Resizing 2d vector - error

Question

For some reason, resizing this two dimensional vector in my class does not work. Thanks for your help!

..
Map::Map(const unsigned int& w,
         const unsigned int& h,
         const unsigned int& s)
    : width(w), height(h), size(s)
{
    squares.resize(width);
    for (unsigned int i = 0; i < width; ++i)
    {
        squares[i].resize(height); // error here
        for (unsigned int j = 0; j < height; ++j)
        {
            squares[i][j] = Square(sf::Vector2f(i * size, j * size));
        }
    }
}

std::vector<std::vector<Square>> squares;
..

Some error messages:

c:\mingw\bin..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_uninitialized.h|481| instanziiert von »static void std::__uninitialized_default_n_1<TrivialValueType>::_uninit_default_n(_ForwardIterator, _Size) [with _ForwardIterator = Square*, _Size = unsigned int, bool _TrivialValueType = false]«|

c:\mingw\bin..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_uninitialized.h|529| instanziiert von »void std::__uninitialized_default_n(_ForwardIterator, _Size) [with _ForwardIterator = Square*, _Size = unsigned int]«|

Yes, it has a default constructor: Square(const sf::Vector2f& pos); — user1429101
– user1429101, Commented Jul 5, 2013 at 18:05
default constructor is a constructor that takes no arguments. what you posted is a constructor that needs one argument and therefore should be marked explicit to avoid implicit conversions — aryan
– aryan, Commented Jul 5, 2013 at 18:07
@aryjczyk multi_array is, by name, an array, not a vector — Bartek Banachewicz
– Bartek Banachewicz, Commented Jul 5, 2013 at 18:07

aryan · Accepted Answer · 2013-07-05 18:56:20Z

1

When you vector.resize(n) and n > vector.size() you are asking it to create n - vector.size() new elements. Vector needs to default construct them, since he doesn't know what arguments to use. If they are not default constructible, resize fails.

However, if you don't want Square to be default constructible you can change failing line to vector.reserve() - that only makes sure there is enough space for new elements, but doesn't actually create them. Then you can create them one by one in a loop using

squares[i].emplace_back(sf::Vector2f(i * size, j * size));

Oh, and by the way passing primitives by reference has suboptimal performance. Pass them by value unless you need them as out parameters.

edited Jul 5, 2013 at 18:56

answered Jul 5, 2013 at 18:28

aryan

6313 silver badges10 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user1429101 Over a year ago

So is it a good practice to only use const references in method parameters when it's not a primitive type?

aryan Over a year ago

in general if you are not passing an out argument it should be either passed by value or by const reference. there are 2 differences 1) performance - passing by value copies the object, passing by ref is under the hood passing by pointer, so there is a level of indirection, so you want to pass small things by value and big ones by reference. i've read somewhere, that 16B is a good threshold on x86, but it's more of a guideline than a rule. if you find yourself writing code that passes a lot of small structs in perf-critical areas you should profile it and see what's best

aryan Over a year ago

2) if you want to perform some intermediate non-const computation on an argument, you would need to copy it anyway, so it's okay to pass it by value even if it's big

Collectives™ on Stack Overflow

Resizing 2d vector - error

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related