Overload resolution (C++ only)

The process of selecting the most appropriate overloaded function or operator is called overload resolution.

Suppose that f is an overloaded function name. When you call the overloaded function f(), the compiler creates a set of candidate functions. This set of functions includes all of the functions named f that can be accessed from the point where you called f(). The compiler may include as a candidate function an alternative representation of one of those accessible functions named f to facilitate overload resolution.

After creating a set of candidate functions, the compiler creates a set of viable functions. This set of functions is a subset of the candidate functions. The number of parameters of each viable function agrees with the number of arguments you used to call f().

The compiler chooses the best viable function, the function declaration that the C++ runtime environment will use when you call f(), from the set of viable functions. The compiler does this by implicit conversion sequences. An implicit conversion sequence is the sequence of conversions required to convert an argument in a function call to the type of the corresponding parameter in a function declaration. The implicit conversion sequences are ranked; some implicit conversion sequences are better than others. The best viable function is the one whose parameters all have either better or equal-ranked implicit conversion sequences than all of the other viable functions. The compiler will not allow a program in which the compiler was able to find more than one best viable function. Implicit conversion sequences are described in more detail in Implicit conversion sequences (C++ only).

When a variable length array is a function parameter, the leftmost array dimension does not distinguish functions among candidate functions. In the following, the second definition of f is not allowed because void f(int []) has already been defined.

void f(int a[*]) {}
void f(int a[5]) {} // illegal

However, array dimensions other than the leftmost in a variable length array do differentiate candidate functions when the variable length array is a function parameter. For example, the overload set for function f might comprise the following:

void f(int a[][5]) {}
void f(int a[][4]) {}
void f(int a[][g]) {}   // assume g is a global int

but cannot include

void f(int a[][g2]) {} // illegal, assuming g2 is a global int

because having candidate functions with second-level array dimensions g and g2 creates ambiguity about which function f should be called: neither g nor g2 is known at compile time.

You can override an exact match by using an explicit cast. In the following example, the second call to f() matches with f(void*):

void f(int) { };
void f(void*) { };

int main() {
   f(0xaabb);            // matches f(int);
   f((void*) 0xaabb);    // matches f(void*)
}