File stream, STL container, and variable length arrays

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

What is the goal you are trying to achieve? Load a variable count of ball coordinates from a single file? — Spook, Feb 06 '13 at 07:53
Yes, I have many images of balls and each image has many balls. — maythe4thbewithu, Feb 07 '13 at 01:24
I have many images and each image has many balls. The ball numbers is fixed for each movie, but it is determined after first identification. I want to track them, 1, 2,.. and save them to 1.txt, 2.txt,..So I keep the file stream for each one for easy access and efficiency. Now, I maintain/new an array of streams outside the class and change "ofstream out" to be "ofstream *out"...Similar to the suggestions below. Thanks for your suggestion. But I wounder if there is a more "nature" way like the suggestion of using C++11. But, my gcc is not support C++11. I can't check so far. still thanks — maythe4thbewithu, Feb 07 '13 at 02:08

Nawaz · Accepted Answer · 2013-02-06T08:03:53.293

3

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!

edited Feb 06 '13 at 08:03

answered Feb 06 '13 at 07:55

Nawaz

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1

Or, since there is hardly ever a reason to use `open` or `close` on a `std::stream`, `balls.emplace_back("./position_1.txt")`, which would require a reasonable `Ball` constructor, though. – Christian Rau Feb 06 '13 at 09:17

score 1 · Answer 2 · answered Feb 06 '13 at 08:02

If your compiler/library are old enough that they don't support move semantics for streams yet (many don't) you might consider storing a (smart) pointer to a stream in the structure instead. You do probably want it to be a smart pointer though -- handling ownership transfer correctly on your own is likely to be quite a bit of work to get right.

score 0 · Answer 3 · answered Feb 06 '13 at 08:04

Then make Balls a vector of pointers to Ball. Allocate the Ball objects with new and push_back the pointers.

Note that, if you use simple pointers, you must delete the Ball objects at the end with a loop. You can avoid this by using smart pointers, like std::tr1::shared_ptr.

File stream, STL container, and variable length arrays

3 Answers3