How to Resolve Ambiguity When Passing Overloaded Functions to std::for_each()?

Passing Overloaded Functions to std::for_each()

When working with overloaded functions in C , you may encounter situations where you need to pass one of these overloads to an algorithm like std::for_each(). The compiler, however, may not automatically determine the correct overload to invoke based on the type of the iterator.

Example:

Consider the following class with two overloaded f() functions:

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

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

The compiler error arises because the call to f() is ambiguous. To resolve this, you need to explicitly specify which overload you intend to use.

Solution 1: Using static_cast<>()

One way to do this is to employ static_cast<>() to cast the function pointer to the appropriate signature, as shown below:

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

Solution 2: Pointer Declarations

Alternatively, you can utilize pointer declarations that implicitly specify the function signature. This approach allows the compiler to automatically deduce the correct overload to call:

// 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

Member Functions

If the f() function is a member function, you will need to use mem_fun or refer to the solution discussed in this Dr. Dobb's article.

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