Are explicit conversion operators allowed in braced initializer lists?

问题

The following code compiles with GCC 4.9.2 but not with Clang 3.5.0:

#include <string>

class Foo
{
public:
  explicit operator std::string() const;
};

std::string bar{Foo{}}; // Works in g++, fails in clang++
std::string baz(Foo{}); // Works in both

clang++ says:

foo.cpp:9:13: error: no matching constructor for initialization of 'std::string'
      (aka 'basic_string<char>')
std::string bar{Foo{}};
            ^  ~~~~~~~
...: note: candidate constructor not viable: no known conversion from 'Foo' to
      'const std::basic_string<char> &' for 1st argument
      basic_string(const basic_string& __str);
      ^

Curiously, it works if std::string is replaced with a primitive type like int.

回答1:

This seems to be a Clang bug. [over.match.list]/1:

When objects of non-aggregate class type T are list-initialized (8.5.4), overload resolution selects the constructor in two phases:

• [..]
• If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list.

Since the second line compiles fine, there is an inconsistency: They should be equivalent when it comes to overload resolution.

回答2:

From [class.conv.fct]/2:

A conversion function may be explicit (7.1.2), in which case it is only considered as a user-defined conversion for direct-initialization (8.5).

So the question is how you initialize your objects. Clearly baz is direct-initialized, so this works. By contrast, bar is direct-list-initialized, but not direct-initialized, and so the explicit conversion is not available.

回答3:

clang doesn't seem to care whether the conversion operator is explicit or not, and I believe it is correct due to the wording in [over.best.ics].

First of all, the direct-initialization

std::string baz(Foo{});

works on both gcc and clang, and is explained by [class.conv.fct]/2 as mentioned in KerrekSB's answer.

The direct-list-initialization

std::string bar{Foo{}};

on the other hand, does not consider any user defined conversions, explicit or not.

Quoting N3337, §13.3.3.1/4 [over.best.ics]

However, when considering the argument of a constructor or user-defined conversion function that is a candidate by 13.3.1.3 when invoked for the copying/moving of the temporary in the second step of a class copy-initialization, by 13.3.1.7 when passing the initializer list as a single argument or when the initializer list has exactly one element and a conversion to some class X or reference to (possibly cv-qualified) X is considered for the first parameter of a constructor of X, or by 13.3.1.4, 13.3.1.5, or 13.3.1.6 in all cases, only standard conversion sequences and ellipsis conversion sequences are considered.

来源：https://stackoverflow.com/questions/27573928/are-explicit-conversion-operators-allowed-in-braced-initializer-lists