In C++, we can use sizeof and decltype on non-static data member of a function, but either of them doesn't work on non-static method's return type, e.g. decltype(std::vector<int>::size()), sizeof(std::vector<int>::size()), any reason why this is restricted? or there are some way works i didn't discovered?
The decltype specifier needs to be given a legal expression - something you could use in actual code. So you can create an instance and call the member on that instance like this:
decltype(std::vector<int>().size())
It is the same for the sizeof operator. It needs to receive a legal expression.