Do you use C or C++?
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Breakaway Games
Former Microsoft XNA and Xbox MVP | Check out my blog for random ramblings on game development
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When life hands you lemons, throw them at God's head.
We have found that using the STL is a nasty performance hit on large scale systems. I also rewrote a string class to use a predefined string size so that I could optimize WindowsNT heap usage. I gues my answer would be that I use both C and C++ as Eva mentioned.
Kressilac
ps The decision usually relies on if I have C++ classes that can perform the same as C and still give the object oriented benefits. Cout, cin, and cerr all end up calling the C io functions so in many cases I skip the C++ layer altogether. As mentioned above CString is horribly slow and has been rewritten by my team. These just provide examples of what I stated above.
We use none of the STL classes, instead we've ended implementing similar things in C++ to take advantage of our memory management libraries. The libraries themselves are C, but there are some thunks in C++ that make them operate transparently in C++.