Is initializer list like this legal in C++11?

问题:

I read the C++ primer 5th edition, which says that newest standard support list initializer.

My test code is like this:

#include <iostream>
#include <string>
#include <cctype>
#include <vector>
using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;
using std::ispunct;
int main(int argc, char *argv[])
{
    vector<int> a1 = {0,1,2};
    vector<int> a2{0,1,2}; // should be equal to a1
    return 0;
}

Then I use Clang 4.0:

bash-3.2$ c++ --version
Apple clang version 4.0 (tags/Apple/clang-421.0.60) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin12.2.0
Thread model: posix

And compile it like this:

c++ -std=c++11 -Wall    playground.cc   -o playground

However, it complains like this:

playground.cc:13:17: error: no matching constructor for initialization of
      'vector<int>'
    vector<int> a1 = {0,1,2};
                ^    ~~~~~~~

 /usr/include/c++/4.2.1/bits/stl_vector.h:255:9: note: candidate constructor
  [with _InputIterator = int] not viable: no known conversion from 'int'
  to 'const allocator_type' (aka 'const std::allocator<int>') for 3rd
  argument;
    vector(_InputIterator __first, _InputIterator __last,
    ^
/usr/include/c++/4.2.1/bits/stl_vector.h:213:7: note: candidate constructor
  not viable: no known conversion from 'int' to 'const allocator_type'
  (aka 'const std::allocator<int>') for 3rd argument;
  vector(size_type __n, const value_type& __value = value_type(),

I checked the C++ support status of Clang, and it looks that it should already support Initializer lists in Clang 3.1. But why does my codes doesn't work. Does anyone have ideas about this?

回答1:

The code is legal, the problem is with your compiler+stdlib setup.

Apple's Xcode ships with the ancient version 4.2.1 of the GNU C++ standard library, libstdc++ (see https://stackoverflow.com/a/14150421/981959 for details) and that version pre-dates C++11 by many years so its std::vector doesn't have an initializer-list constructor.

To use C++11 features you either need to install and use a newer libstdc++, or tell clang to use Apple's own libc++ library, which you do with the -stdlib=libc++ option.