Implicit construction of std::string does not happen during copy-initialization - c++

Question

I'm attempting to copy-initialize my CObj class as follows in the main() function:
#include <string>
#include <iostream>
class CObj
{
public:
CObj(std::string const& str) : m_str(str) { std::cout << "constructor" << std::endl; }
~CObj() { std::cout << "destructor" << std::endl; }
private:
std::string m_str;
};
int main()
{
CObj obj = "hello";
std::cout << "done" << std::endl;
}
However, the line CObj obj = "hello" fails to compile even though std::string is implicitly constructible from a char const*. According to my understanding here, this should work. Any reason why it doesn't? If I do this it works:
CObj obj = std::string("hello");

Answer 1

The literal "Hello" has type const char[6] : in order to call your constructor, two conversions are required : one to std::string and a second one to CObj .
But C++ only allows one user-defined conversion when doing an implicit conversion :
C++ Standard section § 12.3/4 [class.conv]
Type conversions of class objects can be specified by constructors and by conversion functions. These conversions are called user-defined conversions and are used for implicit type conversions
[...]
At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.
This is why this works :
CObj obj = std::string("hello");
Or this:
CObj obj("hello");
Or you could provide a constructor that accepts a const char* :
CObj(const char* cstr) : m_str(cstr) { ... }
I would always advise to make such constructors explicit to avoid unwanted implicit conversions, unless it really brings something to the user of the class.

Answer 2

You are limited to at most one user-defined conversion when instantiating an object (i.e. char[6] -> std::string and std::string -> CObj is one conversion too many).
To fix:
int main()
{
using namespace std::literals::string_literals; // this is necessary
// for the literal conversion
CObj obj = "hello"s; // note the extra "s", (which constructs a std::string)
std::cout << "done" << std::endl;
}