fold_1.cpp

#include <iostream>
#include <numeric>
#include <vector>

int main(void) {
    // Create a vector
    std::vector<std::string> v{"foo", "bar"};

    // Fold by reference
    std::string res = "";
    auto res = std::accumulate(
        v.begin(),
        v.end(),
        res,
        [](std::string & acc, const std::string & val) -> std::string & {
            return acc += val;
        }
    );

    // Outputs to: "foobar"
    std::cout << res << '\n';

    return 0;
}