Question

Does this mean that the item does not exist in the map? I was unable to find a documented answer showing that this was true.

Here is where I add to the map:

void Shader::addAttribute(const string attribute) 
{
    attributeList[attribute] = glGetAttribLocation(program, attribute.c_str());
}

I add to the map with:

shader.addAttribute("position");

Here is where I retrieve data from the map:

//An indexer that returns the location of the attribute
GLuint Shader::operator [](const string attribute) 
{
    return attributeList[attribute];
}

When I print the value in a debug log, I get “location is -1”

Answer 1

The value of whatever key you’re inserting into the map must be -1.

For:

std::map<int, int> map;
std::cout << (map [5]);

If x (in your case, 5) does not match the key of any element in the container, the
function inserts a new element with that key and returns a reference
to its mapped value.

What you’ve done here is you’ve assigned the value of 0 to a key of 5

A map works by keys and values. It doesn’t look like you quite understand how they operate. Normally you would do something like this:

std::map<int, int> map;
map[5] = 1; // map now contains one element, with a key of 5 and a value of 1
std::cout << (map [5]); // Prints 1

You’ve inserted a -1 into you map in this call:

glGetAttribLocation(program, attribute.c_str())

Because, glGetAttribLocation returns -1 when, as Benjamin stated above, “If the named attribute variable is not an active attribute in the specified program object or if name starts with the reserved prefix ‘gl‘, a value of -1 is returned.”_. you should be doing something like the following:

int res = glGetAttribLocation(program, attribute.c_str())
if(res == -1)
{
  // Throw an exception, log an error.. Handle this error somehow.
}
else
{
  // Otherwise store the valid result.
  attributeList[attribute] = res; 
}