loading fstreams into a std::vector in c++

Question

This is a simple and complex question at the same time.

This compiles:

int Test;
vector<int> TEST;
TEST.push_back(Test);
cout << TEST.size();

This does not compile:

fstream Test;
vector<fstream> TEST;
TEST.push_back(Test);
cout << TEST.size();

Is there any particular reason? Is there a way for me to get a dynamic list of fstreams?

The error message:

1>------ Build started: Project: vector_test, Configuration: Debug Win32 ------
1>  vector_test.cpp
1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\fstream(1347): error C2248: 'std::basic_ios<_Elem,_Traits>::basic_ios' : cannot access private member declared in class 'std::basic_ios<_Elem,_Traits>'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ios(176) : see declaration of 'std::basic_ios<_Elem,_Traits>::basic_ios'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          This diagnostic occurred in the compiler generated function 'std::basic_fstream<_Elem,_Traits>::basic_fstream(const std::basic_fstream<_Elem,_Traits> &)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

The object fstream is not copyable.

If you need to record fstreams in a vector you can declare a std::vector<std::fstream*> and push back the address of the object. Remember that if you save the pointer, then you must ensure that, when you access it, the object is still alive.

Answer 2

In C++ 2011 the concrete stream objects are movable. Howver, to take advantage of this you either need to pass a temporary or allow the object to be moved:

std::vector<std::ofstream> files;
files.push_back(std::ofstream("f1"));
std::ofstream file("f2");
files.push_back(std::move(file));

Note that you can't use the file variable after this as the stream was moved into files.

Answer 3

To use a class with a vector, it must be copyable. fstream is not.

See: C++ std::ifstream in constructor problem

Edit: If you need to have multiple references to the same fstream, you can use shared_ptr to manage them. Try something like:

std::vector< std::shared_ptr<fstream> > TEST