Pack indexing (since C++26)
Accesses the element of a pack at a specified index.
Contents |
[edit] Syntax
id-expression ...[ expression ]
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(1) | ||||||||
typedef-name ...[ expression ]
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(2) | ||||||||
typedef-name | - | an identifier or a simple-template-id that names a pack |
id-expression | - | an id-expression that names a pack |
expression | - | a converted constant expression I of type std::size_t designated as index where I is within the range [ 0, sizeof...(P)) for some pack P in pack indexing
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[edit] Explanation
Pack indexing is a pack expansion of the unexpanded pack followed by an ellipsis and index inside the subscript. There are two kinds of pack indexing: pack indexing expression and pack indexing specifier.
Let P
be a non-empty pack containing P0, P1, ..., Pn-1
and I
be a valid index, the instantiation of the expansion P...[I]
yields the pack element PI
of P
.
Indexing a pack with non-constant expression index I
is not allowed.
int runtime_idx(); void bar(auto... args) { auto a = args...[0]; const int n = 1; auto b = args...[n]; int m = 2; auto c = args...[m]; // error: 'm' is not a constant expression auto d = args...[runtime_idx()]; // error: 'runtime_idx()' is not a constant expression }
Indexing a pack of template template parameters is not possible.
template <template <typename...> typename... Temps> using A = Temps...[0]<>; // error: 'Temps' is a pack of template template parameters template <template <typename...> typename... Temps> using B = Temps<>...[0]; // error: 'Temps<>' doesn't denote pack name // although it is a simple-template-id
[edit] Pack indexing expression
id-expression ...[ expression ]
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Pack indexing expression denotes the id-expression, the expression of pack element PI
. The id-expression shall be introduced by the declaration of:
- non-type template parameter pack,
- function parameter pack,
- lambda init-capture pack, or
- structured binding pack.
template <std::size_t I, typename... Ts> constexpr auto element_at(Ts... args) { // 'args' introduced in function parameter pack declaration return args...[I]; } static_assert(element_at<0>(3, 5, 9) == 3); static_assert(element_at<2>(3, 5, 9) == 9); static_assert(element_at<3>(3, 5, 9) == 4); // error: out of bounds static_assert(element_at<0>() == 1); // error: out of bounds, empty pack template <std::size_t I, typename Tup> constexpr auto structured_binding_element_at(Tup tup) { auto [...elems] = tup; // 'elems' introduced in structured binding pack declaration return elems...[I]; } struct A { bool a; int b; }; static_assert(structured_binding_element_at<0>(A {true, 4}) == true); static_assert(structured_binding_element_at<1>(A {true, 4}) == 4); // 'Vals' introduced in non-type template parameter pack declaration template <std::size_t I, std::size_t... Vals> constexpr std::size_t double_at = Vals...[I] * 2; // OK template <std::size_t I, typename... Args> constexpr auto foo(Args... args) { return [...members = args](Args...[I] op) { // 'members' introduced in lambda init-capture pack return members...[I] + op; }; } static_assert(foo<0>(4, "Hello", true)(5) == 9); static_assert(foo<1>(3, std::string("C++"))("26") == "C++26");
Indexing pack of complex expressions other than id-expression is not allowed.
template <std::size_t I, auto... Vals> constexpr auto identity_at = (Vals)...[I]; // error // use 'Vals...[I]' instead template <std::size_t I, std::size_t... Vals> constexpr std::size_t triple_at = (Vals * 3)...[I]; // error // use 'Vals...[I] * 3' instead template <std::size_t I, typename... Args> constexpr decltype(auto) get(Args&&... args) noexcept { return std::forward<Args>(args)...[I]; // error // use 'std::forward<Args...[I]>(args...[I])' instead }
Applying decltype
to pack indexing expression is the same as applying decltype
to id-expression.
void f() { [](auto... args) { using T0 = decltype(args...[0]); // 'T0' is 'double' using T1 = decltype((args...[0])); // 'T1' is 'double&' }(3.14); }
[edit] Pack indexing specifier
typedef-name ...[ expression ]
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Pack indexing specifier denotes the computed-type-specifier, the type of pack element PI
. The typedef-name shall be introduced by the declaration of type template parameter pack.
template <typename... Ts> using last_type_t = Ts...[sizeof...(Ts) - 1]; static_assert(std::is_same_v<last_type_t<>, int>); // error: out of bounds static_assert(std::is_same_v<last_type_t<int>, int>); static_assert(std::is_same_v<last_type_t<bool, char>, char>); static_assert(std::is_same_v<last_type_t<float, int, bool*>, bool*>);
Pack indexing specifier can appear as:
- a simple type specifier,
- a base class specifier,
- a nested name specifier, or
- the type of an explicit destructor call.
Pack indexing specifier can be used in function or constructor parameter list to establish non-deduced contexts in template argument deduction.
template <typename...> struct type_seq {}; template <typename... Ts> auto f(Ts...[0] arg, type_seq<Ts...>) { return arg; } // OK: "Hello" is implicitly converted to 'std::string_view' std::same_as<std::string_view> auto a = f("Hello", type_seq<std::string_view>{}); // Error: "Ok" is not convertible to 'int' std::same_as<int> auto b = f("Ok", type_seq<int, const char*>{});
[edit] Notes
Before C++26, Ts...[N] was a valid syntax for declaring function parameter pack of unnamed arrays of size N, where the parameter types were further adjusted to pointers. Since C++26, Ts...[1] is interpreted as a pack indexing specifier which would change the behavior below to #2. To preserve the first behavior, the function parameter pack must be named, or manually adjusted to a pack of pointer types.
template <typename... Ts> void f(Ts... [1]); template <typename... Ts> void g(Ts... args[1]); template <typename... Ts> void h(Ts*...); // clearer but more permissive: Ts... can contain cv void or function types void foo() { f<char, bool>(nullptr, nullptr); // behavior #1 (before C++26): // calls void 'f<char, bool>(char*, bool*)' (aka 'f<char, bool>(char[1], bool[1])') // behavior #2 (since C++26): // error: supposedly called 'void f<char, bool>(bool)' // but provided with 2 arguments instead of 1 g<char, bool>(nullptr, nullptr); // calls 'g<char, bool>(char*, bool*)' (aka 'g<char, bool>(char[1], bool[1])') h<char, bool>(nullptr, nullptr); // calls 'h<char, bool>(char*, bool*)' }
Feature-test macro | Value | Std | Feature |
---|---|---|---|
__cpp_pack_indexing |
202311L |
(C++26) | Pack indexing |
[edit] Example
#include <tuple> template <std::size_t... Indices, typename Decomposable> constexpr auto splice(Decomposable d) { auto [...elems] = d; return std::make_tuple(elems...[Indices]...); } struct Point { int x; int y; int z; }; int main() { constexpr Point p { .x = 1, .y = 4, .z = 3 }; static_assert(splice<2, 1, 0>(p) == std::make_tuple(3, 4, 1)); static_assert(splice<1, 1, 0, 0>(p) == std::make_tuple(4, 4, 1, 1)); }