How to Get Max Number of Multitextured Units

Question

Lets say I have a function where I want the user to be able to select the appropriate texture in a type safe manner. So instead of using a GLenum of GL_TEXTUREX I define a method as follows.

void activate_enable_bind(uint32_t texture_num)  {
  const uint32_t max_textures = GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS - GL_TEXTURE0;
  const uint32_t actual_texture = (GL_TEXTURE0 + texture_num);

  if (texture_num > max_textures) {
    throw std::runtime_error("ERROR: texture::activate_enable_bind()");
  }

  glActiveTexture(actual_texture);
  glEnable(target_type);
  glBindTexture(target_type, texture_id_);
}

Is this guaranteed to work under all implementations based on the opengl specification, or are implementers allowed to have

`GL_TEXTURE0 - GL_TEXTURE(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS -1)` 

defined in a non-contiguous manner?

I was modifying my code aswell here in what I have:

void activate_enable_bind(uint32_t texture_num = 0)  {
  GLint max_textures = 0;
  glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, &max_textures);
  if (static_cast<GLint>(texture_num) > max_textures - 1) {
    throw std::runtime_error("ERROR: texture::activate_enable_bind()");
  }

  const uint32_t actual_texture = (GL_TEXTURE0 + texture_num);

  glActiveTexture(actual_texture);
  glEnable(target_type);
  glBindTexture(target_type, texture_id_);
}

Answer 1

I think GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS is not on its own a useful value, but is something that you pass to glGet to retrieve the actual value. To account for that, you'd retrieve it like this:

GLint max_combined_texture_image_units;
glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, &max_combined_texture_image_units);
// and then maybe check for errors

As for adding to GL_TEXTURE0, that is safe; §3.8 of the OpenGL 3.2 Core specification says this:

Active­Texture generates the error INVALID_­ENUM if an invalid texture is specified. texture is a symbolic constant of the form TEXTUREi, indicating that texture unit i is to be modified. The constants obey TEXTUREi = TEXTURE0+i (i is in the range 0 to k − 1, where k is the value of MAX_­COMBINED_­TEXTURE_­IMAGE_­UNITS).

Answer 2

Your corrected code, (GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS is an enum):

void activate_enable_bind(uint32_t texture_num)  {
  int value;
  glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS_ARB,&value);

  if (texture_num+1 > value) {
    throw std::runtime_error("ERROR: texture::activate_enable_bind()");
  }

  const uint32_t actual_texture = (GL_TEXTURE0 + texture_num);

  glActiveTexture(actual_texture);
  glEnable(target_type);
  glBindTexture(target_type, texture_id_);
}

EDIT: Also read @icktoofay answer.