c preprocessor - C++ assert: the precedence of the expression in an assert macro

Question

In C++:

  assert(  std::is_same<int , int>::value  ); // does not compile

  assert( (std::is_same<int , int>::value) ); // compiles

Can anyone explain why?

Answer 1

assert is a preprocessor macro. Preprocessor macros are dumb; they don't understand templates. The preprocessor sees 10 tokens within the parentheses:

assert( std :: is_same < int , int > :: value );

It splits at the comma. It doesn't know that this is the wrong place to split at, because it doesn't understand that std::is_same<int and int>::value aren't valid C++ expressions.

The preprocessor is smart enough to not break up the contents of inner pairs of parentheses across multiple arguments. That's why adding the extra parentheses fixes the problem.