Conversion from const char[] to class with constructor from string

Question

I try to pass a string to a function instead an object of class A, but I get an error. Why is it not working and how can I make it work?

#include <string>

class A {
public:
    A(const std::string &str) {

    }
};

void test(const A &a) {

}

int main()
{
    A a("abc"); //it's ok
    test(a); //it's ok
    test("abc"); //error?
    return 0;
}

I don't want to pass a string object to test like test(std::string("abc")). I need convert it from const char*

Answer 1

In the implicit conversion sequence of one type to another, there is a rule that states: At most, only one user defined conversion can be applied. In

test("abc");

You need two user defined conversions. First you need to conversion from const char[N] to std::string, which is considered a user defined conversion¹. Then you need to convert that std::string to a A, which would be a second user defined conversion. That is the reason you get the error

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To fix, you can use

test(A("abc"));
//or
test(std::string("abc"));

and now you have one explicit user defined conversion, and one implicit conversion, satisfying the one implicit conversion rule.

You could also provide a constructor for A that accepts a const char* so that you can directly convert from "abc" to an A.

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1: Even though std::string is part of the standard library, it is not a "built in" type, but a library type. This means it is counted as a user defined type, even though it shipped with your implementation.