Based on this question
i wrote the fallowing code.
I did not completely understood function_traits
! but i was able to
overload the >> operator for lambdas with different number of arguments .
I know it's not the best possible solution , but i wrote it so someone can take it as a starting point ( a variadic template implementation will be awesome !) .
#include<iostream>
#include<string>
#include<tuple>
using namespace std;
template <typename T>
struct function_traits
: public function_traits<decltype(&T::operator())>
{};
template <typename ClassType, typename ReturnType, typename... Args>
struct function_traits<ReturnType(ClassType::*)(Args...) const>
// we specialize for pointers to member function
{
enum { arity = sizeof...(Args) };
// arity is the number of arguments.
typedef ReturnType result_type;
template <size_t i>
struct arg
{
typedef typename std::tuple_element<i, std::tuple<Args...>>::type type;
// the i-th argument is equivalent to the i-th tuple element of a tuple
// composed of those arguments.
};
};
// a place holder class
class database_bind {};
template<int N>
class A {
template<typename F>
static void run(F l);
};
template<>
struct A<1> {
template<typename F>
static void run(F l) {
typedef function_traits<decltype(l)> traits;
typedef typename traits::arg<0>::type type_1;
type_1 col_1;
get_from_db(0, col_1);
l(col_1);
}
};
template<>
struct A<2> {
template<typename F>
static void run(F l) {
typedef function_traits<decltype(l)> traits;
typedef typename traits::arg<0>::type type_1;
typedef typename traits::arg<1>::type type_2;
type_1 col_1;
type_2 col_2;
get_from_db(0, col_1);
get_from_db(1, col_2);
l(col_1, col_2);
}
};
void get_from_db(int col_inx, string& str) { str = "string"; }
void get_from_db(int col_inx, int& i) {i = 123;}
void get_from_db(int col_inx, double& d) { d = 123.456; }
template<typename F>
void operator>>(database_bind dbb, F l)
{
typedef function_traits<decltype(l)> traits;
A<traits::arity>::run(l);
}
And finally :
int main() {
database_bind dbb;
dbb >> [](int i, string s) { cout << i << ' ' << s << endl; };
dbb >> [](int i) { cout << i << endl; };
dbb >> [](string s,double d) { cout << s << ' ' << d << endl; };
}