string_dispatch.cpp

#include <iostream>
#include <string>

template <class T, class F>
auto dispatch(const T&, F&& f) { return f(); }

template <class T, class A, class F, class... Args>
auto dispatch(const T& target, A&& a, F&& f, Args&&... args) {
    if (target == a) return f();
    return dispatch(target, std::forward<Args>(args)...);
}

void f(std::string_view target) {
    int r = dispatch(target,
        "aaa", [] { return 0; },
        "bbb", [] { return 1; },
        "ccc", [] { return 2; },
        []{ return 3; }
    );
    std::cout << r << std::endl;
}

int main() {
    f("aaa");
    f("ccc");
    f("ddd");
}