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

• a parameter pack

• template parameter pack
• function parameter pack

A template parameter pack is a template parameter that accepts zero or more template arguments (constants, types, or templates). A function parameter pack is a function parameter that accepts zero or more function arguments.

The number of elements of a pack is equal to:

• the number of arguments provided for the parameter pack, if the pack is a template or function parameter pack,

A template with at least one parameter pack is called avariadic template.

[edit]Syntax

Template parameter pack (appears inalias template,class template,variable template(since C++14),concept(since C++20) andfunction template parameter lists)

Function parameter pack (a form ofdeclarator, appears in a function parameter list of a variadic function template)

Pack expansion (appears in a body of a template)

[edit]Explanation

A variadic class template can be instantiated with any number of template arguments:

template<class...Types>struct Tuple{}; Tuple<> t0;// Types contains no argumentsTuple<int> t1;// Types contains one argument: intTuple<int,float> t2;// Types contains two arguments: int and floatTuple<0> t3;// error: 0 is not a type

A variadic function template can be called with any number of function arguments (the template arguments are deduced throughtemplate argument deduction):

template<class...Types>void f(Types...args); f();// OK: args contains no argumentsf(1);// OK: args contains one argument: intf(2,1.0);// OK: args contains two arguments: int and double

In a primary class template, the template parameter pack must be the final parameter in the template parameter list. In a function template, the template parameter pack may appear earlier in the list provided that all following parameters can be deduced from the function arguments, or have default arguments:

template<typename U,typename...Ts>// OK: can deduce Ustruct valid;// template<typename... Ts, typename U> // Error: Ts... not at the end// struct Invalid; template<typename...Ts,typename U,typename=void>void valid(U, Ts...);// OK: can deduce U// void valid(Ts..., U); // Can't be used: Ts... is a non-deduced context in this position valid(1.0,1,2,3);// OK: deduces U as double, Ts as {int, int, int}

If every valid specialization of a variadic template requires an empty template parameter pack, the program is ill-formed, no diagnostic required.

[edit]Pack expansion

A pattern followed by an ellipsis, in which the name of at least one pack appears at least once, isexpanded into zero or more instantiations of the pattern, where the name of the pack is replaced by each of the elements from the pack, in order. Instantiations ofalignment specifiers are space-separated, other instantiations are comma-separated.

template<class...Us>void f(Us...pargs){} template<class...Ts>void g(Ts...args){    f(&args...);// “&args...” is a pack expansion// “&args” is its pattern} g(1,0.2,"a");// Ts... args expand to int E1, double E2, const char* E3// &args... expands to &E1, &E2, &E3// Us... pargs expand to int* E1, double* E2, const char** E3

If the names of two packs appear in the same pattern, they are expanded simultaneously, and they must have the same length:

template<typename...>struct Tuple{}; template<typename T1,typename T2>struct Pair{}; template<class...Args1>struct zip{template<class...Args2>struct with{typedef Tuple<Pair<Args1, Args2>...> type;// Pair<Args1, Args2>... is the pack expansion// Pair<Args1, Args2> is the pattern};}; typedef zip<short,int>::with<unsignedshort,unsigned>::type T1;// Pair<Args1, Args2>... expands to// Pair<short, unsigned short>, Pair<int, unsigned int>// T1 is Tuple<Pair<short, unsigned short>, Pair<int, unsigned>> // typedef zip<short>::with<unsigned short, unsigned>::type T2;// error: pack expansion contains packs of different lengths

If a pack expansion is nested within another pack expansion, the packs that appear inside the innermost pack expansion are expanded by it, and there must be another pack mentioned in the enclosing pack expansion, but not in the innermost one:

template<class...Args>void g(Args...args){    f(const_cast<const Args*>(&args)...);// const_cast<const Args*>(&args) is the pattern, it expands two packs// (Args and args) simultaneously     f(h(args...)+ args...);// Nested pack expansion:// inner pack expansion is "args...", it is expanded first// outer pack expansion is h(E1, E2, E3) + args..., it is expanded// second (as h(E1, E2, E3) + E1, h(E1, E2, E3) + E2, h(E1, E2, E3) + E3)}

When the number of elements in a pack is zero (empty pack), the instantiation of a pack expansion does not alter the syntactic interpretation of the enclosing construct, even in cases where omitting the pack expansion entirely would otherwise be ill-formed or would result in a syntax ambiguity. The instantiation produces an empty list.

template<class...Bases>struct X: Bases...{}; template<class...Args>void f(Args...args){    X<Args...> x(args...);} templatevoid f<>();// OK, X<> has no base classes// x is a variable of type X<> that is value-initialized

[edit]Expansion loci

Depending on where the expansion takes place, the resulting comma-separated (or space-separated foralignment specifiers) list is a different kind of list: function parameter list, member initializer list, attribute list, etc. The following is the list of all allowed contexts:

[edit]Function argument lists

A pack expansion may appear inside the parentheses of a function call operator, in which case the largest expression orbrace-enclosed initializer list to the left of the ellipsis is the pattern that is expanded:

f(args...);// expands to f(E1, E2, E3)f(&args...);// expands to f(&E1, &E2, &E3)f(n,++args...);// expands to f(n, ++E1, ++E2, ++E3);f(++args..., n);// expands to f(++E1, ++E2, ++E3, n); f(const_cast<const Args*>(&args)...);// f(const_cast<const E1*>(&X1), const_cast<const E2*>(&X2), const_cast<const E3*>(&X3)) f(h(args...)+ args...);// expands to// f(h(E1, E2, E3) + E1, h(E1, E2, E3) + E2, h(E1, E2, E3) + E3)

[edit]Parenthesized initializers

A pack expansion may appear inside the parentheses of adirect initializer, afunction-style cast, and other contexts (member initializer,new-expression, etc.) in which case the rules are identical to the rules for a function call expression above:

Class c1(&args...);// calls Class::Class(&E1, &E2, &E3)Class c2= Class(n,++args...);// calls Class::Class(n, ++E1, ++E2, ++E3); ::new((void*)p) U(std::forward<Args>(args)...)// std::allocator::allocate

[edit]Brace-enclosed initializers

In a brace-enclosed initializer list, a pack expansion may appear as well:

template<typename...Ts>void func(Ts...args){constint size= sizeof...(args)+2;int res[size]={1, args...,2}; // since initializer lists guarantee sequencing, this can be used to// call a function on each element of a pack, in order:int dummy[sizeof...(Ts)]={(std::cout<< args,0)...};}

[edit]Template argument lists

Pack expansions can be used anywhere in a template argument list, provided the template has the parameters to match the expansion:

template<class A,class B,class...C>void func(A arg1, B arg2, C...arg3){    container<A, B, C...> t1;// expands to container<A, B, E1, E2, E3>    container<C..., A, B> t2;// expands to container<E1, E2, E3, A, B>    container<A, C..., B> t3;// expands to container<A, E1, E2, E3, B>}

[edit]Function parameter list

In a function parameter list, if an ellipsis appears in a parameter declaration (whether it names a function parameter pack (as in,Args...args) or not) the parameter declaration is the pattern:

template<typename...Ts>void f(Ts...){} f('a',1);// Ts... expands to void f(char, int)f(0.1);// Ts... expands to void f(double) template<typename...Ts,int...N>void g(Ts(&...arr)[N]){} int n[1]; g<constchar,int>("a", n);// Ts (&...arr)[N] expands to// const char (&)[2], int(&)[1]

Note: In the patternTs (&...arr)[N], the ellipsis is the innermost element, not the last element as in all other pack expansions.

Note:Ts (&...)[N] is not allowed because the C++11 grammar requires the parenthesized ellipsis to have a name:CWG issue 1488.

[edit]Template parameter list

Pack expansion may appear in a template parameter list:

template<typename...T>struct value_holder{template<T...Values>// expands to a constant template parameterstruct apply{};// list, such as <int, char, int(&)[5]>};

[edit]Base specifiers and member initializer lists

A pack expansion may designate the list of base classes in aclass declaration. Typically, this also means that the constructor needs to use a pack expansion in themember initializer list to call the constructors of these bases:

template<class...Mixins>class X:public Mixins...{public:    X(const Mixins&...mixins): Mixins(mixins)...{}};

[edit]Lambda captures

Pack expansion may appear in the capture clause of alambda expression:

template<class...Args>void f(Args...args){auto lm=[&, args...]{return g(args...);};    lm();}

[edit]Thesizeof... operator

Thesizeof... operator is classified as a pack expansion as well:

template<class...Types>struct count{staticconststd::size_t value= sizeof...(Types);};

template<class...X>void func(int arg)throw(X...){// ... throw different Xs in different situations}

[edit]Alignment specifier

Pack expansions are allowed in both the lists of types and the lists of expressions used by the keywordalignas. The instantiations are space-separated:

template<class...T>struct Align{    alignas(T...)unsignedchar buffer[128];}; Align<int,short> a;// the alignment specifiers after expansion are// alignas(int) alignas(short)// (no comma in between)

[edit]Attribute list

Pack expansions are allowed in the lists ofattributes, if permitted by the attribute's specification. For example:

template<int...args>[[vendor::attr(args)...]]void* f();

template<typename...bases>struct X: bases...{using bases::g...;};X<B, D> x;// OK: B::g and D::g introduced

constevalauto first_plus_last(auto...args){return args...[0]+ args...[sizeof...(args)-1];} static_assert(first_plus_last(5)==10);static_assert(first_plus_last(5,4)==9);static_assert(first_plus_last(5,6,2)==7);

struct C{};struct E{struct Nested;}; template<class...Ts>class R{friend Ts...;}; template<class...Ts,class...Us>class R<R<Ts...>, R<Us...>>{friend Ts::Nested..., Us...;}; R<C, E> rce;// classes C and E are friends of R<C, E>R<R<E>, R<C,int>> rr;// E::Nested and C are friends of R<R<E>, R<C, int>>

[edit]Notes

[edit]Example

The below example defines a function similar tostd::printf, that replace each occurrence of the character% in the format string with a value.

The first overload is called when only the format string is passed and there is no parameter expansion.

The second overload contains a separate template parameter for the head of the arguments and a parameter pack, this allows the recursive call to pass only the tail of the parameters until it becomes empty.

Targs is the template parameter pack andFargs is the function parameter pack.

#include <iostream> void tprintf(constchar* format)// base function{std::cout<< format;} template<typename T,typename...Targs>void tprintf(constchar* format, T value, Targs...Fargs)// recursive variadic function{for(;*format!='\0'; format++){if(*format=='%'){std::cout<< value;            tprintf(format+1, Fargs...);// recursive callreturn;}std::cout<<*format;}} int main(){    tprintf("% world% %\n","Hello",'!',123);}

Hello world! 123

[edit]Defect reports

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

[edit]See also

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