initializer list as argument to operator[]

Question

This question is related to the one discussed here.

I try to use an initializer list to create an argument to be passed to operator[].

#include <string>
#include <vector>

struct A {

std::string& operator[](std::vector<std::string> vec)
{
  return vec.front();
}

};

int main()
{
    // ok
    std::vector<std::string> vec {"hello", "world", "test"};

    A a;
    // error: could not convert '{"hello", "world", "test"}' to 'std::vector...'
    a[ {"hello", "world", "test"} ];
}

My Compiler (GCC 4.6.1) complains:

g++ -std=c++0x test.cpp
test.cpp: In function ‘int main()’:
test.cpp:20:8: error: expected primary-expression before ‘{’ token
test.cpp:20:8: error: expected ‘]’ before ‘{’ token
test.cpp:20:8: error: expected ‘;’ before ‘{’ token
test.cpp:20:35: error: expected primary-expression before ‘]’ token
test.cpp:20:35: error: expected ‘;’ before ‘]’ token

Should this be valid C++11?

Interestingly, when using operator() instead of operator[] it works.

Sou

Most definitely, it is compiler bug, as there should be absolutely no difference between a.f({"aa", ""bb"}) and a[{"aa", ""bb"}]. – Sanity

Passing a temporary explicitly compiles, though: a[ std::vector<std::string>({"hello", "world", "test"}) ]; – Bestial

Answer 1

Yes, it is valid C++11 and should work in any compliant compiler.

Please note that C++11 support in gcc is quite immature, and this code example will not compile in any 4.6 version, but only in 4.7 svn snapshot versions.

Emmott

Using GCC 4.7 from svn trunk revision 179769 doesn't work either. Same error report as with GCC 4.6. – Sou

@MichelSteuwer: Right before posting this I tried with my local build of gcc 4.7 which is r182904 (I think), and it worked. – Emmott

Answer 2

Yes, it is valid. The version of GCC you're using doesn't have full support for C++11 yet, so it fails to compile, even when it's standard-compliant.

A more recent version of GCC compiles your code. See live example at Compiler Explorer.

Ophiology