在基于范围的for循环中重新声明变量

Question

This code fails with GCC 4.8.1 but works with MSVC2013:

#include <vector>
#include <string>

int main()
{
  std::vector<int> V{1,2,3,4,5};

  for (auto i : V)
  {
    std::string i = "oups";
  }
}

GCC 4.8.1 tells:

prog.cpp:10:17: error: redeclaration of ‘std::string i’
     std::string i = "oups";
                 ^

Is it some bug in the MSVC 2013 compiler?

Answer 1

Yes, it's a bug, but in GCC. C++11[stmt.ranged] clearly states that your range-based for loop is equivalent to this:

{
  auto && __range = (V);
  for ( auto __begin = __range.begin(),
             __end = __range.end();
        __begin != __end;
        ++__begin ) {
    auto i = *__begin;
    {
      std::string i = "oups";
    }
  }
}

So the inner i should simply hide the loop-control i without any problems.

And, as this live example shows, when spelled out like this, GCC actually accepts it just fine.

Answer 2

Its a bug in GCC, as well as Clang

See Range Based For-loop

for ( range_declaration : range_expression ){ loop_statement }

will be equivalent to

{
auto && __range = range_expression ; 
     for (auto __begin = begin_expr,
        __end = end_expr; 
        __begin != __end; ++__begin) { 
           range_declaration = *__begin;
            {  // Notice brace
               loop_statement 
            } // Notice ending brace
} 
} 

On another note, on Visual C++ 2013

for (auto i : V) std::string i = "oups"; 
       /* match this with equivalent for-loop
          loop-statement aren't in braces
      */

shouldn't compile.

Answer 3

c++11 [stmt.ranged] tells that the range-for loop expands into this :

{
  auto && __range = range-init;
  for ( auto __begin = begin-expr,
        __end = end-expr;
        __begin != __end;
        ++__begin ) {
    for-range-declaration = *__begin;
    statement
  }
}

Contrary to other answers, I am claiming there are no scopes for the statements, and that this is a MSVC's bug (not gcc or clang).