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First , let us see the follow two functions :

int max(int nX, int nY)
{
    return (nX > nY) ? nX : nY;
}

double max(double dX, double dY)
{
    return (dX > dY) ? dX : dY;
}

these two functions are almost the same , beside the parameter type and return type.
although function overloading can help us deal with the problem which the parameter type is different , 
but it can not deal with different return type.
So , that is why we need function template.

In C++, function templates are functions that serve as a pattern for creating other similar functions.
first , we need to make a function template declaration :

template <typename Type> // this is the template parameter declaration
Type max(Type tX, Type tY)
{
    return (tX > tY) ? tX : tY;
}

then we can start using the function template.

int nValue = max(3, 7); // returns 7
double dValue = max(6.34, 18.523); // returns 18.523
char chValue = max('a', '6'); // returns 'a'

ref : http://www.learncpp.com/cpp-tutorial/141-function-templates/

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