Passing an int to a function, then using that int to create an array

Question

I'm trying to create a textureLoader class for my openGL project and I can't initialize an array of textures inside of my class constructor because the the array won't accept anything unless it's a const int.

To paint you a simple picture...

myFunction(NUM)
  {
  GLuint textures[NUM];
  }

My past Failures

myConstructor(const int& num)
  {
   GLuint textures[num] //error: expression must have a constant value
  }

myConstructor(int num)
{
std::vector <GLuint> textures(num);//works but wait
glGenTextures(num, textures) // <--- doesn't work cause vectors. 
}

myConstructor(int num)
{
const int PLEASE_WORK = num;
GLuint textures[PLEASE_WORK]; // doesn't work.

What you tried (except the vector method) is called Variable-length arrays, which is supported in C99 but not in C++ IIRC. — user12205
– user12205, Commented Jun 15, 2015 at 23:35
I will look that up, thank you. I was thinking at least the third option should have worked. — Bryan the Lion
– Bryan the Lion, Commented Jun 15, 2015 at 23:40
it doesn't because the size of the array is not known during compilation. — user12205
– user12205, Commented Jun 15, 2015 at 23:42

Cory Kramer · Accepted Answer · 2015-06-15 23:34:07Z

3

Your second options is close, you can get at the underlying array of the vector by calling .data()

myConstructor(int num)
{
    std::vector <GLuint> textures(num);
    glGenTextures(num, textures.data());
}

Assuming glGenTextures has a signature like

void glGenTextures(int, GLuint*)

I don't know much about this function, but be careful who owns that array. The vector will fall out of scope after your constructor so I'm hoping that glGenTextures will copy whatever it needs. Otherwise if the array needs to persist

myConstructor(int num)
{
    GLuint* textures = new GLuint[num];
    glGenTextures(num, textures);
}

But then I'm not sure who is supposed to clean up that memory.

answered Jun 15, 2015 at 23:34

Cory Kramer

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7 Comments

Bryan the Lion Over a year ago

Yes, thank you, I will keep the scope of those variables in mind. Is there any way to do it using a const int& or anything other than vectors? Or is that my only alternative.

Tas Over a year ago

FYI: std::vector::data is c++11

Cory Kramer Over a year ago

@BryantheLion See the second half of my answer (using new), that is your alternative.

Cory Kramer Over a year ago

@Tas It is halfway through 2015, I would hope people are using (at least) C++11 compilers, otherwise they should make a point of specifying which compiler they are targeting.

Cory Kramer Over a year ago

@BryantheLion Sure that is a valid option. Keep a pointer to that array as a member variable, then call delete[] your_array in your destructor.

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