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Clotters · Jul 20th, 2005, 12:01 AM

I'm steadily progressing through C++ and am looking at the moment at Pointers vs. References. I have a little question and turned here for it to be answered ...

(Toggle Plain Text)

 

#include <iostream>

using namespace std;

short Factor(int n, int* pSquared, int* pCubed);

int main()
{
	int number, squared, cubed;
	short error;

	cout << "Enter a number (0 - 20): ";
	cin >> number;

	error = Factor(number, &squared, &cubed);

	if (error)
	{
		cout << "Number: " << number << endl;
		cout << "Square: " << squared << endl;
		cout << "Cubed " << cubed << endl;
	}
	else
		cout << "Error encountered!!" << endl;
	return 0;
}

short Factor(int n, int *pSquared, int *pCubed)
{
	short Value = 1;
	if (n > 20)
		Value = 0;
	else
	{
		*pSquared = n*n;
		*pCubed = n*n*n;
		
	}
	return Value;
}

My problem arizes because I'm used to seeing things (such as pointers) clearly initialized, yet here ... I can't seem to see line that initializes the pointers that are used.

I can see they're mentioned in the prototype of Factor, but that doesn't initialize them as usable pointers that can be legitimately used from there on ... does it? Even so, I can't see a place where the pointers are tied to gether with the 'squared' and 'cubed' variable integers anyway.

I'd be pleased with a short, clear and concise explanation if possible.

EverLearning · Jul 20th, 2005, 12:12 AM

(Toggle Plain Text)

 
short Factor(int n, int* pSquared, int* pCubed);
int main()
{
...
	error = Factor(number, &squared, &cubed);
...
}

short Factor(int n, int *pSquared, int *pCubed)
{
	short Value = 1;
	if (n > 20)
		Value = 0;
	else
	{
		*pSquared = n*n;
		*pCubed = n*n*n;
		
	}
	return Value;
}

Quote:

My problem arizes because I'm used to seeing things (such as pointers) clearly initialized, yet here ... I can't seem to see line that initializes the pointers that are used.

the addresses of variables are passed, the variables already are declared and exist, they are integers.

Quote:

but that doesn't initialize them as usable pointers that can be legitimately used from there on ... does it?

what's happening is that whatever that address contains is being manipulated, which is why * is used, i.e. the contents of the address.

Quote:

Even so, I can't see a place where the pointers are tied to gether with the 'squared' and 'cubed' variable integers anyway.

in red.

ps: check out this thread

Clotters · Jul 20th, 2005, 7:13 PM

Thanks, for that. A new issue has arrived though.

You might want to bear in mind that this is the first time I'm being ontroduced to copy constructors.

(Toggle Plain Text)

// Passing Objects by Reference
#include &lt;iostream&gt;

using namespace std;
class SimpleCat
{
  public:
     SimpleCat ();           // constructor
     SimpleCat (SimpleCat&); //copy consructor (why have SimpleCat& inside the brackets? Does the & do anything)
     ~SimpleCat();           // destructor
};

SimpleCat::SimpleCat()
{   
   cout &lt;&lt; "Simple Cat Constructor..." &lt;&lt; endl;
}

SimpleCat::SimpleCat(SimpleCat&)
{
   cout &lt;&lt; "Simple Cat Copy Constructor.." &lt;&lt; endl;
}

SimpleCat::~SimpleCat()
{
  cout &lt;&lt; "Simple Cat Destructor.." &lt;&lt; endl;
}

SimpleCat FunctionOne (SimpleCat theCat);   //
SimpleCat* FunctionTwo (SimpleCat *theCat); // 

int main()
{
   cout &lt;&lt; "Making a cat..." &lt;&lt; endl;
   SimpleCat Frisky;
   cout &lt;&lt; "Calling FunctionOne..." &lt;&lt; endl;
   FunctionOne(Frisky);
   cout &lt;&lt; "Calling FunctionTwo..." &lt;&lt; endl;
   FunctionTwo(&Frisky);
   char response;
   cin &gt;&gt; response;
   return 0;
}

// FunctionOne passes by value
SimpleCat FunctionOne(SimpleCat theCat)
{
   cout &lt;&lt; "Function One. Returning..." &lt;&lt; endl;
   return theCat;
}

// FunctionTwo passes by reference
SimpleCat* FunctionTwo(SimpleCat *theCat)
{
   cout &lt;&lt; "Function Two. Returning..." &lt;&lt; endl;
   return theCat;
}

Now the output SHOULD, in theory, be ...

Making a cat ...
Simple Cat Constructor...
Calling Function One...
Simple Cat Copy Constructor..
FunctionOne. Returning...
Simple Cat Copy Constructor...
Simple Cat Destructor...
Calling Function Two...
Function Two. Returning...
Simple Cat Destructor...

Now, would someone please be kind enough tell me what is going on and the basic message of this example that I need to understand.

Thanks in advance.

DaWei · Jul 20th, 2005, 7:43 PM

A pointer is a reference in the dictionary sense, as is a nickname. In C++, the term "reference" was chosen (perhaps unwisely) to indicate an alias, and not just any old reference. A pointer is a separate variable which contains the address of a thing. I have a tutorial regarding pointers at the link in my signature. It does not cover references. A reference (C++ sense) is an alias, merely another name for an existing item. When you deal with a pointer, you deal with an item indirectly. When you deal with a reference, you deal with an item directly, but you call it "redneck" instead of "Jim Bob." When you copy an object you might get by with a "shallow" copy, which means it might contain pointers to other objects and you just copy those pointers. If the referred to object is deleted, all pointers to it become invalid. A non-shallow copy might make a copy of the objects and assign them new pointers. That way, if the original were destroyed, there would still be a valid object for the copy.

The use of the "&" has to be derived from context (in C++). It might be used as the "address of" operator and it might be used to assign an alias to an existing object. When you pass a reference to a function, the parameter is actually a pointer to the object, but when you pass a pointer you are actually creating a copy of a pointer. The effective references differ by one level of indirection. The compiler takes this into consideration so that you use one directly (syntactically speaking) inside the function and the other indirectly.

I'm not sure if this addresses your question. Post back if not. A pointer is a reference, but not a "C++ reference." A convict and his number are not the same, but "Bent Nose" Billy the Kid and William Bonney might very well be.

Clotters · Jul 20th, 2005, 7:50 PM

Thanks, that helps. One other thing I was worried about was that I know references can not be null. Some compilers might still compile it, but if a nulled reference is used, it will cause a problem.

My question is how do you create a null reference, so I know what they are.

Simple assigning a reference to a variable containing the null number (0) doesn't crash the system, like I was hoping.

This may seem odd, but my logic is if I know how to make a null referece, I'll know what they are .. and be able to stop them from occuring.

EverLearning · Jul 20th, 2005, 7:52 PM

Example you posted definitely does not demonstrate purpose and usefullness of a copy constructor.

tempest · Jul 20th, 2005, 7:53 PM

(Toggle Plain Text)

#include <iostream>

int main(int argc, char *argv[]) {
	int *pntr = NULL;
	*pntr = 82;
	std::cout << pntr << std::endl;
	return 0;
}

I think the code above will throw an error.

DaWei · Jul 20th, 2005, 8:02 PM

Tempest's example doesn't even contain a reference. A reference (C++) can't be NULL because you make a reference by saying "ThisName = &ThatName". If you assign zero to ThisName you just change the value of ThisName AND ThatName (the same bloke, you see), you don't create a null reference.

A null pointer is another horse. Whether or not a null pointer causes a problem depends upon the circumstance. See the last section of my tutorial for material on NULL pointers.

Clotters · Jul 20th, 2005, 9:54 PM

Thanks for that help.

I think I understand the fundamentals of pointers and references. At the very least I can interpet most pieces of code using them now.

I've been on them for a while, what I think I need to do is take a break. I'll finish the rest of my book, and then go over in detail the pointer and reference chapters again.

After a small break I think I'll be able to go back and tackle the tougher issues better than I am able to now, anyway.

Once again, thanks for the help.

Jul 20th, 2005, 12:01 AM	#1
Clotters Programmer Join Date: May 2005 Posts: 60 Rep Power: 4	Pointers to return multiple values I'm steadily progressing through C++ and am looking at the moment at Pointers vs. References. I have a little question and turned here for it to be answered ... (Toggle Plain Text) #include <iostream> using namespace std; short Factor(int n, int* pSquared, int* pCubed); int main() { int number, squared, cubed; short error; cout << "Enter a number (0 - 20): "; cin >> number; error = Factor(number, &squared, &cubed); if (error) { cout << "Number: " << number << endl; cout << "Square: " << squared << endl; cout << "Cubed " << cubed << endl; } else cout << "Error encountered!!" << endl; return 0; } short Factor(int n, int pSquared, int pCubed) { short Value = 1; if (n > 20) Value = 0; else { pSquared = nn; pCubed = nnn; } return Value; } #include <iostream> using namespace std; short Factor(int n, int pSquared, int* pCubed); int main() { int number, squared, cubed; short error; cout << "Enter a number (0 - 20): "; cin >> number; error = Factor(number, &squared, &cubed); if (error) { cout << "Number: " << number << endl; cout << "Square: " << squared << endl; cout << "Cubed " << cubed << endl; } else cout << "Error encountered!!" << endl; return 0; } short Factor(int n, int pSquared, int pCubed) { short Value = 1; if (n > 20) Value = 0; else { pSquared = nn; pCubed = nn*n; } return Value; } My problem arizes because I'm used to seeing things (such as pointers) clearly initialized, yet here ... I can't seem to see line that initializes the pointers that are used. I can see they're mentioned in the prototype of Factor, but that doesn't initialize them as usable pointers that can be legitimately used from there on ... does it? Even so, I can't see a place where the pointers are tied to gether with the 'squared' and 'cubed' variable integers anyway. I'd be pleased with a short, clear and concise explanation if possible.

Jul 20th, 2005, 7:13 PM	#3
Clotters Programmer Join Date: May 2005 Posts: 60 Rep Power: 4	Thanks, for that. A new issue has arrived though. You might want to bear in mind that this is the first time I'm being ontroduced to copy constructors. (Toggle Plain Text) // Passing Objects by Reference #include <iostream> using namespace std; class SimpleCat { public: SimpleCat (); // constructor SimpleCat (SimpleCat&); //copy consructor (why have SimpleCat& inside the brackets? Does the & do anything) ~SimpleCat(); // destructor }; SimpleCat::SimpleCat() { cout << "Simple Cat Constructor..." << endl; } SimpleCat::SimpleCat(SimpleCat&) { cout << "Simple Cat Copy Constructor.." << endl; } SimpleCat::~SimpleCat() { cout << "Simple Cat Destructor.." << endl; } SimpleCat FunctionOne (SimpleCat theCat); // SimpleCat* FunctionTwo (SimpleCat theCat); // int main() { cout << "Making a cat..." << endl; SimpleCat Frisky; cout << "Calling FunctionOne..." << endl; FunctionOne(Frisky); cout << "Calling FunctionTwo..." << endl; FunctionTwo(&Frisky); char response; cin >> response; return 0; } // FunctionOne passes by value SimpleCat FunctionOne(SimpleCat theCat) { cout << "Function One. Returning..." << endl; return theCat; } // FunctionTwo passes by reference SimpleCat FunctionTwo(SimpleCat theCat) { cout << "Function Two. Returning..." << endl; return theCat; } // Passing Objects by Reference #include <iostream> using namespace std; class SimpleCat { public: SimpleCat (); // constructor SimpleCat (SimpleCat&); //copy consructor (why have SimpleCat& inside the brackets? Does the & do anything) ~SimpleCat(); // destructor }; SimpleCat::SimpleCat() { cout << "Simple Cat Constructor..." << endl; } SimpleCat::SimpleCat(SimpleCat&) { cout << "Simple Cat Copy Constructor.." << endl; } SimpleCat::~SimpleCat() { cout << "Simple Cat Destructor.." << endl; } SimpleCat FunctionOne (SimpleCat theCat); // SimpleCat FunctionTwo (SimpleCat theCat); // int main() { cout << "Making a cat..." << endl; SimpleCat Frisky; cout << "Calling FunctionOne..." << endl; FunctionOne(Frisky); cout << "Calling FunctionTwo..." << endl; FunctionTwo(&Frisky); char response; cin >> response; return 0; } // FunctionOne passes by value SimpleCat FunctionOne(SimpleCat theCat) { cout << "Function One. Returning..." << endl; return theCat; } // FunctionTwo passes by reference SimpleCat FunctionTwo(SimpleCat *theCat) { cout << "Function Two. Returning..." << endl; return theCat; } Now the output SHOULD, in theory, be ... Making a cat ... Simple Cat Constructor... Calling Function One... Simple Cat Copy Constructor.. FunctionOne. Returning... Simple Cat Copy Constructor... Simple Cat Destructor... Calling Function Two... Function Two. Returning... Simple Cat Destructor... Now, would someone please be kind enough tell me what is going on and the basic message of this example that I need to understand. Thanks in advance.

Jul 20th, 2005, 7:43 PM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	A pointer is a reference in the dictionary sense, as is a nickname. In C++, the term "reference" was chosen (perhaps unwisely) to indicate an alias, and not just any old reference. A pointer is a separate variable which contains the address of a thing. I have a tutorial regarding pointers at the link in my signature. It does not cover references. A reference (C++ sense) is an alias, merely another name for an existing item. When you deal with a pointer, you deal with an item indirectly. When you deal with a reference, you deal with an item directly, but you call it "redneck" instead of "Jim Bob." When you copy an object you might get by with a "shallow" copy, which means it might contain pointers to other objects and you just copy those pointers. If the referred to object is deleted, all pointers to it become invalid. A non-shallow copy might make a copy of the objects and assign them new pointers. That way, if the original were destroyed, there would still be a valid object for the copy. The use of the "&" has to be derived from context (in C++). It might be used as the "address of" operator and it might be used to assign an alias to an existing object. When you pass a reference to a function, the parameter is actually a pointer to the object, but when you pass a pointer you are actually creating a copy of a pointer. The effective references differ by one level of indirection. The compiler takes this into consideration so that you use one directly (syntactically speaking) inside the function and the other indirectly. I'm not sure if this addresses your question. Post back if not. A pointer is a reference, but not a "C++ reference." A convict and his number are not the same, but "Bent Nose" Billy the Kid and William Bonney might very well be. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 20th, 2005, 7:52 PM	#6
EverLearning Hobbyist Programmer Join Date: May 2005 Location: Indiana Posts: 130 Rep Power: 4	Example you posted definitely does not demonstrate purpose and usefullness of a copy constructor. __________________ got math? yumm... "All men by nature desire to know" -Aristotle, Metaphysics

Jul 20th, 2005, 7:53 PM	#7
tempest Programming Guru Join Date: Oct 2004 Posts: 1,041 Rep Power: 5	(Toggle Plain Text) #include <iostream> int main(int argc, char argv[]) { int pntr = NULL; pntr = 82; std::cout << pntr << std::endl; return 0; } #include <iostream> int main(int argc, char argv[]) { int pntr = NULL; pntr = 82; std::cout << pntr << std::endl; return 0; } I think the code above will throw an error. __________________

Jul 20th, 2005, 7:50 PM	#5
Clotters Programmer Join Date: May 2005 Posts: 60 Rep Power: 4	Thanks, that helps. One other thing I was worried about was that I know references can not be null. Some compilers might still compile it, but if a nulled reference is used, it will cause a problem. My question is how do you create a null reference, so I know what they are. Simple assigning a reference to a variable containing the null number (0) doesn't crash the system, like I was hoping. This may seem odd, but my logic is if I know how to make a null referece, I'll know what they are .. and be able to stop them from occuring.

Jul 20th, 2005, 8:02 PM	#8
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Tempest's example doesn't even contain a reference. A reference (C++) can't be NULL because you make a reference by saying "ThisName = &ThatName". If you assign zero to ThisName you just change the value of ThisName AND ThatName (the same bloke, you see), you don't create a null reference. A null pointer is another horse. Whether or not a null pointer causes a problem depends upon the circumstance. See the last section of my tutorial for material on NULL pointers. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 20th, 2005, 9:54 PM	#9
Clotters Programmer Join Date: May 2005 Posts: 60 Rep Power: 4	Thanks for that help. I think I understand the fundamentals of pointers and references. At the very least I can interpet most pieces of code using them now. I've been on them for a while, what I think I need to do is take a break. I'll finish the rest of my book, and then go over in detail the pointer and reference chapters again. After a small break I think I'll be able to go back and tackle the tougher issues better than I am able to now, anyway. Once again, thanks for the help.

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Pointers to return multiple values