How is ambiguity determined in the overload resolution algorithm?

Question

I'm trying to understand the overloading resolution method.

Why is this ambiguous:

void func(double, int, int, double) {} void func(int, double, double, double) {}  void main() {     func(1, 2, 3, 4); } 

but this isn't?

void func(int, int, int, double) {} void func(int, double, double, double) {}  void main() {     func(1, 2, 3, 4); } 

In the first case there are 2 exact parameters matches and 2 conversions against 1 exact match and 3 conversions, and in the second case there are 3 exact matches and 1 conversion against 1 exact matches and 3 conversions.

So why is one ambiguous and one is not? What is the logic here?

Answer 1

The overload resolution rules only define a partial order on the set of all matches - if an overload F1 is not a better match than F2, it does not imply that F2 is a better match than F1. The exact partial order can be thought of as comparing two points in k dimensions, where the number of arguments is k. Lets define this partial order on points in k-dim space - (x_1, x_2,..., x_k) < (y_1, y_2,..., y_k) if x_i <= y_i for all i and x_j < y_j for at least one j. This is exactly the partial order on candidate non-template functions defined by the standard.

Lets look at your examples :

void func(double, int,    int,    double) {}                   vvv     vvv       vvv                  better  better    equal void func(int,    double, double, double) {}           vvv                       vvv          better                    equal 

So neither overload is strictly better than the other.

In your second example:

void func(int,   int,   int,   double) {}           vvv    vvv    vvv     vvv          equal  better better  equal void func(int, double, double, double) {}           vvv          equal 

Now, the first overload is better than the second in all but one argument AND is never worse than the second. Thus, there is no ambiguity - the partial order does indeed declare the first one better.

(The above description does not consider function templates. You can find more details at cppreference.)