutput
{
  static inline long bytecount = 50;
};