How do I use move-semantics to reallocate resources from class constructor?-CodePudding

To enable me to read a binary file I have made this class. I would like to not copy a temporary for which I am trying to use move semantics. But this code produces "access violation error".

#include <fstream>
#include <iostream>

class myClass
{
public:
    std::istream& myStream;
    static myClass create(const char* path);
    myClass(myClass&&) noexcept = default;
    myClass(std::istream& input);
    myClass() = default;
};

myClass myClass::create(const char* path)
{
    std::ifstream input{ path, std::ios::binary | std::ios::in };
    if (!input.is_open()) throw std::runtime_error("Error - couldn't open file");
    return std::move(myClass(input));
}

myClass::myClass(std::istream& input) :
    myStream(input)
{

}
int main()
{
    const char* path = R"(file.bin)";
    myClass cr{myClass::create(path)};

    for (int i = 0; i < 10;   i)
        cr.myStream.seekg(i);

    return 0;
}

CodePudding user response：

You are accessing the stream after the temporary has been destroyed. You need to store the stream in the myClass object:

#include <fstream>
#include <iostream>

class myClass
{
public:
    std::istream& myStream;
    static myClass create(const char* path);
    myClass(myClass&& other) noexcept;
    myClass(std::istream& input);
    myClass() = default;
};

myClass myClass::create(const char* path)
{
    std::ifstream input{ path, std::ios::binary | std::ios::in };
    if (!input.is_open()) throw std::runtime_error("Error - couldn't open file");
    return std::move(myClass(input));
}

myClass::myClass(myClass&& other) noexcept :
    myStream(other.myStream)
    {
    other.myStream = std::cin;
}

myClass::myClass(std::istream& input) :
    myStream(input)
{

}
int main()
{
    const char* path = R"(file.bin)";
    myClass cr{myClass::create(path)};

    for (int i = 0; i < 10;   i)
        cr.myStream.seekg(i);

    return 0;
}

CodePudding user response：

According to my tests, the error comes from the file path, you used a relative path when defining the path and the program cannot search for this file. After I changed the path to an absolute path, the program has no error.

You need to replace the path in const char* path = R"(C:\\Users\\Admin\\Desktop\\test.bin)"; with the path in your computer.

#include <fstream>
#include <iostream>

class myClass
{
public:
    std::istream& myStream;
    static myClass create(const char* path);
    myClass(myClass&&) noexcept = default;
    myClass(std::istream& input);
    myClass() = default;
};

myClass myClass::create(const char* path)
{
    std::ifstream input{ path, std::ios::binary | std::ios::in };
    if (!input.is_open()) throw std::runtime_error("Error - couldn't open file");
    return std::move(myClass(input));
}

myClass::myClass(std::istream& input) :
    myStream(input)
{

}
int main()
{
    const char* path = R"(C:\\Users\\Admin\\Desktop\\test.bin)";
    myClass cr{ myClass::create(path) };

    for (int i = 0; i < 10;   i)
        cr.myStream.seekg(i);

    return 0;
}