Templates

2024-07-14 09:29| 来源: 网络整理| 查看: 265

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A template is a C++ entity that defines one of the following:

a family of classes (class template), which may be nested classes a family of functions (function template), which may be member functions an alias to a family of types (alias template) (since C++11) a family of variables (variable template) (since C++14) a concept (constraints and concepts) (since C++20)

Templates are parameterized by one or more template parameters, of three kinds: type template parameters, non-type template parameters, and template template parameters.

When template arguments are provided, or, for function and class(since C++17) templates only, deduced, they are substituted for the template parameters to obtain a specialization of the template, that is, a specific type or a specific function lvalue.

Specializations may also be provided explicitly: full specializations are allowed for class, variable(since C++14) and function templates, partial specializations are only allowed for class templates and variable templates(since C++14).

When a class template specialization is referenced in context that requires a complete object type, or when a function template specialization is referenced in context that requires a function definition to exist, the template is instantiated (the code for it is actually compiled), unless the template was already explicitly specialized or explicitly instantiated. Instantiation of a class template does not instantiate any of its member functions unless they are also used. At link time, identical instantiations generated by different translation units are merged.

The definition of a class template must be visible at the point of implicit instantiation, which is why template libraries typically provide all template definitions in the headers (e.g. most boost libraries are header-only).

Contents 1 Syntax 2 Template identifiers 3 Templated entity 4 Defect reports 5 See also [edit] Syntax template requires-clause (optional) declaration (1) export template declaration (2) (until C++11) template concept concept-name = constraint-expression ; (3) (since C++20) parameter-list - a non-empty comma-separated list of the template parameters, each of which is either non-type parameter, a type parameter, a template parameter, or a parameter pack of any of those(since C++11). requires-clause - (since C++20) a requires-clause that specifies the constraints on the template arguments. declaration - declaration of a class (including struct and union), a member class or member enumeration type, a function or member function, a static data member at namespace scope, a variable or static data member at class scope(since C++14), or an alias template(since C++11). It may also define a template specialization. concept-nameconstraint-expression - see constraints and concepts

export was an optional modifier which declared the template as exported (when used with a class template, it declared all of its members exported as well). Files that instantiated exported templates did not need to include their definitions: the declaration was sufficient. Implementations of export were rare and disagreed with each other on details.

(until C++11) This section is incompleteReason: core syntax, template parameters, and instantiations, take content common between class_template and function_template

[edit] Template identifiers

A template identifier has one of the following syntaxes:

template-name (1) operatorop (2) operator "" identifier (3) (since C++11)(deprecated) operator user-defined-string-literal (4) (since C++11) 1) A simple template identifier. 2) An operator function template identifier. 3,4) A literal operator function template identifier. template-name - an identifier that names a template op - an overloadable operator identifier - an identifier user-defined-string-literal - "" followed by an identifier

A simple template identifier that names a class template specialization names a class.

A template identifier that names an alias template specialization names a type.

A template identifier that names a function template specialization names a function.

If all following conditions are satisfied, a template identifier is valid :

There are at most as many arguments as there are parameters or a parameter is a template parameter pack(since C++11). There is an argument for each non-deducible non-pack(since C++11) parameter that does not have a default template argument. Each template argument matches the corresponding template parameter. Substitution of each template argument into the following template parameters (if any) succeeds. If the template identifier is non-dependent, the associated constraints are satisfied as specified below. (since C++20)

An invalid simple template id is a compile-time error, unless it names a function template specialization (in which case SFINAE may apply).

template class X; struct S { using type = int; }; using T1 = X; // error: too many arguments using T2 = X; // error: no default argument for first template parameter using T3 = X; // error: value 1 does not match type-parameter using T4 = X; // error: substitution failure for second template parameter using T5 = X; // OK

When the template-name of a simple template id names a constrained non-function template or a constrained template template parameter, but not a member template that is a member of an unknown specialization, and all template arguments in the simple template id are non-dependent, the associated constraints of the constrained template must be satisfied:

template concept C1 = sizeof(T) != sizeof(int); template struct S1 {}; template using Ptr = T*; S1* p; // error: constraints not satisfied Ptr p; // error: constraints not satisfied template struct S2 { Ptr x; }; // error, no diagnostic required template struct S3 { Ptr x; }; // OK, satisfaction is not required S3 x; // error: constraints not satisfied template struct S4 { X x; // error, no diagnostic required }; template concept C2 = sizeof(T) == 1; template struct S {}; template struct S; // error: constraints not satisfied template struct S {}; // error: constraints not satisfied (since C++20)

If all following conditions are satisfied, two template identifiers are same :

Their template-name s or operators refer to the same template. Their corresponding type template arguments are the same type. The template parameter values determined by their corresponding non-type template arguments are template-argument-equivalent. Their corresponding template template arguments refer to the same template.

Two template identifier that are the same refer to the same variable,(since C++14) class, or function.

[edit] Templated entity

A templated entity (or, in some sources, "temploid") is any entity that is defined (or, for a lambda expression, created)(since C++11) within a template definition. All of the following are templated entities:

a class/function/variable(since C++14) template a concept (since C++20) a member of a templated entity (such as a non-template member function of a class template) an enumerator of an enumeration that is a templated entity any entity defined or created within a templated entity: a local class, a local variable, a friend function, etc the closure type of a lambda expression that appears in the declaration of a templated entity (since C++11)

For example, in

template struct A { void f() {} };

the function A::f is not a function template, but is still considered to be templated.

A templated function is a function template or a function that is templated.

A templated class is a class template or a class that is templated.

A templated variable is a variable template or a variable that is templated.

(since C++14) [edit] Defect reports

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

DR Applied to Behavior as published Correct behavior CWG 2293 C++98 the rules of determining whether a templateidentifier is valid were not provided provided CWG 2682 C++98C++14 the definitions of templated function/template class(C++98)/templated variable (C++14) were missing added P2308R1 C++98 two template identifiers were different if theircorresponding non-type template argumentsare not template-argument-equivalent they are different if their correspondingnon-type template parameter valuesare not template-argument-equivalent [edit] See also C documentation for Generic selection

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