Capturing reference variable by copy in C++0x lambda

Question

According to the answers and comments for this question, when a reference variable is captured by value, the lambda object should make a copy of the referenced object, not the reference itself. However, GCC doesn't seem to do this.

Using the following test:

#include <stddef.h> #include <iostream>  using std::cout; using std::endl;  int main(int argc, char** argv) {     int i = 10;     int& ir = i;      [=]     {         cout << "value capture" << endl              << "i: " << i << endl              << "ir: " << ir << endl              << "&i: " << &i << endl              << "&ir: " << &ir << endl              << endl;     }();      [&]     {         cout << "reference capture" << endl              << "i: " << i << endl              << "ir: " << ir << endl              << "&i: " << &i << endl              << "&ir: " << &ir << endl              << endl;     }();          return EXIT_SUCCESS; } 

Compiling with GCC 4.5.1, using -std=c++0x, and running gives the following output:

value capture i: 10 ir: -226727748 &i: 0x7ffff27c68a0 &ir: 0x7ffff27c68a4  reference capture i: 10 ir: 10 &i: 0x7ffff27c68bc &ir: 0x7ffff27c68bc 

When captured by copy, ir just references junk data. But it correctly references i when captured by reference.

Is this a bug in GCC? If so, does anyone know if a later version fixes it? What is the correct behavior?

EDIT

If the first lambda function is changed to

[i, ir] {     cout << "explicit value capture" << endl          << "i: " << i << endl          << "ir: " << ir << endl          << "&i: " << &i << endl          << "&ir: " << &ir << endl          << endl; }(); 

then the output looks correct:

explicit value capture i: 10 ir: 10 &i: 0x7fff0a5b5790 &ir: 0x7fff0a5b5794 

This looks more and more like a bug.

Answer 1

This has just been fixed in gcc-4.7 trunk and gcc-4.6 branch. These should be available in gcc-4.7.0 (a while from now - still in stage 1) and gcc-4.6.2 (alas 4.6.1 just came out.)

But the intrepid could wait for the next snapshots or get a subversion copy.

See audit trail for details.

Answer 2

Compiled with VS 2010 gives:

 value capture i: 10 ir: 10 &i: 0012FE74 &ir: 0012FE78  reference capture i: 10 ir: 10 &i: 0012FF60 &ir: 0012FF60 

Looks like a bug for me.