template specialization

In some cases, it isn’t possible or desirable for a template to define exactly the same code for any type. In such cases you can define a specialization of the template for that particular type. When a user instantiates the template with that type, the compiler uses the specialization to generate the class, and for all other types, the compiler chooses the more general template. 
* Specializations in which all parameters are specialized are complete specializations. 
* If only some of the parameters are specialized, it is called a partial specialization.
```
#include <string>
#include <iostream>
using namespace std;

template <typename K, typename V> class MyMap{
public:    
    MyMap() { cout<<"KV"<<endl;}
};

// partial specialization for string keys
template<typename V> class MyMap<string, V>{
public:    
    MyMap() { cout<<"V"<<endl;}
};

int main() {
    MyMap<int, string> strings1; // uses original template
    MyMap<string, string> strings2{}; // uses the partial specialization
}
```
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