Determining the Candidate and Viable Functions

The first step of function matching identifies the set of overloaded functions considered for the call. The functions in this set are the candidate functions. A candidate function is a function with the same name as the called function and for which a declaration is visible at the point of the call. In this example, there are four candidate functions named f.

The second step selects from the set of candidate functions those functions that can be called with the arguments in the given call. The selected functions are the viable functions. To be viable, a function must have the same number of parameters as there are arguments in the call, and the type of each argument must match—or be convertible to—the type of its corresponding parameter.

We can eliminate two of our candidate functions based on the number of arguments. The function that has no parameters and the one that has two int parameters are not viable for this call. Our call has only one argument, and these functions have zero and two parameters, respectively.

The function that takes a single int and the function that takes two doubles might be viable. Either of these functions can be called with a single argument. The function taking two doubles has a default argument, which means it can be called with a single argument.

Having used the number of arguments to winnow the candidate functions, we next look at whether the argument types match those of the parameters. As with any call, an argument might match its parameter either because the types match exactly or because there is a conversion from the argument type to the type of the parameter. In this example, both of our remaining functions are viable:

• f(int) is viable because a conversion exists that can convert the argument of type double to the parameter of type int.

• f(double, double) is viable because a default argument is provided for the function’s second parameter and its first parameter is of type double, which exactly matches the type of the argument in the call.