Class template
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A class template defines a family of classes.
Inhaltsverzeichnis |
[Bearbeiten]Syntax
template<parameter-list>declaration | |||||||||
[Bearbeiten]Erklärung
declaration defines or declares a class (including struct and union), a member class or member enumeration type, a function or member function, a static data member of a class template, or a type alias. It may also define a Template-Spezialisierung. This page focuses on class templates.
parameter-list is 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. This page focuses on the parameters that are not parameter packs.
[Bearbeiten]Non-type template parameter
| typename | (1) | ||||||||
typename= default | (2) | ||||||||
type...name | (3) | (seit C++11) | |||||||
type is one of the following types (optionally cv-qualified, the qualifiers are ignored)
- integral type
- enumeration
- pointer to object or to function
- lvalue reference to object or to function
- pointer to member object or to member function
- std::nullptr_t(seit C++11)
Array and function types may be written in a template declaration, but they are automatically replaced by pointer to data and pointer to function as appropriate.
When the name of a non-type template parameter is used in an expression within the body of the class template, it is an unmodifiable prvalue unless its type was an lvalue reference type.
[Bearbeiten]Type template parameter
typenamename | (1) | ||||||||
classname | (2) | ||||||||
typename|classname= default | (3) | ||||||||
typename|class...name | (4) | (seit C++11) | |||||||
| This section is incomplete |
[Bearbeiten]Template template parameter
template<parameter-list>name | (1) | ||||||||
template<parameter-list>name=default | (2) | ||||||||
template<parameter-list>...name | (3) | (seit C++11) | |||||||
| This section is incomplete |
[Bearbeiten]Class template instantiation
A class template by itself is not a type, or an object, or any other entity. No code is generated from a source file that contains only template definitions. In order for any code to appear, a template must be instantiated: the template arguments must be provided so that the compiler can generate an actual class (or function, from a function template).
[Bearbeiten]Explicit instantiation
templateclassname<argument-list> ; | (1) | ||||||||
externtemplateclassname<argument-list> ; | (2) | (seit C++11) | |||||||
An explicit instantiation definition forces instantiation of the class, struct, or union they refer to. It may appear in the program anywhere after the template definition, and for a given argument-list, is only allowed to appear once in the program.
An explicit instantiation declaration (an extern template) prevents implicit instantiations: the code that would otherwise cause an implicit instantiation has to use the explicit instantiation definition provided somewhere else in the program.
[Bearbeiten]Implicit instantiation
When code refers to a template in context that requires a completely defined type, or when the completeness of the type affects the code, and this particular type has not been explicitly instantiated, implicit instantiation occurs. For example, when an object of this type is constructed, but not when a pointer to this type is constructed.
This applies to the members of the class template: unless the member is used in the program, it is not instantiated, and does not require a definition.
template<class T>struct Z {
void f(){}
void g();// never defined
};// template definition
templatestruct Z<double>;// explicit instantiation of Z<double>
Z<int> a;// implicit instantiation of Z<int>
Z<char>* p;// nothing is instantiated here
p->f();// implicit instantiation of Z<char> and Z<char>::f() occurs here.
// Z<char>::g() is never needed and never instantiated: it does not have to be defined
[Bearbeiten]Non-type template parameters
The following limitations apply when instantiating class templates that have non-type template parameters:
- For integral and arithmetic types, the template argument provided during instantiation must be a constant expression.
- For pointers to objects, the template arguments have to designate the address of an object with static storage duration and a linkage (either internal or external), or a constant expression that evaluates to the appropriate null pointer value.
- For pointers to functions, the valid arguments are pointers to functions with linkage (or constant expressions that evaluate to null pointer values).
- For lvalue reference parameters, the argument provided at instantiation cannot be a temporary, an unnamed lvalue, or a named lvalue with no linkage.
- For pointers to members, the argument has to be a pointer to member expressed as &Class::Member or a constant expression that evaluates to null pointer value.
In particular, this implies that string literals, addresses of array elements, and addresses of non-static members cannot be used as template arguments to instantiate templates whose corresponding non-type template parameters are pointers to data.
[Bearbeiten]Beispiel
template<typename T>struct S {template<typename U>void foo(){}}; template<typename T>void bar(){ S<T>s; s.foo<T>();// error: < parsed as less than operator s.template foo<T>();// OK}
[Bearbeiten]Non-type template parameters
#include <iostream> // simple non-type template parametertemplate<int N>struct S {int a[N];}; template<constchar*>struct S2 {}; // complicated non-type exampletemplate<char c, // integral typeint(&ra)[5], // lvalue reference to object (of array type)int(*pf)(int), // pointer to functionint(S<10>::*a)[10]// pointer to member object (of type int[10])>struct Complicated {// calls the function selected at compile time// and stores the result in the array selected at compile timevoid foo(char base){ ra[4]= pf(c - base);}}; // S2<"fail"> s2; // Error: string literal cannot be usedchar okay[]="okay";// static object with linkage// S2< &okay[0] > s2; // Error: array element has no linkage S2<okay> s2;// works int a[5];int f(int n){return n;}int main(){ S<10> s;// s.a is an array of 10 int s.a[9]=4; Complicated<'2', a, f, &S<10>::a> c; c.foo('0'); std::cout<< s.a[9]<< a[4]<<'\n';}
Output:
42
[Bearbeiten]Siehe auch
- deklariert eine Funktion VorlageOriginal:Funktions-Template Erklärung</div> declares a function templateOriginal:function template declarationThe text has been machine-translated via [http://translate.google.com Google Translate].
You can help to correct and verify the translation. Click [http://en.cppreference.com/w/Cppreference:MachineTranslations here] for instructions.The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions. - definiert eine bestehende Vorlage für einen bestimmten TypOriginal:Template-Spezialisierung</div> defines an existing template for a specific typeOriginal:template specializationThe text has been machine-translated via [http://translate.google.com Google Translate].
You can help to correct and verify the translation. Click [http://en.cppreference.com/w/Cppreference:MachineTranslations here] for instructions.The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions. - ermöglicht die Verwendung von Listen von Typen in Vorlagen (seit C++11)Original:Parameter Packs</div> allows the use of lists of types in templates (seit C++11)Original:parameter packsThe text has been machine-translated via [http://translate.google.com Google Translate].
You can help to correct and verify the translation. Click [http://en.cppreference.com/w/Cppreference:MachineTranslations here] for instructions.The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.
