How to Resolve Ambiguity When Passing Overloaded Function Pointers in C ?

Overloaded Function Pointers in C

In C , an overloaded function is a function with multiple implementations with different signatures. When an overloaded function is passed to a generic algorithm, it can be challenging to specify which implementation should be used.

Consider the following example:

class A {
    void f(char c);
    void f(int i);

    void scan(const std::string& s) {
        std::for_each(s.begin(), s.end(), f); // Overloaded function passed to 'for_each'
    }
};

Here, we have a class A with two overloaded member functions named f that take a char and an int as parameters, respectively. The scan member function of A attempts to use the for_each algorithm to iterate over a string, calling the f function for each character.

Problem:

However, the compiler cannot automatically determine which implementation of f should be used when passed to std::for_each. This is because the algorithm expects a function pointer with a specific signature, and the overloaded function signatures cannot be distinguished from each other based on the generic function pointer type.

Solution:

To specify which overload of f to use, we can employ one of the following techniques:

1. Static Cast with Function Signature:

We can use static_cast<>() to cast the function pointer to the specific signature required by std::for_each:

// Uses the void f(char c); overload
std::for_each(s.begin(), s.end(), static_cast<void (*)(char)>(f));

// Uses the void f(int i); overload
std::for_each(s.begin(), s.end(), static_cast<void (*)(int)>(f));

2. Function Pointer Declaration:

Alternatively, we can declare function pointers with the desired signature and assign them to the overloaded function:

void (*fpc)(char) = &A::f; // Function pointer for void f(char c)
std::for_each(s.begin(), s.end(), fpc); // Uses the void f(char c); overload

void (*fpi)(int) = &A::f; // Function pointer for void f(int i)
std::for_each(s.begin(), s.end(), fpi); // Uses the void f(int i); overload

For Member Functions (Optional):

If the overloaded function is a member function, the mem_fun utility can be employed, or the solution provided in the linked Dr. Dobb's article can be used to specify the desired overload.

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