C++ template Creating a Library

#include <iostream> 
#include <cmath> 

using namespace std; 

namespace MyNamespace 
{ 

template<typename T> 
T Area(T height, T length) /*  w  ww. j a  va 2 s  .  c o  m*/
{ 
    return height * length; 
} 

const double PI = 4.0*atan(1.0); 

template<typename T> 
T CircleArea(T radius) 
{ 
    double result; 

    result = PI * radius * radius; 

    // This version truncates the value. 
    return (T)result; 
} 

template<typename T> 
T TriangleArea(T base, T height) 
{ 
    double result; 

    result = base * height * 0.5; 

    return (T)result; 
} 

} 

#include <iostream>

using namespace std;
using namespace MyNamespace;

int main()
{
    cout << "4 X 4 Areas:" << endl;
    cout << "Square: " << Area<int>(4, 4) << endl;
    cout << "Circle: " << CircleArea<int>(2) << endl;
    cout << "Triangle: " << TriangleArea<int>(4, 4) << endl;
    cout << "Using a value of pi of: " << PI << endl;
    return 0;
}