Storing pointer to functions in a map - c++

Question

Can anyone explain why this piece of code is generating the error as shown below. I am trying to store pointer to function in a map. The code would work fine if I keep everything in main() function. Also I would appreciate it if someone shows how to fix it.
#include <boost/variant.hpp>
#include <map>
#include <iostream>
#include <map>
using namespace boost;
class obj {
public:
int num1;
int num2;
std::string s1;
std::string s2;
};
typedef boost::variant<int, float, double, std::string> MultiType;
typedef MultiType(*FnPtr)(obj);
class c {
public:
MultiType add(obj o);
std::map<std::string, FnPtr> metricToFuncMap = { { "i", add } };
};
MultiType c::add(obj o) {
{ MultiType x; x = o.num1 + o.num2; return x; }
}
int main() {
obj o;
//add
MultiType d;
d = 1;
o.num1 = 1;
o.num2 = 2;
//concat
o.s1 = "hello";
o.s2 = "world";
c c;
MultiType x = c.metricToFuncMap["i"](o);
std::cout << get<int>(x);
return 0;
}
Error:
E0289 no instance of constructor "std::map<_Kty, _Ty, _Pr, _Alloc>::map [with _Kty=std::string, _Ty=FnPtr, _Pr=std::less<std::string>, _Alloc=std::allocator<std::pair<const std::string, FnPtr>>]" matches the argument list

Answer 1

Here's a fairly minimal example.
#include <iostream>
#include <functional>
#include <map>
class Foo {
public:
Foo() {}
};
typedef std::function<std::string (const Foo &)> MyFunction;
typedef std::map<std::string, MyFunction> MyMap;
int main(int, char **) {
Foo foo;
MyFunction f = [](const Foo &) { return "Hello"; };
MyMap myMap { {"x", f} };
MyFunction blah = myMap["x"];
std::cout << "Got: " << blah(foo) << "\n";
}
Compile and run:
$ g++ --std=c++17 Blah.cpp -o Blah && Blah
Got: Hello
The advantage of using std::function is that while there's a little overhead, it's far more powerful. You can (as I have) used lambdas. Your lambdas can capture this, meaning you can call non-static member methods. You can also use std::bind(), if you prefer, although I don't care for the syntax and will usually use a lambda instead.