Vector of input streams?

Question

Im looking to create a vector that stores a list of different input streams including cin and a few text files. So far I have this.

vector<istream> inStreams;
vector<istream>::iterator streamsIterator;

streamsIterator = inStreams.begin();
streamsIterator = inStreams.insert(streamsIterator, cin); ////this has error!
for (char i = 1; argv[i] != NULL; i++) {
    streamsIterator = inStreams.insert(streamsIterator, ifstream(argv[i], ifstream::in));
    }

The problem is that the compiler spits out a big error that goes deep into the template library that I just cant decipher.

try to store pointers to streams in the vector instead of values — Alexander, May 18 '15 at 06:55
Just a question, why are you using a iterator for that, why not a simple push_back? e.g. for(int i = 0; i < argc ; ++i) inStreams.push_back(ifstream(argv[i],ifstream::in)); — Dinari, May 18 '15 at 07:00

score 2 · Accepted Answer · answered May 18 '15 at 08:02

There are a few problems here. First is streams are not copyable so you can't copy them into a container. Although if you have a very recent compiler that supports it, streams are moveable.

However because you want to store different types in the one container that are polymorphic you can't even move them in. You can't store a std::fstream in the space allocated to a std::istream because you get slicing (corruption).

One way to do this is using pointers to store in your container:

int main(int, char* argv[])
{
    std::vector<std::istream*> istreams;

    istreams.push_back(&std::cin);

    for(char** arg = argv + 1; *arg; ++arg)
    {
        istreams.push_back(new std::ifstream(*arg));
    }
}

Unfortunately now you have a problem of ownership. The std::ifstream objects you added that were created with new need to be deleted but the global std::cin object should not be deleted.

How you solve that problem depends on what you are trying to do overall but mixing pointers that need deleting with those that should not be deleted is problematic.

Excellent, thank you both of you, these answers clear a lot of things up. — Tristus, May 19 '15 at 04:05

score 1 · Answer 2 · edited May 18 '15 at 08:00

This program does not compile, because you have no default constructor for std::istream class.

This code would compile if you use references instead of raw type. Make sure you free items when you remove them from the vector:

#include <vector>
#include <iostream>
#include <fstream>
#include <istream>

using namespace std;

int  main(int argc, char* argv[])
{
    vector<istream*> inStreams;
    vector<istream*>::iterator streamsIterator;

    streamsIterator = inStreams.begin();
    streamsIterator = inStreams.insert(streamsIterator, &cin); ////this has no error now
    for (char i = 1; argv[i] != NULL; i++) {
            auto str = new ifstream(argv[i], ifstream::in);
            streamsIterator = inStreams.insert(streamsIterator, str);
    }

    return 0;
}

Vector of input streams?

2 Answers2