c++ - 大括号初始值设定项列表中是否允许显式转换运算符？

Question

以下代码可以使用 GCC 4.9.2 编译，但不能使用 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++ 说:

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);
      ^

奇怪的是，如果将 std::string 替换为像 int 这样的原始类型，它会起作用。

Answer 1

这似乎是一个 Clang 错误。 [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.

由于第二行编译良好，因此存在不一致:在重载决议方面它们应该是等价的。