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Pack indexing (since C++26)

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Accesses the element of a parameter pack at a specified index.

Contents

[edit] Syntax

id-expression ...[ expression ] (1)
typedef-name ...[ expression ] (2)
1) Pack indexing expression
2) Pack indexing specifier
typedef-name - an identifier or a simple-template-id that names parameter pack
id-expression - an id-expression that names parameter pack
expression - a converted constant expression I of type std::size_t designated as index where I is within the range [0sizeof...(P)) for some pack P in pack indexing

[edit] Explanation

Pack indexing is a pack expansion of the unexpanded parameter 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 parameter 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 ]

Pack indexing expression denotes the id-expression, the expression of pack element PI. The id-expression shall be introduced by the declaration of:

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
 
// '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 ]

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:

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