What is the purpose of the ## operator in C++, and what is it called?

Question

I was looking through the DXUTCore project that comes with the DirectX March 2009 SDK, and noticed that instead of making normal accessor methods, they used macros to create the generic accessors, similar to the following:

#define GET_ACCESSOR( x, y )  inline x Get##y()  { DXUTLock l; return m_state.m_##y;};
...

GET_ACCESSOR( WCHAR*, WindowTitle );

It seems that the ## operator just inserts the text from the second argument into the macro to create a function operating on a variable using that text. Is this something that is standard in C++ (i.e. not Microsoft specific)? Is its use considered good practice? And, what is that operator called?

Answer 1

Token-pasting operator, used by the pre-processor to join two tokens into a single token.

Answer 2

This is also standard C++, contrary to what Raldolpho stated.

Here is the relevant information:

16.3.3 The ## operator [cpp.concat]

1 A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition.

2 If, in the replacement list, a parameter is immediately preceded or followed by a ## preprocessing token, the parameter is replaced by the corresponding argument’s preprocessing token sequence.

3 For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token. If the result is not a valid preprocessing token, the behavior is undefined. The resulting token is available for further macro replacement. The order of evaluation of ## operators is unspecified.