The big reasons for avoiding the STL are (in my experience) 1. Holdover from when STL implementations were buggy 2. Holdover from C 3. Avoiding exceptions it

The big reasons for avoiding the STL are (in my experience) 1. Holdover from when STL implementations were buggy 2. Holdover from C 3. Avoiding exceptions it might throw if you compile with exceptions off 4. Many STLs are slow and unoptimized. 5. Many parts of the STL are slow, no matter which STL you use 6. Poor allocator support (better, not perfect in c++11) 7. You need guarantees that the STL doesnt provide. there are more, but those are the big ones.