Overloaded function. Why it's ambiguous here?

Question

I have my function overloaded in code below:

void function(char x, double y) {
    cout << "char, double" << endl;
}

void function(int x, int y) {
    cout << "int, int" << endl;
}

int main() {
    function('a', 'b');
    return 0;
}

When i try to compile it says me: "[Warning] ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second"

How does compiler make implicit conversions here so that it's ambiguous which candidate is right?

Answer 1

The literal constants 'a' and 'b' have type char, so there is no exact match. The ambiguity occurs because the first parameter matched the first function, but the preferred conversion of the second is to int, matching the second function.

GCC is very explicit about this, issuing the following diagnostic:

warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second

GCC seems to be saying, I could resolve this for you, but ISO C++ won't allow it. Type agreement is however important to good code quality and avoidance of errors.

You can coerce the selection by casting:

function( static_cast<int>('a'), static_cast<int>('b') );

or by supplying a function( char, char ) overload.

Answer 2

There are implicit conversions between double, int, and char in C++, so you should use static_cast<int> to convert data from char to int for example.

function( static_cast<int>(c), static_cast<int>(d) );

this will call function( int, int );

In your particular case, you use 'a' and 'b' character literals, which, as I mentioned above, have implicit conversions to int and double, because a char variable represents the ASCII value of the character entered in the assignment operator. Hence, we can initialize an unsigned char variable, or a char in the ways below:

unsigned char a = 97; // the ASCII decimal code for "a"
unsigned char b = 'a'; // translates into 97

Since an unsigned char or char variables is an 8 bits variable, and int is a 32 bit value, they have implicit conversions.