C++ vector of objects of custom class - copy constructor deleted - std::ifstream

Question

I'm trying to create a vector of objects of a custom class A:

class A {
    std::ifstream _file;
    std::string _fileName;
public:
    A(std::string &fileName) : _fileName(fileName) {
        this->_file = std::ifstream(this->_fileName, std::ios::in);
    }
    ~A() {
        this->_file.close();
    }
};

In the main I'm pushing n objects of class A with a for loop iterating a vector of file names.

Example:

#include <iostream>
#include <string>
#include <vector>
#include "A.hpp"

int main() {

    std::vector<A> AList;
    std::vector<std::string> fileList = { "file1", "file2", "file3" };

    for (auto &f : fileList) {
        std::cout << f << std::endl;
        A tempObj(f);
        AList.emplace_back(tempObj);
    }

    return 0;
}

But I get this error: /usr/include/c++/9.1.0/bits/stl_construct.h:75:7: error: use of deleted function ‘A::A(const A&)’

If I'm not wrong, since I have a member std::ifstream inside my class A, the copy constructor is deleted (Ref: https://en.cppreference.com/w/cpp/io/basic_ifstream/basic_ifstream)

How can I solve it? What I'm doing wrong?

Thanks for your help

Answer 1

As noted in the other answer, class A is non-copyable because its member std::ifstream _file; is non-copyable.

But std::ifstream is movable, and normally your class would be movable as well, but providing a custom destructor prevents the implicit generation of a move constructor and a move assignment operator (see the chart here).

So, step 1: Make your class movable by removing the custom destructor (the implicitly generated destructor will do the same thing anyway).

Alternatively, if you want to keep the destructor for some reason, you need to ask your compiler to generate a move constructor and a move assignment operator:

A(A &&) = default;
A &operator=(A &&) = default;

Step 2: When adding an instance of class A to the vector, move it instead of copying:

A tempObj(f);
AList.emplace_back(std::move(tempObj));
//                 ^^^^^^^^^^       ^

Alternatively, construct it directly in the vector:

AList.emplace_back(f);