The C++ standard library provides a wide range of facilities that are usable in standard C++.

[edit]Category

Thelanguage support library provides components that are required by certain parts of the C++ language, such as memory allocation (new/delete) andexception processing.

Thediagnostics library provides a consistent framework for reporting errors in a C++ program, includingpredefined exception classes.

Thememory management library provides components for memory management, includingsmart pointers andscoped allocator(since C++11).

Thegeneral utilities library includes components used by other library elements, such as apredefined storage allocator for dynamic storage management, and components used as infrastructure in C++ programs, such astuples and(since C++11)function wrappers.

Thecontainers,iterators,ranges(since C++20), andalgorithms libraries provide a C++ program with access to a subset of the most widely used algorithms and data structures.

Thestrings library provides support for manipulating text represented as homogeneous sequences of following types:char,char8_t(since C++20),char16_t,char32_t(since C++11),wchar_t, and any other character-like types.

Thetext processing library providesregular expression matching and searching(since C++11), utilities fortext formatting(since C++20),identifying text encodings(since C++26) andlocalization facilities.

Thenumerics library providesnumeric algorithms andcomplex number components that extend support for numeric processing. Thevalarray component provides support for n-at-a-time processing, potentially implemented as parallel operations on platforms that support such processing. Therandom number component provides facilities for generating pseudo-random numbers.(since C++11)

Thetime library provides generally useful time utilities.

Theinput/output library provides theiostream components that are the primary mechanism for C++ program input and output. They can be used with other elements of the library, particularly strings, locales, and iterators.

[edit]Library contents

The C++ standard library provides definitions for theentities andmacros described in the synopses of theC++ standard library headers, unless otherwise specified.

All library entities exceptoperator new andoperator delete are defined within the namespacestd ornamespaces nested within namespacestd (except the entities for the C standard library facilities, see below). It is unspecified whether names declared in a specific namespace are declared directly in that namespace or in aninline namespace inside that namespace.(since C++11)

[edit]Headers

Each element of the C++ standard library is declared or defined (as appropriate) in aheader. A header is not necessarily a source file, nor are the sequences delimited by< and> in header names necessarily valid source file names.

The C++ standard library provides theC++ library headers andadditional C++ headers for C library facilities (see “headers” page for descriptions):

Afreestanding implementation has an implementation-defined set of headers, seehere for the minimal requirement on the set of headers.

[edit]C standard library

The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety. The descriptions of many library functions rely on the C standard library for the semantics of those functions.

In some cases, the signatures specified in standard C++ may be different from the signatures in the C standard library, and additional overloads may be declared, but the behavior and the preconditions(including those implied by C'srestrict)(since C++17) are the same unless otherwise stated.

For compatibility with the C standard library, the C++ standard library provides the C headers listed below. The intended use of these headers is for interoperability only. It is possible that C++ source files need to include one of these headers in order to be valid ISO C. Source files that are not intended to also be valid ISO C should not use any of the C headers. Seehere for descriptions.

Except otherwise noted, the contents of each headercxxx is the same as that of the corresponding headerxxx.h as specified in theC standard library. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope of the namespacestd. It is unspecified whether these names (including any overloads added) are first declared within the global namespace scope and are then injected into namespacestd by explicitusing-declarations.

Names which are defined as macros in C (assert,offsetof,setjmp,va_arg,va_end andva_start) must be defined as macros in the C++ standard library, even if C grants license for implementation as functions.

Names that are defined as functions in C must be defined as functions in the C++ standard library. This disallows the practice, allowed in C, of providing a masking macro in addition to the function prototype. The only way to achieve equivalent inline behavior in C++ is to provide a definition as an externinline function.

Identifiers that are keywords or operators in C++ cannot be defined as macros in C++ standard library headers. In particular, including the standard header<iso646.h> has no effect.

[edit]Names associated with safe functions in standard C(since C++17)

If any C++ header is included, it is implementation-defined whether any of the following C standard Annex K names is declared in the global namespace (none of them is declared in namespacestd):

[edit]Using the library

[edit]Including headers

The entities in the C++ standard library are defined in headers, whose contents are made available to a translation unit when it contains the appropriate#include preprocessing directive.

A translation unit may include library headers in any order. Each may be included more than once, with no effect different from being included exactly once, except that the effect of including either<cassert> or<assert.h> depends each time on the lexically current definition ofNDEBUG.

A translation unit can only include a header outside of any declaration or definition, and lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.

[edit]Linkage

Entities in the C++ standard library havestorage duration#external linkage. Unless otherwise specified, objects and functions have the defaultextern"C++"linkage.

Whether a name from the C standard library declared with external linkage hasextern"C" orextern"C++" linkage is implementation-defined. The C++ standard recommends usingextern"C++" in this case.

Objects and functions defined in the library and required by a C++ program are included in the program prior to program startup.

[edit]Requirements on standard library implementations

[edit]Guarantees

A C++ header must providedeclarations anddefinitions that appear in

• the synopsis of that header, or
• the synopsis of another header which is appeared to be included in the synopsis of that header.

For types and macros defined in multiple headers (such asNULL), including any number of these headers in any order never violates theone definition rule.

Unless otherwise specified, allobject-like macros defined by the C standard library that expand to integralconstant expressions can be used in#if preprocessing directives.

Calling a standard library non-member function signature always results in actually calling that function. Therefore a conforming standard library implementation cannot define additional non-member functions that may be called by a valid C++ program.

Non-member function signatures are never declared with additionaldefault arguments.

Unless otherwise specified, calls made by functions in the standard library to non-operator, non-member functions do not use functions from anothernamespace which are found throughargument-dependent name lookup.

For eachfriend declaration of a function (template) within a class (template) definition, no other declaration is provided for that function (template).

For each class defined in the C++ standard library required to bederived from another class defined in the C++ standard library,

• the base class must bevirtual if it is specified asvirtual,
• the base class cannot be virtual if it is not specified asvirtual, and
• unless otherwise specified, types with distinct names shall be distinct types.

If a function defined in the C++ standard library is specified to throw anexception (in a particular situation) of a given type, the exception thrown can only have that type or a type derived from that type so that an exception handler for the base type can catch it.

Functions from the C standard library can only throw exceptions when such a function calls a program-supplied function that throws an exception (qsort() andbsearch() meet this condition).

Destructor operations defined in the C++ standard library never throw exceptions. Every destructor in the C++ standard library behaves as if it had anon-throwing exception specification.

[edit]Implementation freedom

It is unspecified whether any member or non-member functions in the C++ standard library are defined asinline.

For a non-virtual C++ standard library member function, a different set of member function signatures can be declared, provided that any call to that member function that would select an overload from the given set of declarations behaves as if that overload was selected. This allows, for instance:

• adding parameters with default arguments,
• replacing a member function with default arguments with two or more member functions with equivalent behavior, or
• adding additional signatures for a member function name.

Unless otherwise specified, it is implementation-defined which functions in the C++ standard library may be recursively reentered.

It is unspecified whether any function signature or class in the C++ standard library is a friend of another class in the C++ standard library.

The names and global function signatures describedhere are reserved to the implementation.

Any class in the C++ standard library can be derived from a class with a name reserved to the implementation. If a class defined in the C++ standard library is required to be derived from other classes in the C++ standard library, that class can be derived directly from the required base or indirectly through a hierarchy of base classes with names reserved to the implementation.

If a function defined in the C++ standard library is not specified to throw an exception but does not have a non-throwing exception specification, the exception thrown is implementation-defined, but its type should bestd::exception or any type derived fromstd::exception.

The exception specification for a non-virtual function can be strengthened by adding a non-throwing exception specification.

[edit]Notes

libstdc++,libc++, andSTL all support using standard library modules in C++20 mode.

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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