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Can I Take the Address of a Standard Library Function Like `std::tolower`?
Can I Take the Address of a Standard Library Function Like `std::tolower`?
Dec 17, 2024 pm 08:22 PM
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
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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