C++ :: Unordered Map With Class Data

I'm trying to make an unordered_map with a key of std::string and a data type of my own class, "Variable". This is my code that I've done:

#include <iostream>
#include <sstream>
#include <string>
#include <unordered_map>
class Variable { // The class that I want to have as data type
public:
Variable(double value) : _value(value) {}

[Code] .....

This code works just fine when I use double instead of "Variable", but the moment I put in the class "Variable" instead it gives my this massive error:

(cut for clarity)main.cpp|25|error: conversion from 'std::_Hashtable<std::basic_string<char>, std::pair<const std::basic_string<char>, Variable>, std::allocator<std::pair<const std::basic_string<char>, Variable> >, std::_Select1st<std::pair<const std::basic_string<char>, Variable> >, std::equal_to<std::basic_string<char> >, std::hash<std::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, true, false, true>::_Insert_Return_Type {aka std::pair<std::__detail|