How to use std::bind with std::function and std::map - c++11

Question

I am trying to create a map of std::function and then trying to bind it few parameters, but it is giving an error.
So, my std::function definition is
using abc = std::function<double(const double& t1, const double& t2)>;
and the map is
std::map<std::pair<std::string, std::string>, abc> conversion_;
The way I am trying to insert in this map is
conversion_.emplace(
std::make_pair("a", "b"),
std::bind(conversion, 3, std::placeholders::_1));
conversion_.find(std::make_pair("a", "b"))->second(4); -- Access
I have defined the function conversion, but when I am trying to access the function it is giving the below error
error: no match for call to ‘(const std::function<double(const double&, const double&)>) (int)’
Adding full code:
conversion function is defined in a separate file :
namespace x
{
double conversion(const double& a, const double& b);
}
Header file where I am trying to define map
namespace x
{
class main
{
public:
using abc = std::function<double(const double& t1, const double& t2)>;
main();
private:
std::map<std::pair<std::string, std::string>, abc> conversion_;
};
}
The CPP file
namespace x
{
main::main()
{
conversion_.emplace(
std::make_pair("a", "b"),
std::bind(conversion, 3, std::placeholders::_1));
auto m = conversion_.find(std::make_pair("a", "b"))->second(4);
}
}

Answer 1

I guess conversion is function as double(const double& t1, const double& t2). If so, std::bind(conversion, 3, std::placeholders::_1) define a functor which requires one argument a double (the other argument of conversion is fixed as 3) and return double. You just need modify abc into std::function<double(const double& t1)>;