Relationship between STL and C++

Question

So apparently STL is known as standard template library, which includes common data structures, classes, functions, or methods. However, the STL is not built into C++ language even though it holds the code to use such common things in the C++ language? I thought all these common data structures and methods were built into C++ language itself, but we must keep including the preprocessor directives to access them. Also, is there one preprocessor directive for all of the STL? Why have separate preprocessor directives that built up STL collectively. Should not the STL be represented by one thing?

Answer 1

The STL is the name of a software library, developed originally by Alexander Stepanov, and proposed for consideration by the C++ standardisation committee in 1993.

During the standardisation process, which eventually resulted in the first C++ standard being ratified in 1998, the specification of the library evolved. The C++ standard specifies the C++ standard library.

Because of this history, the STL influenced the specification of the C++ standard library. During the standardisation process before 1998, the STL was was evolved and extended. In 1994, this work resulted in a proposal for a C++ standard library by Alexander Stepanov and Meng Lee being voted and incorporated into the (then) draft C++ standard.

Technically, it is possible to distinguish between the C++ language (rules of syntax, semantics, etc) and the C++ standard library (which provides a set of types and functions that build on and support the language). A lot of the C++ standard library can be implemented in the C++ language (e.g. templated parts of the library, such as standard algorithms, which originated in STL). Some elements (e.g. the specification of the range of integral types represented by the templated std::numeric_limits) are implementation-defined. Some parts cannot be implemented in C++ at all (e.g. at some level they access "compiler magic", or use facilities of the host system (OS-specific API, machine instructions, etc).

There is not a single preprocessor directive for the C++ standard library, and never was for the STL. The philosophy is that code only accesses functionality it needs (e.g. if doing console-based I/O, it includes <iostream> but doesn't need to include <numeric> (which provides common math functions)). Practically, with most implementations, including parts of the standard library that are not needed by a program, tends to increase the time and resources needed for preprocessing and subsequent phases of translation (parsing the contents of the headers), and therefore increases build times by a substantial amount. Since significant projects can have build-from-scratch times measured in weeks or months, and using "include everything headers" can easily increase build times by orders of magnitude, it is usually considered good practice to avoid them.

Answer 2

The "Standard Template Library" is no more. It's the "Standard Library" now. STL was from the SGI era and has been significantly reworked and developed since then.

The reason it's broken up into various components instead of one gigantic #include file is because processing these files comes with a cost, and it can also pollute your root namespace if you're using namespace std.

C++, like C, requires inclusion of header files for the tools that you're using, nothing is supplied automatically. This isn't all that unusual, other languages force you to require or import or use other code which serves the same purpose.

Now the Standard Library is not part of the C++ language per-se, the language does not require using it, but it is an expected feature of any standard-compliant C++ compiler and the Standard Library has been improved in conjunction with the language through each major release. It's not built into the language, but it is built into the compiler, if you care for such distinctions. It'd also something that makes C++ far more useful than having just the core language.

So the Standard Library is the baseline for a C++ compiler. Many developers choose to go beyond that using things like Boost, where Boost itself is an unofficial standard library of sorts. Many features from Boost have been reworked and absorbed into the Standard Library, a trend that's likely to continue.

You can make an omni-include file that includes everything if you prefer, but you'll probably see much, much slower build times. The entire library is a considerable amount of header information to process.

Answer 3

So apparently STL is known as standard template library

Yes, but STL is nowadays a misnomer. It is best to call it the standard library.

However, the STL is not built into C++ language even though it holds the code to use such common things in the C++ language?

The standard library is part of the ISO C++ language.

Even for freestanding implementations (e.g. without an operating system), the standard requires implementing a fair amount of headers and things intended to support the language.

I thought all these common data structures and methods were built into C++ language itself

For hosted implementations (the ones you usually deal with unless you work on things like embedded), they are!

but we must keep including the preprocessor directives to access them.

That does not mean they are not part of the language, just that you need to include the headers like for any other library.

The language could have an "auto-detection" feature for things in the std namespace (and sometimes compilers have one to suggest you things on errors), but it is not the case.

Also, is there one preprocessor directive for all of the STL?

No, but that is not usually a problem: you can make one if you really want that (although it is usually best to just declare whatever you need in each translation unit).

Why have separate preprocessor directives that built up STL collectively. Should not the STL be represented by one thing?

The language could have specified such a thing (some languages have a "prelude" of sorts), but currently that isn't the case and you need to be explicit.

Answer 4

Per Wikipedia

In the C++ programming language, the C++ Standard Library is a collection of classes and functions, which are written in the core language and part of the C++ ISO Standard itself.