Question

Originally, I have some code that looks like

class Ball
{
public:
    double x , y ;
    ofstream out ;
} ;

int main()
{
    Ball array[N] ;

    array[0].out.open("./position_1.txt") ;
    array[1].out.open("./position_2.txt") ;

    ......
}

where N is a run-time determined constant.
But it suffers variable length arrays problem recently.

I try to follow the suggestion by this post Can't set variable length with variable by using STL container.

int main()
{
    vector<Ball> Balls ;
    Ball b ;

    b.out.open( "./position_1.txt" ) ;
    Balls.push_back( b ) ;
    ......
}

It fails at push_bak(), since stream can not be copied.

I can’t determine the number of balls before running and I have to store the file stream instead of path for efficiency (preventing opening and closing files).

Is there any way to achieve the goal?
Thanks

Answer 1

C++ streams are not copyable, but they’re movable, so you can do this instead:

Balls.push_back( std::move(b) );

//DO NOT use b after push_back, as it has been moved!

and the default move-semantic generated by compiler for your class will work fine with this.

In C++11, you can write this:

std::vector<Ball> balls(N); //N is known at runtime

balls[i].out.open( "./position_1.txt" ); //i <= i < N

Note that this will not work in C++03, as in C++03 vector’s constructor creates N copies of a default created object of type Ball. In C++11, however, it doesn’t make any copy!