C++ Getting parameter pack from tuple via std::experimental::apply function - c++

Question

I have following question about std::experimental::apply. From what I understand it takes function object and tuple, and than expands the tuple into parameter pack, which is applied to given functor. Unluckily, I have type mismatch in the code below when I try to compile (function invocation).
template<std::size_t population_size, std::size_t generations, typename Func,
class Compare=std::greater<>,
class Generator=std::default_random_engine,
class Distribution=std::uniform_real_distribution<>>
constexpr auto optimize(Func function, const std::pair<auto,auto>... range){ //CHECKING IF FUNCTION IS A FUNCTOR, TBD!
static_assert(sizeof...(range)>2, "Function needs at least two arguments");
//RANDOM NUMBER GENERATOR AND CHECKING ARITHMETIC TYPES
constexpr decltype(auto) generate_number=[&](std::pair<auto,auto> range){
static_assert(std::is_arithmetic<std::tuple_element_t<0, decltype(range)>>(),
"First argument of std::pair has to be arithmetic!");
static_assert(std::is_arithmetic<std::tuple_element_t<1, decltype(range)>>(),
"Second argument of std::pair has to be arithmetic!");
return std::bind(Distribution(range.first,range.second),Generator());
};
//SINGLE RANDOM INDIVIDUAL GENERATOR
decltype(auto) generate_random_individual=[&](){ //RUN THIS FUNCTION ASYNCHRONOUSLY, TBD!
auto genotype=std::make_tuple(generate_number(range)()...); //WORKS FINE
//CRITICAL FUNCTION AND THE TOPIC OF THIS POST
auto key=std::experimental::apply(function,genotype);
//CRITICAL FUNCTION AND THE TOPIC OF THIS POST
//return std::make_pair(key,genotype);
};
generate_random_individual();
}
Here is my test case (should compile fine without experimental::apply).
#include<utility>
#include<iostream>
#include"optimalization.hpp"
class Function{
public:
Function()=default;
double operator()(double x, double y, double z){
return (std::exp(x+1.25)*std::pow(y,z))/std::exp((x*y)/z);
}
};
int main(){
Function f{};
double x=optimize<100, 200>(f, std::make_pair(-21, 37), std::make_pair(22.5, 88.11), std::make_pair(-13, 37));
std::cout << x << std::endl;
}

Answer 1

The core of the problem is that Function::operator() is not const. There are some other issues, such as constexpr auto optimize(Func function, const std::pair<auto, auto>... range) being invalid syntax, but those basically fall into place once the main issue is fixed.
Working code:
#include <cstddef>
#include <functional>
#include <random>
#include <experimental/tuple>
#include <type_traits>
#include <utility>
template<
std::size_t population_size, std::size_t generations,
typename Func,
typename Compare = std::greater<>,
typename Generator = std::default_random_engine,
typename Distribution = std::uniform_real_distribution<>,
typename... RangeElemT
>
auto optimize(Func function, std::pair<RangeElemT, RangeElemT> const... range) {
static_assert(sizeof...(range) > 2, "Function needs at least two arguments");
auto generate_number = [&](auto range) {
static_assert(
std::is_arithmetic<std::tuple_element_t<0, decltype(range)>>{},
"First argument of std::pair has to be arithmetic!"
);
static_assert(
std::is_arithmetic<std::tuple_element_t<1, decltype(range)>>{},
"Second argument of std::pair has to be arithmetic!"
);
return std::bind(Distribution(range.first, range.second), Generator{});
};
auto genotype = std::make_tuple(generate_number(range)()...);
auto key = std::experimental::apply(function, genotype);
return std::make_pair(key, genotype);
}
#include <cmath>
struct Function {
double operator ()(double x, double y, double z) const {
return std::exp(x + 1.25) * std::pow(y, z) / std::exp(x * y / z);
}
};
#include <iostream>
#include <boost/type_index.hpp>
int main() {
namespace bti = boost::typeindex;
Function f{};
auto x = optimize<100, 200>(
f,
std::make_pair(-21, 37),
std::make_pair(22.5, 88.11),
std::make_pair(-13, 37)
);
std::cout
<< bti::type_id_with_cvr<decltype(x)>().pretty_name() << '\n'
<< x.first << " :: "
<< std::get<0>(x.second) << ", "
<< std::get<1>(x.second) << ", "
<< std::get<2>(x.second) << '\n';
}
Online Demo
Also note that in both places you used decltype(auto), auto can (and for clarity, should) be used instead.