编写时

std::cout << toupper('a');

发生以下情况:

1. int toupper(int ch) 会被调用，并返回一个整数，其值为'A'( 0x41 ).
2. std :: basic_ostream :: operator<<(std::cout，0x41) 被调用，这是 int (2)重载，因为提供了 int .

1. int toupper(int ch) is called, and returns an integer whose value is 'A' (0x41).
2. std::basic_ostream::operator<<(std::cout, 0x41) is called, that is the int (2) overload since an int was provided.

总体上，它显示"65".

Overall, it prints "65".

作为解决方案，您可以将大写字母回退为 char :

As a solution, you can cast back your upper case to a char:

std::cout << static_cast<char>(toupper('a'));