How to Convert a C++ String to Uppercase

You need to put a double colon before toupper:

transform(input.begin(), input.end(), input.begin(), ::toupper);

Explanation:

There are two different toupper functions:

1. toupper in the global namespace (accessed with ::toupper), which comes from C.

2. toupper in the std namespace (accessed with std::toupper) which has multiple overloads and thus cannot be simply referenced with a name only. You have to explicitly cast it to a specific function signature in order to be referenced, but the code for getting a function pointer looks ugly: static_cast<int (*)(int)>(&std::toupper)

Since you’re using namespace std, when writing toupper, 2. hides 1. and is thus chosen, according to name resolution rules.