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<h1 id="use-stdinteger_sequence-and-stdindex_sequence"><a class="header" href="#use-stdinteger_sequence-and-stdindex_sequence">use std::integer_sequence and std::index_sequence</a></h1>
<blockquote>
<p>they use integer constants in template header but not types.</p>
</blockquote>
<blockquote>
<p>need a sequence of integers in your template impl which is generally variadic then use this.</p>
</blockquote>
<p>rule: let make_index_sequence method to create an index_sequence<ints...> instance and provide it to another wrapper which will capture it's INTs in its own header and then you can unpack it.</p>
<blockquote>
<p>index_sequence is template alias of integer_sequence and both supports makers.</p>
</blockquote>
<pre><code class="language-c++">#include <print>
// capture Ints here
template <size_t... Ints>
constexpr auto get_total(std::index_sequence<Ints...>) {
return (0 + ... + Ints);
}
// let make_index_sequence to generate a wrapper of Ints with given param
template <size_t N, typename Indices = std::make_index_sequence<N + 1>>
constexpr auto total = get_total(Indices{}); // pass it another variadic helper to capture Ints and unpack it
int main() {
std::print("{}\n", total<3>);
std::print("{}\n", total<4>);
std::print("{}\n", total<5>);
return 0;
}
</code></pre>
<p>or integer_sequence:</p>
<pre><code class="language-c++">
#include <iostream>
#include <utility>
// Function to print elements of an integer sequence
template <typename T, T... Ints>
void print_sequence(std::integer_sequence<T, Ints...>) {
((std::cout << Ints << ' '), ...);
std::cout << '\n';
}
int main() {
// Create an integer sequence of type int with values 0, 1, 2, 3, 4
std::integer_sequence<int, 0, 1, 2, 3, 4> seq;
print_sequence(seq); // Output: 0 1 2 3 4
}
</code></pre>
<pre><code class="language-c++">#include <iostream>
#include <utility>
// Function to print elements of a tuple
template <typename Tuple, std::size_t... Is>
void print_tuple(const Tuple& t, std::index_sequence<Is...>) {
((std::cout << std::get<Is>(t) << ' '), ...);
std::cout << '\n';
}
int main() {
auto myTuple = std::make_tuple(1, 2.5, "Hello");
// Create an index sequence for the tuple
print_tuple(myTuple, std::index_sequence_for<int, double, const char*>{}); // Output: 1 2.5 Hello
return 0;
}
</code></pre>
<p>definitions in std:</p>
<pre><code class="language-c++">template <class...Ts>
index_sequence_for = make_index_sequence<sizeof...(Ts)>;
template <size_t N>
make_index_sequence = make_integer_sequence<size_t, N>;
template <class T, T N>
make_integer_sequence = integer_sequence<T, Ints...>
template <size_t...Ints>
index_sequence = integer_sequence<size_t, Ints...>;
template <class T, T...Ints>
class integer_sequence;
</code></pre>
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