C++ Templet qustion....

[chaturchand]

Can you please help me with following c++ Templet question ?
But when you try to use the two together:

the compiler gives a lot of rather indecipherable error messages, telling you:
could not deduce template argument for 'const class td::basic_string<_E,_Tr,_A>' from 'class Ingredient'
and so on.
What's the problem? How would can i fix it?

// add.h    
template <class TYPE>  
TYPE add(TYPE &a, TYPE &b)  
{   
  return a + b;  
}  


// ingredient.h    
#include <string>  
using namespace std;    

class Ingredient  

{  
public:   
  Ingredient(double amount, string unit)    
   : m_amount(amount), m_unit(unit)   
  {}     

private:   
  double m_amount;   
  string m_unit;  
};  

#include <iostream>  
using namespace std;  
#include "ingredient.h"  
#include "add.h"    

int main()  
{   
  Ingredient oil(3, "gallons");   
  Ingredient water(2, "gallons");   

  Ingredient result = add(oil, water);      
  return 0;  
}  

[chaturchand]

Please help me..

[jsmith]

You need to overload operator+ for Ingredient.

class Ingredient {
  friend Ingredient operator+( Ingredient const& i1, Ingredient const& i2 ) {
      // Add them together and return result;
  }
};

Otherwise the compiler does not know how to perform a + b in add() with
TYPE=Ingredient.

[chaturchand]

JSmith.....
This strategy doesnt work....try to plug in code....It keep on erroring..

[jsmith]

The following code compiles fine for me using
g++ (GCC) 4.1.1 20070105 (Red Hat 4.1.1-51)

#include <iostream>

using namespace std;

template <class TYPE>
TYPE add(TYPE &a, TYPE &b)
{
  return a + b;
}

class Ingredient

{
public:
  Ingredient(double amount, string unit)
   : m_amount(amount), m_unit(unit)
  {}

  // Function I added:
  friend Ingredient operator+( const Ingredient&, const Ingredient& ) {}

private:
  double m_amount;
  string m_unit;
};

int main()
{
  Ingredient oil(3, "gallons");
  Ingredient water(2, "gallons");

  Ingredient result = add(oil, water);
  return 0;
}