Can I Take the Address of a Standard Library Function Like `std::tolower`?

Can I Take the Address of a Function Defined in the Standard Library?

Standard C functions are powerful tools, but their interaction with syntax like std::invoke can sometimes reveal unexpected behavior. Consider the following code:

#include <cctype>
#include <functional>
#include <iostream>

int main() {
    std::invoke(std::boolalpha, std::cout); // #1

    using ctype_func = int(*)(int);
    char c = std::invoke(static_cast<ctype_func>(std::tolower), 'A'); // #2
    std::cout << c << "\n";
}

The first call to std::invoke with std::boolalpha works as expected, resulting in a call to std::cout.setf(std::ios_base::boolalpha) and setting the Boolean output format. However, the second call with std::tolower raises concerns.

Is the Expected Output Guaranteed?

Unfortunately, the expected output of this code is not guaranteed in C 20. According to [namespace.std](https://en.cppreference.com/w/cpp/header/namespace/std), taking the address of a standard library function (unless it's designated as addressable) is undefined behavior. The std::tolower function falls under this category.

Why is that?

The reason lies in the fact that the C standard library header is included verbatim in C . This means that the behavior of standard C functions like tolower must be consistent with the C standard, which specifically prohibits taking their addresses.

Conclusion

In conclusion, even though std::tolower behaves similarly to a standard library function with addressable properties in the C code snippet, its underlying implementation as a C function makes taking its address undefined behavior. This behavior is not specific to std::tolower but applies to any standard library function not explicitly designated as addressable.

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