Question

I have a problem with std::map performance. In my C++ project I have a list of GUIObjects which also includes Windows. I draw everything in for loop, like this:

unsigned int guiObjectListSize = m_guiObjectList.size();
for(unsigned int i = 0; i < guiObjectListSize; i++)
{
    GUIObject* obj = m_guiObjectList[i];
    if(obj->getParentId() < 0)
    obj->draw();                                
}

In this case when I run a project, it works smoothly. I have 4 windows and few other components like buttons etc.

But I would like to take care of drawing windows separately, so after modifications, my code looks like this:

// Draw all objects except windows
unsigned int guiObjectListSize = m_guiObjectList.size();
for(unsigned int i = 0; i < guiObjectListSize; i++)
{
    GUIObject* obj = m_guiObjectList[i];
    if((obj->getParentId() < 0) && (dynamic_cast<Window*>(obj) == nullptr))
        obj->draw();        // GUIManager should only draw objects which don't have parents specified
                            // And those that aren't instances of Window class
                            // Rest objects will be drawn by their parents
                            // But only if that parent is able to draw children (i.e. Window or Layout)
}

// Now draw windows
for(int i = 1; i <= m_windowList.size(); i++)
{
    m_windowList[i]->draw(); // m_windowList is a map!
}

So I created a std::map<int, Window*>, because I need z-indexes of Windows to be set as keys in a map. But the problem is that when I run this code, it’s really slow. Even though I have only 4 windows (map size is 4), I can see that fps rate is very low. I can’t say an exact number, because I don’t have such counter implemented yet.

Could anyone tell me why this approach is so slow?

Answer 1

This is what virtual functions are for. Not only do you eliminate the slow dynamic_cast, but you get a more flexible type check.

// Draw all objects except windows
unsigned int guiObjectListSize = m_guiObjectList.size();
for(unsigned int i = 0; i < guiObjectListSize; i++)
{
    GUIObject* obj = m_guiObjectList[i];
    if(obj->getParentId() < 0)
        obj->drawFirstChance();
}

// Now draw windows
for(int i = 1; i <= m_windowList.size(); i++)
{
    m_windowList[i]->drawSecondChance();
}

Where drawFirstChance doesn’t do anything for windows and other floating objects.

The next optimization opportunity is to make the window list a vector and perform z-order sorting only when it changes (assuming windows are created/destroyed/reordered much less often than they are drawn).