Why isn't the compiler able to promote char to int& but has no problem when passing it by reference-to-const (char to int const&)?
Example code:
#include <iostream>
using namespace std;
void func1(int & i) { cout << "int & " << i << endl; }
void func2(int const & i) { cout << "const int & " << i << endl; }
int main() {
char mychar = 'a';
func1(mychar); // compiler-error
func2(mychar); // no error
return 0;
}
This is allowed:
char c = 'X';
const int& i = c;
We're implicitly promoting c to int and binding i to that temporary. This can't really lead to any confusing behavior. i has the same value c did, it's just a different type.
But what would happen if the same logic were allowed with non-const:
char c = 'X';
int& i = c;
i = 'J';
i can't bind directly to c - it's the wrong type. We'd still need to promote c to and int and bind i to that temporary. But when we assign i, we're just modifying that temporary we bound - c would still be X. That would be incredibly confusing, non-intuitive, and error-prone. So the language forbids it.
Promotions and conversions create a new temporary object. Non-const references cannot bind to temporaries.
