Question

How does C++ determine implicit conversion/construction of objects few levels deep?
for example:

struct A {};
struct B: A {};
struct C { operator B() { return B(); } };

void f(A a) {}

int main(void)
{
    f(C());
}

Does it create tree of all possible conversions and chooses appropriate terminal? Something else? Thanks

Answer 1

The call to f() would need two conversions, one user-defined conversion (C to B) and one built-in conversion (derived-to-base: B to A). Calls with non-matching arguments succeed when they would need zero or one user-defined conversions. If different conversions (built-in or user-defined) would succeed, then, if all possible ways are equal in the number/kind of conversions they need, the call is ambiguous and the compiler needs to emit a diagnostic.

How compilers implement this isn’t specified by the standard.