Header file creation?

[Vish0203]

Hey!
i've been trying to create a header file using dev C++ but i'm stuck with sum errors. Can anyone help me out in this?

#ifndef SUM_H
#define SUM_H
template <class T>
class add
{
    private: 
        const T a,b;
    public:
        T sum(T,T);
        void displaysum();  
};
template <class T>
T add::sum(T x,T y)
{
    a=x;b=y;
    return a+b;
}

template <class T>
void add::displaysum()
{
    cout<<endl<<"sum is "<<right<<a+b;
}

#endif

I get an error on compiling..

[Error] expected '=', ',', ';', 'asm' or '__attribute__' before '<' token

[deceptikon]

It looks okay aside from the class name not being fully qualified in your definitions. They should look like this (notice the template argument is included on the class name):

template <class T>
T add<T>::sum(T x,T y)
{
    a=x;b=y;
    return a+b;
}
template <class T>
void add<T>::displaysum()
{
    cout<<endl<<"sum is "<<right<<a+b;
}

[Vish0203]

ohkay! i made it correct now. but, still there is some linking problem.

#ifndef SUM_H
#define SUM_H
#include "iostream"
using namespace std;
template <class T>
class add
{
  private:
    const T a,b;
  public:
    T sum(T,T);
    void displaysum();
};
template <class T>
T add<T>::sum(T x,T y)
{
  a=x;b=y;
  return a+b;
}
template <class T>
void add<T>::displaysum()
{
  cout<<endl<<"sum is "<<right<<a+b;
}
#endif

i tried to make a program including sum.h

#include<iostream>
#include "sum.h"
using namespace std;
int main()
{
    sum(5,4);
    displaysum();
    cin.get();
}

here are the errors i get.

[Error] 'sum' was not declared in this scope 
[Error] 'displaysum' was not declared in this scope 

[deceptikon]

sum() and displaysum() are both member functions. You need an add object.

[Vish0203]

yeah i had a thought of it.. But that doesn't happen with other header files isn't it?? We directly use the functions?

[deceptikon]

But that doesn't happen with other header files isn't it??

It has nothing to do with header files, the rules of C++ don't change just because you break your program down into different files. In fact, headers are nothing more than a glorified cut and paste. When you say #include "sum.h", the entire contents of the sum.h header file are textually pasted over that directive. So this:

#include<iostream>
#include "sum.h"
using namespace std;
int main()
{
    sum(5,4);
    displaysum();
    cin.get();
}

Becomes this:

// The entire contents of iostream go here
// The entire contents of sum.h go here
using namespace std;
int main()
{
    sum(5,4);
    displaysum();
    cin.get();
}

We directly use the functions?

You can directly use a member function without qualification inside the class definition because this is implied. So this:

class Foo {
    void Bar() { ... }
    void Baz()
    {
        Bar();
    }
};

Is equivalent to this:

class Foo {
    void Bar() { ... }
    void Baz()
    {
        this->Bar();
    }
};

Outside of a class definition, no such implicit object exists and the compiler will assume that an unqualified function call refers to a non-member function.

[Vish0203]

oh.. so that's what actually happpens!! I was thinking that I'm creating a header.. foolish me!! :P
thanks! I'll checkout for this pointer!

[deceptikon]

I was thinking that I'm creating a header..

Well, you are creating a header. It's just that a "header" in C++ doesn't really mean very much. It's just a file with C++ code (usually declarations) that can be easily and conveniently reused with the #include preprocessor directive.