A template parameter pack is a template parameter that accepts zero or more template arguments. [ Example:
template<class ... Types> struct Tuple { };
Tuple<> t0; Tuple<int> t1; Tuple<int, float> t2; Tuple<0> error;
— end example ]
A function parameter pack is a function parameter that accepts zero or more function arguments. [ Example:
template<class ... Types> void f(Types ... args); f(); f(1); f(2, 1.0);
— end example ]
A parameter pack is either a template parameter pack or a function parameter pack.
For the purpose of determining whether a parameter pack satisfies a rule regarding entities other than parameter packs, the parameter pack is considered to be the entity that would result from an instantiation of the pattern in which it appears.
[ Example:
template<class ... Types> void f(Types ... rest);
template<class ... Types> void g(Types ... rest) {
f(&rest ...); }
— end example ]
A parameter pack whose name appears within the pattern of a pack expansion is expanded by that pack expansion. An appearance of the name of a parameter pack is only expanded by the innermost enclosing pack expansion. The pattern of a pack expansion shall name one or more parameter packs that are not expanded by a nested pack expansion; such parameter packs are called unexpanded parameter packs in the pattern. All of the parameter packs expanded by a pack expansion shall have the same number of arguments specified. An appearance of a name of a parameter pack that is not expanded is ill-formed. [ Example:
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;
};
};
typedef zip<short, int>::with<unsigned short, unsigned>::type T1;
typedef zip<short>::with<unsigned short, unsigned>::type T2;
template<class ... Args>
void g(Args ... args) { f(const_cast<const Args*>(&args)...); f(5 ...); f(args); f(h(args ...) + args ...); }
— end example ]
The instantiation of a pack expansion that is not a sizeof... expression produces a list E1, E2, ..., EN, where N is the number of elements in the pack expansion parameters. Each Ei is generated by instantiating the pattern and replacing each pack expansion parameter with its ith element. Such an element, in the context of the instantiation, is interpreted as follows:
if the pack is a template parameter pack, the element is a template parameter ([temp.param]) of the corresponding kind (type or non-type) designating the type or value from the template argument; otherwise,
if the pack is a function parameter pack, the element is an id-expression designating the function parameter that resulted from the instantiation of the pattern where the pack is declared.
All of the Ei become elements in the enclosing list. [ Note: The variety of list varies with the context: expression-list, base-specifier-list, template-argument-list, etc. — end note ] When N is zero, the instantiation of the expansion produces an empty list. Such an instantiation does not alter the syntactic interpretation of the enclosing construct, even in cases where omitting the list entirely would otherwise be ill-formed or would result in an ambiguity in the grammar. [ Example:
template<class... T> struct X : T... { };
template<class... T> void f(T... values) {
X<T...> x(values...);
}
template void f<>();
— end example ]
The instantiation of a sizeof... expression ([expr.sizeof]) produces an integral constant containing the number of elements in the parameter pack it expands.
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