When should you use an STL other than the one that comes with your compiler?

I've never had a need to use an alternative STL, but I could envision some scenarios where it might be useful to use, for example, the Apache version, if you need small executables because you're developing for an embedded platform.

Another reason might be to use an STL version that guarantees certain things which are not necessarily guaranteed by the standard. For example, to ensure you have non-COW strings so you can write thread-safe code.

The C++ standard library can be implemented in variety of ways. Some implementers try to cope with modern ideas. So, using an optimized implementation may result in faster and smaller executables.

Take SCARY for example. Some implementers didn't do it yet, although it reduces the bloat of STL to a great extent. When you do the following:

vector<int> f;
vector<int, MyAllocator> s;

size_t fc = count(f.begin(), f.end(), SomeValue);
size_t sc = count(s.begin(), s.end(), SomeOtherValue);

An "old" implementation could produce two different count functions in the result executable, because the type of f is not the same as of s. Thats because the iterator type depends on the type of the vector itself, although it doesn't need to be like that. A better idea is to separate the type of the iterator in a separate class, and provide a typedef in vector, and the compiler would produce one count only. That was just an example, but I think there are more to say about the quality of some implementations.

Folks mentioning STLport have cited performance and portability, but there's also a pretty good debug mode available. I think that's a great reason to be using a different STL, if your current compiler's library is limited in this way.

Aaand ... it looks like Max and I tied on mentioning debugging! ;^)~