Exact match

An overload without conversions needed for parameter types or only conversions needed between types that are still considered exact matches is preferred over an overload that requires other conversions in order to call.

void f(int x);
void f(double x);
f(42); // calls f(int)

When an argument binds to a reference to the same type, the match is considered to not require a conversion even if the reference is more cv-qualified.

void f(int& x);
void f(double x);
int x = 42;
f(x); // argument type is int; exact match with int&

void g(const int& x);
void g(int x);
g(x); // ambiguous; both overloads give exact match

For the purposes of overload resolution, the type “array of T” is considered to match exactly with the type “pointer to T”, and the function type T is considered to match exactly with the function pointer type T*, even though both require conversions.

void f(int* p);
void f(void* p);

void g(int* p);
void g(int (&p)[100]);

int a[100];
f(a); // calls f(int*); exact match with array-to-pointer conversion
g(a); // ambiguous; both overloads give exact match