Question

I am trying to write a scientific graph library, it works but I have some performance problems. When creating a graph I use a template class for the nodes and do something like

for(unsigned int i = 0; i < l_NodeCount; ++i) 
                m_NodeList.push_back(Node<T>(m_NodeCounter++));

Even though in the constructor of the node class almost nothing happens (a few variables are asigned) this part is a major bottleneck of my program (when I use over a million of nodes), especially in the debug mode it becomes too inefficient to run at all.

Is there a better way to simultaneusly create all those template classes without having to call the constructor each time or do I have to rewrite it without templates?

Answer 1

There are things that happen in your code:

• as you add elements to the vector, it occasionally has to resize the internal array, which involves copying all existing elements to the new array
• the constructor is called for each element

The constructor call is unavoidable. You create a million elements, you have a million constructor calls. What you can change is what the constructor does.

Adding elements is obviously unavoidable too, but the copying/resizing can be avoided. Call reserve on the vector initially, to reserve enough space for all your nodes.

Depending on your compiler, optimization settings and other flags, the compiler may do a lot of unnecessary bounds checking and debug checks as well.

• You can disable this for the compiler (_SECURE_SCL=0 on VS2005/2008, _ITERATOR_DEBUG_LEVEL=0 in VS2010. I believe it’s off by default in GCC, and don’t know about other compilers).
• Alternatively, you can rewrite the loop to minimize the amount of debug checking that needs to be done. Using the standard library algorithms instead of a raw loop allows the library to skip most of the checks (typically, a bounds check will then be performed on the begin and the end iterator, and not on the intervening iterations, whereas on a plain loop, it’ll be done every time an iterator is dereferenced)