Calling an Empty std::function

If you attempt to call a std::function object that is empty, it will throw a std::bad_function_call exception. This exception is derived from std::exception and is defined in the <functional> header.

For example:

1#include <functional>
2#include <iostream>
3
4int main() {
5  std::function<void()> Callable;
6
7  try {
8    Callable();
9  } catch (const std::bad_function_call& e) {
10    std::cout << "Caught exception: " << e.what();
11  }
12}

1Caught exception: bad function call

To avoid this exception, you should always check if a std::function is empty before calling it. You can do this by using the operator bool() of std::function, which returns false if the std::function is empty, and true otherwise.

1#include <functional>
2#include <iostream>
3
4int main() {
5  std::function<void()> Callable;
6
7  if (Callable) {
8    Callable();
9  } else {
10    std::cout << "Callable is empty";
11  }
12}

1Callable is empty

By checking the state of the std::function before calling it, you can avoid the std::bad_function_call exception and handle the case where the std::function is empty in a way that's appropriate for your program's logic.