std::ranges::fold_left

#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>

int main()
{
    namespace ranges = std::ranges;

    std::vector v{1, 2, 3, 4, 5, 6, 7, 8};
    
    int sum = ranges::fold_left(v.begin(), v.end(), 0, std::plus<int>()); // (1)
    std::cout << "sum: " << sum << '\n';
    
    int mul = ranges::fold_left(v, 1, std::multiplies<int>()); // (2)
    std::cout << "mul: " << mul << '\n';
    
    // get the product of the std::pair::second of all pairs in the vector:
    std::vector<std::pair<char, float>> data {{'A', 2.f}, {'B', 3.f}, {'C', 3.5f}};
    float sec = ranges::fold_left
    (
        data | ranges::views::values, 2.0f, std::multiplies<>()
    );
    std::cout << "sec: " << sec << '\n';
    
    // use a program defined function object (lambda-expression):
    std::string str = ranges::fold_left
    (
        v, "A", [](std::string s, int x) { return s + ':' + std::to_string(x); }
    );
    std::cout << "str: " << str << '\n';

    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 1}, {2, 0}, {3, 0}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto res = ranges::fold_left(nums, {7, 0}, std::multiplies{}); // (2)
    #else
        auto res = ranges::fold_left(nums, CD{7, 0}, std::multiplies{}); // (2)
    #endif
    std::cout << "res: " << res << '\n';
}