accessing tuple elements using C++ 20 views

[juandent]

An interesting capability for dealing with tuples is now available in C++ 20 (part of the support for views), like this:

#include <sqlite_orm/sqlite_orm.h>

#include <iostream>
#include <string>
#include <vector>
#include <ranges>
#include <numbers>    // for math constants
#include <algorithm>  // for sort()

void print(std::ranges::input_range auto&& coll)
{
    for (const auto& elem : coll) {
        std::cout << elem << ' ';
    }
    std::cout << '\n';
}



struct Constant
{
    int id;
    std::string name;
    double value;
};



void elements_view()
{
    using namespace sqlite_orm;

    auto storage = make_storage("constants.sqlite",
        make_table("constants", make_column("id", &Constant::id, primary_key().autoincrement()),
            make_column("name", &Constant::name),
            make_column("value", &Constant::value)));
    storage.sync_schema();

    storage.remove_all<Constant>();

    std::vector<Constant> constants = {
        {-1, "pi", std::numbers::pi },
        {-1, "e", std::numbers::e },
        {-1, "golden-ratio", std::numbers::egamma },
        {-1, "euler-constant", std::numbers::phi } };

    storage.insert_range(constants.begin(), constants.end());


    auto lines = storage.select(columns(&Constant::id, &Constant::name, &Constant::value));


    // print all elements #2 in the tuple (the Constant::values)
    print(lines | std::views::elements<2>);                                          // HERE!
}

Nice ah?

[fnc12]

yes it looks nice and as I see requires nothing to write to support this. The only thing I can't understand is why we need the word auto here:

void print(std::ranges::input_range auto&& coll)

?

[trueqbit]

ranges::input_range is a concept, auto is the other way to write a template function!

[juandent]

as @trueqbit points out, this is a template function. It is equivalent to:

template<typename T>
requires std::ranges::input_range<T>
void print(T&& coll)
{
    for (const auto& elem : coll) {
        std::cout << elem << ' ';
    }
    std::cout << '\n';
}