A standard library for a programming language is the library that is conventionally made available in every implementation of that language. In some cases, the library is described directly in the programming language specification; in other cases, the contents of the standard library are determined by more informal social practices in the programming community.
Depending on the constructs made available by the host language, a standard library may include:
Most standard libraries include definitions for at least the following commonly used facilities:
Philosophies of standard library design vary widely. For example, Bjarne Stroustrup, designer of C++, writes:
What ought to be in the standard C++ library? One ideal is for a programmer to be able to find every interesting, significant, and reasonably general class, function, template, etc., in a library. However, the question here is not, "What ought to be in some library?" but "What ought to be in the standard library?" The answer "Everything!" is a reasonable first approximation to an answer to the former question but not the latter. A standard library is something every implementer must supply so that every programmer can rely on it.[1]
This suggests a relatively small standard library, containing only the constructs that "every programmer" might reasonably require when building a large collection of software. This is the philosophy that is used in the C and C++ standard libraries.
By contrast, Guido van Rossum, designer of Python, has embraced a much more inclusive vision of the standard library; in the Python tutorial, he writes:
Python has a "batteries included" philosophy. This is best seen through the sophisticated and robust capabilities of its larger packages.[2]
Van Rossum goes on to list libraries for processing XML, XML-RPC, email messages, and localization, facilities that the C++ standard library omits. This other philosophy is often found in Scripting languages (as in Python or Ruby) or languages that use a Virtual machine, such as Java or the .NET Framework languages.