How does this generate algorithm work?

[SirSkorpan]

Hi,
I'm fairly new to c++ I'm at the end of my first course in object oriented c++ programming. In one of my assignments I want to fill a vector<int> with numbers from 1 - 16. I found that the generate algorithm presented here: http://www.cplusplus.com/reference/algorithm/generate/ serves my purpose well. The problem is I can't really wrap my head around the example code on the page I linked to and I don't really feel comfortable with using code that I don't understand.

The code I use to fill my vector is this:

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

// class generator:
struct c_unique {
  int current;
  c_unique() {current=0;}
  int operator()() {return ++current;}
} UniqueNumber;


int main () {
  
  vector<int> myvector (16);

  generate (myvector.begin(), myvector.end(), UniqueNumber);

  cout << "\nmyvector contains:";
  for (unsigned int i = 0; i < myvector.size(); i++)
    cout << " " << myvector.at(i);

  cout << endl;
 
  return 0;
}

I think there are two problems here I'm not really sure what is done with the struct UniqueNumber, and then there is the generate() function that I'm not too sure about.

I realize that I could use a loop where I fill my vector, which would probably work just as well and be easier for me to understand. But I would really like to understand this example.

Thanks in advance!
/SirSkorpan

[guestgulkan]

The generate function takes 3 parameters.
The first two are iterators for the particular container (the first one is a start iterator and the second one is the ending iterator).

The third parameter (in this case it is an object of struct c_unique) is what is called a function object (also known as a functor).

In basic terms, a function object is something that can be called using the function calling syntax.
Two things spring to mind:

1. A function. (this one is obvious)
2. An object of a class which has an overloaded function call operator () .

This second method is the the one used in the above example.

Here is the generate function template definition from MSVC 2008 header files:

		// TEMPLATE FUNCTION generate
template<class _FwdIt,
	class _Fn0> inline
	void _Generate(_FwdIt _First, _FwdIt _Last, _Fn0 _Func)
	{	// replace [_First, _Last) with _Func()
	_DEBUG_RANGE(_First, _Last);
	_DEBUG_POINTER(_Func);
	for (; _First != _Last; ++_First)
		*_First = _Func();
	}

I have underlined the bits to do with the function object.
As you can see - *_First =_Func() - the function object is 'called' using the function call syntax.



With regard to your example.
I have added a function to do the same thing as the struct - andas you can see, they both give the same output.

// class generator: ------- Function Object
struct c_unique {
  int current;
  c_unique() {current=0;}
  int operator()() {return ++current;}
} UniqueNumber;


//Class generator  -   Function
int generate_number()
{
    static int num=1;

    return num++;
}



int main()
{

    
  vector<int> myvector (16);

  
  //Using function object
  
  generate (myvector.begin(), myvector.end(), UniqueNumber);

  cout << "\nmyvector contains:";
  for (unsigned int i = 0; i < myvector.size(); i++)
    cout << " " << myvector.at(i);

  cout << endl;
 
    
 //Using a function
  generate (myvector.begin(), myvector.end(), generate_number);

  cout << "\nmyvector contains:";
  for (unsigned int i = 0; i < myvector.size(); i++)
    cout << " " << myvector.at(i);

  cout << endl;
}
 
    

For further information - look up the following topics.

1. Operator overloading
2. Function objects.

[SirSkorpan]

Thank you very much guestgulkan, with your response and a little further reading in my book is very clear! Much appreciated!