c++ - The macro `assert`, why doesn't it compile in the global scope? -

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this snippet in cppreference doesn't compile. understand problem has fact assert macro expanded in global scope. is, if include part of code below, starting assert(sieof(s)==8); inside function, main() example, code work.

#include <cassert> struct s {     char c;  // 1 byte value              // 3 bytes padding     float f; // 4 bytes value     bool operator==(const s& arg) const { // value-based equality         return c == arg.c && f == arg.f;     } }; assert(sizeof(s) == 8); s s1 = {'a', 3.14}; s s2 = s1; reinterpret_cast<char*>(&s1)[2] = 'b'; // change 2nd byte assert(s1 == s2); // value did not change

but want understand why code doesn't compile, stated in original code. example in vs2013, macro defined follows:

    #define assert(_expression) (void)( (!!(_expression)) ||              (_wassert(_crt_wide(#_expression), _crt_wide(__file__), __line__), 0) )

and compiler complains following error message: 

error c2062: type 'void' unexpected

the obvious question is: why void accepted inside main not in global scope?

see error messages in clang.

global scope can contain declarations. assert macro expands expression statement, cannot appear @ global scope.


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