How Can I Specify a Pointer to an Overloaded Function in C ?

Specifying a Pointer to Overloaded Functions

In the realm of C , it is not uncommon to encounter overloaded functions that share the same name but differ in their input parameters. This presents a challenge when attempting to pass such functions as arguments to generic algorithms like std::for_each().

Nested Overloaded Functions Within a Class

Consider the following example, where overloading occurs within a class:

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

    void scan(const std::string& s) {
        std::for_each(s.begin(), s.end(), f);
    }
};

Here, calling the scan() method with a string leaves the compiler uncertain about which implementation of f to use. By default, f() will take a character as an argument. However, the user may intend to use the overload that accepts an integer.

Explicit Pointer Assignment

To resolve this ambiguity, it is possible to explicitly specify the desired overload using static_cast<>():

// 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));

By casting the function pointer to the appropriate type, you can explicitly indicate to the compiler which overload should be called.

Alternative Method Using Function Pointers

Another approach is to declare function pointers explicitly:

// The compiler will figure out which f to use according to
// the function pointer declaration.
void (*fpc)(char) = &f;
std::for_each(s.begin(), s.end(), fpc); // Uses the void f(char c); overload
void (*fpi)(int) = &f;
std::for_each(s.begin(), s.end(), fpi); // Uses the void f(int i); overload

By creating function pointers with specific signatures, the compiler can automatically determine the correct overload to use.

Member Function Handling

If the overloaded function is a member function, a different approach is necessary. Consider using the mem_fun template or refer to additional resources, such as the Dr. Dobb's article linked in the provided answer.

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