Got error C2280 when using std::vector.push_back()

Question

I got error "C2280: attempting to reference a deleted function" compiling error when doing following code:

std::ofstream ofs(m_headerFileName, std::ios::binary, std::ios_base::app);
            m_ofsHeader.push_back(ofs);

where

std::vector<std::ofstream>      m_ofsHeader;

I don't understand why I cannot push a ofstream instance into an ofstream vector. Someone gives some hint? Thanks. I am on Windows 7 and Visual Studio 2015. Also, what is the walkaround here if there is any?

I am trying to keep a bunch of ifstream/ofstream with each one having their own file to read/write.

Streams can not be copied, there can only be one single instance of a stream. — Some programmer dude, Dec 15 '17 at 16:16
It depends, why do you want to do this? What is the actual problem you want to solve with this solution? What is the use-case? Please [read about the XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem) and take some time to reflect how this question could be one. — Some programmer dude, Dec 15 '17 at 16:17
@pm100 Could show a little code example to get me started? Appreciate it. — Nick X Tsui, Dec 15 '17 at 16:21
Possible duplicate of [C++ copy a stream object](https://stackoverflow.com/questions/7903903/c-copy-a-stream-object) — underscore_d, Dec 15 '17 at 16:21

Christian Hackl · Answer 1 · 2017-12-15T16:54:10.793

First of all, the following is wrong, because there is no std::ofstream constructor taking three arguments:

std::ofstream ofs(m_headerFileName, std::ios::binary, std::ios_base::app);

You probably meant:

std::ofstream ofs(m_headerFileName, std::ios::binary | std::ios::app)

And then, there's the storage problem. Streams cannot be copied, that's why your push_back fails.

You could just move the streams instead:

#include <fstream>
#include <vector>
#include <ios>

int main()
{
    std::vector<std::ofstream> streams;

    std::ofstream os("foo.txt", std::ios::binary | std::ios::app);
    streams.push_back(std::move(os));
}

Note the std::move, which casts os so that the && overload of push_back is used.

Or you store std::unique_ptrs to the streams in the vector:

#include <fstream>
#include <memory>
#include <vector>
#include <ios>

int main()
{
    std::vector<std::unique_ptr<std::ofstream>> streams;

    auto os = std::make_unique<std::ofstream>("foo.txt", std::ios::binary | std::ios::app);
    streams.push_back(std::move(os));
}

Your answer is much better than mine :) – Dumbo Dec 15 '17 at 20:08 — Dumbo, Dec 15 '17 at 20:08

score 3 · Accepted Answer · answered Dec 15 '17 at 16:16

3

Streams have no copy constructor. You might implement your own or possibly move semantics.

answered Dec 15 '17 at 16:16

Dumbo

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*"You might implement your own"* - What do you mean? You cannot implement your own copy constructors for standard classes. – Christian Hackl Dec 15 '17 at 16:45
@ChristianHackl Hmmm...driving from stream and overwriting thing like overflow, underflow, etc...it is gonna be hard but still possible (and by the time of my answer it was not clear what OP wants hence the XY problem occurred) – Dumbo Dec 15 '17 at 20:07

score 2 · Answer 3 · answered Dec 15 '17 at 16:20

2

Streams cannot be copied. Their copy constructor and copy assigner are mark as delete. Consider using move semantics as well as std::vector::emplace_back():

m_ofsHeader.emplace_back(std::move(ofs));
            ^~~~~~~      ^~~~

answered Dec 15 '17 at 16:20

iBug

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Got error C2280 when using std::vector.push_back()

3 Answers3