私有std :: fstream成员无法访问？

Question

Why is it unable to be accessed? I cannot understand this error at all. It's extremely vague and poorly explained by the compiler.

Error   1   error C2248: 'std::basic_fstream<_Elem,_Traits>::basic_fstream' : cannot 
access private member declared in class 'std::basic_fstream<_Elem,_Traits>' c:\users
\user\documents\visual studio 2012\projects\queue\queue\main.cpp    69  1   queue

Line 69 contains nothing but a closing brace with a semicolon.

class queue {
public:
    queue() {
        back = NULL;
        front = NULL;
    }
    void setfile(const char* to) {
        file.open(to, std::fstream::in | std::fstream::out | std::fstream::app);
    }
    void push(const char* msg) {
        file << msg << QueryPerformanceCounter(&nano::end);
        if(front == NULL) {
            back = new item(msg);
            front = back;
        }
        else {
            back->setprev(new item(msg));
            back = back->getprev();
        }
    }
    void* pop(const char* msg) {
        file << msg << QueryPerformanceCounter(&nano::end);
        if(front == NULL) {
            return false;
        }
        else {
            item* temp = front;
            front = front->getprev();
            delete temp;
        }
    }
private:
    std::fstream file;
    item* back;
    item* front;
};

That final brace at the end of the class definition is where the error points me to. You might be wondering if I've tried to assign or copy the private std::fstream object, but I haven't. All the code using it is inline within the class definition. The setfile() function is the only code that interacts with it from the outside, and the constant character argument it takes is just the filename.

Using MSVC++ 2012 at the moment.

Answer 1

std::fstream has not public copy constructors or assignment operator. So implicit generated copy constructor for queue class can not copy of file object.

First, you can deny copying of queue object by explicit declaration of private copy constructor. Second solution is to include your fstream by pointer not by value. To prevent memory problems, use shared_ptr instead of simple pointers.

UPDATE I

f no user-defined copy constructors are provided for a class type (struct, class, or union), the compiler will always declare a copy constructor as an inline public member of its class.

From cpp reference

So even if you doesn't copy your queue objects, compiler will generate copy constructor anyway. And this constructor cannot copy std::fstream object.