The Standard Template Library (STL) is a software library included in the C++ Standard Library. It provides containers, iterators, and algorithms. More specifically, the C++ Standard Library is based on the STL published by SGI. Both include some features not found in the other. SGI's STL is rigidly specified as a set of headers, while ISO C++ does not specify header content, and allows implementation either in the headers, or in a true library.

The STL provides a ready-made set of common classes, such as containers and associative arrays, that can be used with any built-in type and with any user-defined type that supports some elementary operations (such as copying and assignment). STL algorithms are independent of containers, which significantly reduces the complexity of the library.

The STL achieves its results through the use of templates. This approach provides compile-time polymorphism that is often more efficient than traditional run-time polymorphism. Modern C++ compilers are tuned to minimize any abstraction penalty arising from heavy use of the STL.

The STL was created as the first library of generic algorithms and data structures, with four ideas in mind: generic programming, abstractness without loss of efficiency, the Von Neumann computation model, and value semantics.