C++ function overload priority [duplicate] - c++

Question

This question already has answers here:
Unexpected overload resolution in visual studio involving void*, string and const char[]
(3 answers)
Closed 7 years ago.
Why would the code give an output: bool ?
Is there any way I could make the const char* to match the string version?
#include <string>
#include <iostream>
void func(bool)
{
std::cout << "bool" << std::endl;
}
void func(const std::string&)
{
std::cout << "string" << std::endl;
}
int main(int argc, char* argv[])
{
func("hello");
}

Answer 1

This happens because the compiler will prefer built-in conversions to user-defined conversions. The conversion from a pointer to a bool is built-in, so that overload is selected rather than constructing a std::string.
You could add an overload which takes a const char* and forwards it to the std::string version:
void func(const char* arg)
{
func(std::string{arg});
}

Answer 2

To answer the why:
Function matching is the process by which the compiler selects which function to call among an overload set.
Here, there are two viable candidates (the two functions you defined). To pick one, the compiler ranks the conversion they imply.
The first candidate void func(bool) implies an array-to-pointer conversion followed by a boolean conversion (from const char[6] to const char* to bool)
The second candidate implies a user-defined conversion (calling the std::string ctor taking a const char*)
The second conversion has lower ranking, so the first candidate is selected as the best match.

Answer 3

According to the standard N4431 §13.3.3.2/2 Ranking implicit conversion sequences [over.ics.rank] (emphasis mine):
When comparing the basic forms of implicit conversion sequences (as
defined in 13.3.3.1) (2.1) — a standard conversion sequence
(13.3.3.1.1) is a better conversion sequence than a user-defined
conversion sequence or an ellipsis conversion sequence, and (2.2) — a
user-defined conversion sequence (13.3.3.1.2) is a better conversion
sequence than an ellipsis conversion sequence (13.3.3.1.3).
Consequently, because the char const * to bool is a standard implicit conversion compared to the implicit conversion to std::string which is a user-defined conversion is a better conversion and is preferred in overload resolution.
In order to force overload resolution to choose the std::string version:
func(std::string("hello"));