Standard libraries

A standard library in computer programming is the library made available across implementations of a programming language. These libraries are conventionally described in programming language specifications; however, contents of a language's associated library may also be determined (in part or whole) by more informal practices of a language's community.

A language's standard library is often treated as part of the language by its users, although the designers may have treated it as a separate entity. Many language specifications define a core set that must be made available in all implementations, in addition to other portions which may be optionally implemented. The line between a language and its libraries therefore differs from language to language. Indeed, some languages are designed so that the meanings of certain syntactic constructs cannot even be described without referring to the core library. For example, in Java, a string literal is defined as an instance of the java.lang.String class; similarly, in Smalltalk, an anonymous function expression (a "block") constructs an instance of the library's BlockContext class. Conversely, Scheme contains multiple coherent subsets that suffice to construct the rest of the language as library macros, and so the language designers do not even bother to say which portions of the language must be implemented as language constructs, and which must be implemented as parts of a library.

Standard libraries typically include definitions for commonly used algorithms, data structures, and mechanisms for input and output. 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.

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