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In C++:
assert( std::is_same<int , int>::value ); // does not compile
assert( (std::is_same<int , int>::value) ); // compiles
Can anyone explain why?
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C library macro - assert() The C library macro void assert(int expression) allows diagnostic information to be written to the standard error file. In other words, it can be used to add diagnostics in your C program.
The assert() function tests the condition parameter. If it is false, it prints a message to standard error, using the string parameter to describe the failed condition. It then sets the variable _assert_exit to one and executes the exit statement. The exit statement jumps to the END rule.
The assert macro prints a diagnostic message when expression evaluates to false (0) and calls abort to stop program execution. No action is taken if expression is true (nonzero). The diagnostic message includes the failed expression, the name of the source file and line number where the assertion failed.
By making assert a macro instead, you can eliminate the calculation entirely when assertions are turned off. Apple's swift tackles this problem of having to resolve arguments before calling the function a bit but they don't have a precompiler, after all.
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.
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