Re: [Boost-users] formal review of Switch library ends tomorrow (Wednesday, January 9th) - reviews needed

Steven Watanabe wrote:

AMDG

Tobias Schwinger writes:

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I don't think the comma operator is a completely fool-proof way to detect void. It is.

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It could cause an ambiguity if the other type also defines an overloaded comma operator. No, because the operator is binary and one operand can have a reserved type which is only internally used by the test code.

I hope I haven't promised too much :-)...

Ok. Let's make this concrete. Here is what I imagine:

<code> Try the attached code - does it work for you? Regards, Tobias #include <iostream> #include <cassert> template< typename T > struct reserved { char i[2]; template< typename U > friend char operator,(T const&, reserved<U> const&); }; #define IS_VOID(expr,target_type) \ (sizeof(( (expr) , reserved() )) != 1) // void void_(); int nonvoid(); struct X {}; struct Y { Y(X); Y(); }; template<class T> T operator,(X, T); // int main() { assert( IS_VOID(void_(),int) ); assert( IS_VOID(void_(),X) ); assert( IS_VOID(void_(),Y) ); assert( !IS_VOID(nonvoid(),int) ); assert( !IS_VOID(nonvoid(),X) ); assert( !IS_VOID(nonvoid(),Y) ); assert( !IS_VOID(X(),int) ); assert( !IS_VOID(X(),X) ); assert( !IS_VOID(X(),Y) ); assert( !IS_VOID(Y(),int) ); assert( !IS_VOID(Y(),X) ); assert( !IS_VOID(Y(),Y) ); return 0; }