c++ - How is ambiguity determined in the overload resolution algorithm? -

i'm trying understand overloading resolution method.

why ambiguous:

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

but isn't?

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

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

so why 1 ambiguous , 1 not? logic here?

the overload resolution rules define partial order on set of matches - if overload f1 not better match f2, not imply f2 better match f1. exact partial order can thought of comparing 2 points in k dimensions, number of arguments k. lets define 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 , x_j < y_j @ least 1 j. partial order on candidate non-template functions defined standard.

lets @ 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 strictly better other.

in 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, first overload better second in 1 argument , never worse second. thus, there no ambiguity - partial order indeed declare first 1 better.

(the above description not consider function templates. can find more details @ cppreference.)