Why can't I emplace ifstreams in a vector?

Question

The following example program doesn't compile for me on either clang 3.1 or gcc 4.8:

#include <fstream>
#include <vector>

using namespace std;

int main()
{
    vector<ifstream> bla;
    bla.emplace_back("filename");
    return 0;
}

However, I thought emplace_back should

"Insert a new element at the end of the vector, right after its current last element. This new element is constructed in place using args as the arguments for its construction."

Does anyone know why this doesn't compile then? did I misunderstand or are the library implementations not yet complete?

Answer 1

Streams in c++11 are movable, so in theory you should be able to do what you want, problem is that movable streams have not yet been implemented in gcc/libstdc++.

To back up my answer further please take a look at gcc/libstdc++ c++11 status: http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2011

Especially 27.5, 27.8, 27.9

Answer 2

That's a bug in the implementation of std::basic_istream. It should be movable, but isn't; and only movable types can be stored in a vector.

Here is the bug report against GCC.