为什么我可以有一个std :: vector <std::ofstream*> 但不是std :: vector <std::ofstream> ？

Question

I have the following test code in which I have a parameter fS which is a container of ofstream s:

    #include <fstream>
    #include <vector>
    #include <cstdlib>
    #include <cstdio>

    int main()
    {
        // my container of ofstream(s)
        std::vector<std::ofstream> fS;

        // instantiate an ofstream
        std::ofstream of("myfile.txt");

        // push back to my container
        fS.push_back(of);

        return 0;
    }

This does not compile at all. Whereas when I change the container of ofstream into a container of pointers to ofstream s, the code compiles:

    #include <fstream>
    #include <vector>
    #include <cstdlib>
    #include <cstdio>

    int main()
    {
        // my container of ofstream(s)
        std::vector<std::ofstream*> fS;

        // instantiate an ofstream
        std::ofstream * of = new std::ofstream("myfile.txt");

        // push back to my container
        fS.push_back(of);

        return 0;
    }

Why is this?

Answer 1

When you call push_back(of) on a vector , it tries to add a copy of the object of to the vector. (C++ loves making copies of things). In this case, you're trying to copy an ofstream , which isn't permitted. Intuitively, it's not clear what it would mean to have a copy of an ofstream , so the spec prohibits it.

On the other hand, suppose you have a vector of ofstream* s. Now, if you try to push_back a pointer to an ofstream , then C++ interprets this to mean that you should put a copy of the pointer into the vector , and that's okay because pointers can easily be copied around.

There's a third option, though, if you have a recent compiler. C++ recently introduced the idea of move semantics , that instead of trying to copy the file stream into the vector, you can move the file stream into the vector. You could therefore write something like this:

int main()
{
    // my container of ofstream(s)
    std::vector<std::ofstream> fS;

    // instantiate an ofstream
    std::ofstream of("myfile.txt");

    // push back to my container
    fS.push_back(std::move(of));

    return 0;
}

After doing this, the variable of won't refer to the original file stream anymore; it'll instead just have some dummy value. However, the vector will now effectively contain the stream that used to be stored in of .