C++ Templates

What are templates?

• Templates provide a mechanism to handle different data types in the same function or class.
• When used in functions they are referred as function templates.
• When used with classes they are referred as class templates.
• Template arguments are specified either with keywords "class" or "typename".
• Main advantages of templates are that they make the code listing smaller and maintainence burden is reduced since changes are done at one place only.

EXAMPLE: Demonstrate the usage of function templates.

#include <iostream>
using namespace std;

// Template function
template <typename T>
T max ( T a, T b ) {
    return a<b?b:a;
}

// Overloaded
template <typename T>
T max ( T a, T b, T c ) {
    return max(max(a,b),c);
}

// Args of different types
template <typename T, typename S>
void func ( T a, S b ) {
    cout << a << " " << b << endl;
}

void main()
{
    // Call to 2 arg template function
    cout << max(1,2) << endl;
    cout << max<int>(1,2) << endl;
    cout << max(1.1,2.1) << endl;
    cout << max<double>(1,2.1) << endl;
    cout << max(static_cast<double>(1),2.1) << endl;
    cout << max("hello", "world") << endl;

    /* Compilation error when different types are used
      cout << max(1, 1.2) << endl;
      Error E2285 template.cpp 26: Could not find a match for 'max<_T>(int,double)' in function main()
     */

    // Call to overloaded template function
    cout << max(1,2,3) << endl;

    // Call to template function with different arg types
    func(1, 1.2);
}

OUTPUT:
2
2
2.1
2.1
2.1
world
3
1 1.2

EXAMPLE: Demonstrate the usage of class templates.

#include <iostream>
#include <vector>
#include <stdexcept>
using namespace std;

template <typename T>
class MyClass {
    vector<T> data;

  public:
    void Add(T& a) {
        data.push_back(a);
    }

    T Get() {
        if ( data.empty() )
        {
      throw out_of_range("Out of range");
        }
        return data.back();
    }
};

void main()
{
    // Class used with int
    int val1 = 100, val2 = 200;
    MyClass<int> intObj;
    intObj.Add(val1);
    intObj.Add(val2);
    cout << intObj.Get() << endl;

    // Class used with string
    string str1 = "Hello", str2 = "World";
    MyClass<string> strObj;
    strObj.Add(str1);
    strObj.Add(str2);
    cout << strObj.Get() << endl;
}

OUTPUT:
200
World

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