List-initialization and failed overload resolution of initializer_list constructor

问题

The below fails to compile with clang35 -std=c++11:

#include <iostream>
#include <string>
#include <initializer_list>

class A
{
 public:
  A(int, bool) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
  A(int, double) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
  A(std::initializer_list<int>) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
};

int main()
{
  A a1 = {1, 1.0};
  return 0;
}

with error

init.cpp:15:14: error: type 'double' cannot be narrowed to 'int' in initializer list [-Wc++11-narrowing]
  A a1 = {1, 1.0};
             ^~~
init.cpp:15:14: note: insert an explicit cast to silence this issue
  A a1 = {1, 1.0};
             ^~~
             static_cast<int>( )

OTOH, it warns about the narrowing and compiles on g++48 -std=c++11

init.cpp: In function ‘int main()’:
init.cpp:15:17: warning: narrowing conversion of ‘1.0e+0’ from ‘double’ to ‘int’ inside { } [-Wnarrowing]
   A a1 = {1, 1.0};
                 ^
init.cpp:15:17: warning: narrowing conversion of ‘1.0e+0’ from ‘double’ to ‘int’ inside { } [-Wnarrowing]

and produces the result

A::A(std::initializer_list<int>)

Does either behaviour make sense? Quoting from cppreference

All constructors that take std::initializer_list as the only argument, or as the first argument if the remaining arguments have default values, are examined, and matched by overload resolution against a single argument of type std::initializer_list

If the previous stage does not produce a match, all constructors of T participate in overload resolution against the set of arguments that consists of the elements of the braced-init-list, with the restriction that only non-narrowing conversions are allowed. If this stage produces an explicit constructor as the best match for a copy-list-initialization, compilation fails (note, in simple copy-initialization, explicit constructors are not considered at all)

Since narrowing conversions aren't allowed, I would expect the overload resolution step to not match the A(std::initializer_list<int>) constructor and instead match the A(int, double) one. For example, changing A(std::initializer_list<int>) to A(std::initializer_list<std::string>) compiles with both clang35 and g++48 and prints

A::A(int, double)

as expected.

回答1:

The behavior makes sense. Scott Meyers has an example almost exactly like this in Effective Modern C++ (emphasis in original):

If, however, one or more constructors declare a parameter of type std::initializer_list, calls using the braced initialization syntax strongly prefer the overloads taking std;:initializer_lists. Strongly. If there's any way for compilers to construe a call using a braced initializer to be a constructor taking a std::initializer_list, compilers will employ that interpretation.

Example using this class:

class Widget {
public:
    Widget(int, bool);
    Widget(int, double);
    Widget(std::initializer_list<long double>);
};

Widget w1(10, true); // calls first ctor
Widget w2{10, true}; // calls std::initializer_list ctor
Widget w3(10, 5.0); // calls second ctor
Widget w4{10, 5.0}; // calls std::initializer_list ctor

Those two calls call the initializer_list ctor even though they involve converting BOTH arguments - and even though the other constructors are perfect matches.

Furthermore:

Compilers' determination to match braced initializers with constructors taking std::initializer_lists is so strong, it prevails even if the best-match std::initializer_list constructor can't be called. For example:
class Widget {
public:
    Widget(int, bool); // as before
    Widget(int, double); // as before
    Widget(std::initializer_list<bool> ); // now bool
};

Widget w{10, 5.0}; // error! requires narrowing conversions

Both compilers pick the correct overload (the initializer_list one) - which we can see is required from the standard (§13.3.1.7):

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

(1.1) — Initially, the candidate functions are the initializer-list constructors (8.5.4) of the class T and the argument list consists of the initializer list as a single argument.
(1.2) — 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.

But calling that particular constructor involves a narrowing. In 8.5.1:

If the initializer-clause is an expression and a narrowing conversion (8.5.4) is required to convert the expression, the program is ill-formed.

So the program is ill-formed. In this case, clang chooses to throw an error while gcc chooses to issue a warning. Both compilers are conforming.

来源：https://stackoverflow.com/questions/28058371/list-initialization-and-failed-overload-resolution-of-initializer-list-construct

标签

c++

c++11

overload-resolution

list-initialization