c++ unordered map using templates - templates

Question

I am trying to declare an unordered map using templates in c++. However, as I expect the object to be one of the primitive datatypes I don't want to declare a custom class for only one object.
I've tried using :
template <class T> std::unordered_map<int, T> storedObj;
but I keep getting the error: ‘storedObj’ was not declared in this scope
a code snippet is below
#include<iostream>
#include<unordered_map>
#include<deque>
std::deque<int> freeIds;
template <class T> std::unordered_map<int, T> storedObj;
unsigned static int objIdCount=0;
const unsigned long int MAXID = 1000000000;
Can you please tell me what's wrong? Thanks.

Answer 1

You are doing partial template specialisation here (see http://en.cppreference.com/w/cpp/language/partial_specialization). Probably what you had in mind is a typedef-like construction but that doesn't work with templates.
With partial template specialisation you can (partially) re-implement or re-define a templated type.
One way to achieve your goal could be:
template <class T>
class my_unordered_map :
public std::unordered_map<int, T>
{
};
int main( void ) {
my_unordered_map<float> mf;
return 0;
}

Answer 2

Templates are used for compile-time polymorphism. You cannot instantiate a template-based class without specifying an actual datatype for that template.
A basic way to use templates would be:
#include <iostream>
using namespace std;
template <class T>
class MyClass {
T foo;
public:
MyClass(T bar) {
foo = bar;
}
void print() {
cout << foo;
}
};
int main() {
MyClass<int> c(5);
c.print(); // prints 5
return 0;
}
You can achieve what you're trying to do by extending (deriving) the unordered_map class, like this:
#include <iostream>
#include <unordered_map>
using namespace std;
template <class T>
class MyClass: public unordered_map<int, T> {
// optional extra code here
};
int main() {
MyClass<string> cstring;
cstring[0] = "foo";
cout << cstring[0] << "\n"; // prints "foo"
MyClass<double> cdouble;
cdouble[0] = 3.14; // prints 3.14
cout << cdouble[0];
return 0;
}