How to set an array size at runtime without constucting objects?

Question

How do I create an array of GLTexture nullptrs with a size calculated at runtime? . The implementation below creates an array GLTexture pointers initialised with nullptr with a constant size of [11][32]. I want the 11 and 32 shown in the header file below to be interchanged with a value calculated at runtime.

Header File

#pragma once
    #include <GLEW\glew.h>
    #include "GLTexture.h"

        namespace Nova
        {
            class TextureBinder
            {
            private:
                GLuint      m_activeUnit;
                GLint       m_maxTextureUnits;
                GLint       m_maxTextureTargets;
                GLTexture*  m_boundTextures[11][32] = {nullptr};

            public:

                static TextureBinder& GetInstance()
                {
                    static TextureBinder binder;
                    return binder;
                }

                TextureBinder(TextureBinder const&) = delete;
                void operator=(TextureBinder&) = delete;


            private:
                TextureBinder();
            };
        }

CPP File

#pragma once
#include "TextureBinder.h"

namespace Nova
{
    /* zero is the default opengl active texture unit
    - glActiveTexture(unit) only needs to be called for multitexturing
*/
            TextureBinder::TextureBinder()
        :
        m_activeUnit(0),
        m_maxTextureTargets(11)
    {
        glGetIntegerv(GL_MAX_TEXTURE_IMAGE_UNITS, &m_maxTextureUnits);
    }   
}

Which variable is the array size you want? Your code is lengthy. — Arif Burhan
– Arif Burhan, Commented Mar 13, 2016 at 20:07
@ArifBurhan sorry missed that! m_boundTextures[m_maxTextureTargets][m_maxTextureUnits] — DanielCollier
– DanielCollier, Commented Mar 13, 2016 at 20:08
are the m_boundTextures pointers pointing to textures that this class owns, or are they managed elsewhere? — M.M
– M.M, Commented Mar 13, 2016 at 20:16
@M.M this class does not own them they are managed elsewhere. so they should not be constructed just assigned — DanielCollier
– DanielCollier, Commented Mar 13, 2016 at 20:18

Jacob H · Accepted Answer · 2016-03-13 20:13:22Z

2

Assuming you actually want a dynamically sized array (i.e. can be calculated at runtime and works with different sizes), you need to use a loop in your constructor:

GLTexture*  **m_boundTextures;

TextureBinder() {
    /* ... */
    m_boundTextures = new GLTexture* *[HEIGHT];
    for(int i = 0; i < HEIGHT; i++) {
        m_boundTextures[i] = new GLTexture* [WIDTH];
        for(int j = 0; j < WIDTH; j++) {
            m_boundTextures[i][j] = nullptr;
        }
    }
    /* ... */
}

And of course make sure to clean up the memory using delete[] (in the same format) in your destructor.

answered Mar 13, 2016 at 20:13

Jacob H

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2 Comments

Aiman Al-Eryani Over a year ago

Or maybe use std::vector with which you can set a default value when you resize it ;)

DanielCollier Over a year ago

I prefer the other answer but +1 because I learnt something new !

M.M · Accepted Answer · 2016-03-13 20:22:35Z

1

This simplest alternative to code is to use:

std::vector< std::vector<GLTexture*> > m_boundTextures;

and add to the ctor-initializer list

// Numbers can be replaced by variables,
// or you can set the size later using 'resize' member function
m_boundTextures(11, std::vector<GLTexture*>(32));

Another option to consider is using a single vector of size 11 * 32 (or whatever your dimensions are) and then using multiplication to access the right index; you could make a helper function for this, e.g.:

GLTexture* & lookup(size_t row, size_t col) { return m_boundTextures.at(col + row * row_length); }

answered Mar 13, 2016 at 20:22

M.M

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2 Comments

DanielCollier Over a year ago

if I call the resize member function for the m_boundTextures like so m_boundTextures.resize(11); how would I change the size of std::vector<GLTexture*> that is within it?

M.M Over a year ago

for a single one, m_boundTextures[n].resize(32);. For all at once, for (auto& x : m_boundTextures) x.resize(32);

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