C++ :Pointers, I need an explanation.

[JumBie]

I have run into another mind bogglement.

// pointers to structures 
#include <iostream> 
#include <string> 
#include <sstream> 
using namespace std;  

struct movies_t {
   string title;  
   int year; 
};  

int main () 
{   
string mystr;   
 
movies_t amovie;   
movies_t * pmovie;   
pmovie = &amovie;    

cout << "Enter title: ";   
getline (cin, pmovie->title);   
cout << "Enter year: ";   
getline (cin, mystr);   
(stringstream) mystr >> pmovie->year;    

cout << "\nYou have entered:\n";   
cout << pmovie->title;   
cout << " (" << pmovie->year << ")\n";    
return 0; 
}

Now the pointer is confusing me again...

pmovie->title

(*pmovie).title

*pmovie.title

*(pmovie.title)

What do each of these mean in simple terms?

 

[mv2devnull]

1. Access member of an object via a pointer.

2. Access an object by dereferencing a pointer and then access the member of the object. Same as 1.

3. Syntax error in you example. Member access has higher precendence than the dereference.

4. Same as 3, so an error. If pmovie is an object, then access the member, and dereference (assuming 'title' can be dereferenced).

 

[soccerballtux]

this helpful http://en.wikipedia.org/wiki/Operators_in_C_and_C++

 

[rameshms]

Jumbee, look for operator precedence in the link soccerballtux gave you..
http://en.wikipedia.org/wiki/Operators_in_C_and_C++#Operator_precedence

Per the precedence, *pmovie.title is same as *(pmovie.title)

 

[JumBie]

Oh wow, thanks soccerballtux, and rameshms. That link has a TON of great information which I can use as a reference as my C++ book does not have an appendix. I think it would be a good idea to read more about the theory as well as the order of operations, I think that will make things a LOT easier!<br>

 

[JumBie]

1. Access member of an object via a pointer.

2. Access an object by dereferencing a pointer and then access the member of the object. Same as 1.

3. Syntax error in you example. Member access has higher precendence than the dereference.

4. Same as 3, so an error. If pmovie is an object, then access the member, and dereference (assuming 'title' can be dereferenced).

When we say "member" are we talking about the value of the object?
And dereferencing implies that we access the value as opposed to the address, am i correct?

or...

Object pmovie and title is a member of that object? I'm just confused when we say "access an object by dereferencing a pointer"

 

[JumBie]

New question.

int main()
{
int x= 6;
int r=4;

cout << "int x before: " << x << "\n";
cout << "int r before: " << r << "\n";

swap(x,r);

cout << "int x after: " << x << "\n";
cout << "int r after: " << r << "\n";


  return 0;
}
void swap (int& x,int& r)
{
    int temp;
    temp=x; // hold x temporarily
    x=r; // assign r to x
    r=temp;//assign the value of x to r

    return ;
}

why do I need to use a reference to swap the two variables. Can't this function just swap them with out a reference.

 

[LevelSea]

New question.

int main()
{
int x= 6;
int r=4;

cout << "int x before: " << x << "\n";
cout << "int r before: " << r << "\n";

swap(x,r);

cout << "int x after: " << x << "\n";
cout << "int r after: " << r << "\n";


  return 0;
}
void swap (int& x,int& r)
{
    int temp;
    temp=x; // hold x temporarily
    x=r; // assign r to x
    r=temp;//assign the value of x to r

    return ;
}

why do I need to use a reference to swap the two variables. Can't this function just swap them with out a reference.

Because if you pass it as a normal variable, it's copied. That's called passing by value. If you want to change the values passed by parameter, you need to pass by reference, hence the reference variables.

Check this, about half way down in the section "Arguments passed by value and by reference". http://www.cplusplus.com/doc/tutorial/functions/


For even more fun with pointers, check out Duff's Device. http://en.wikipedia.org/wiki/Duff's_device

 

[JumBie]

C++
Screen *screen = new Screen();
ScreenLabel *Label = new ScreenLabel ("Hello, Polycode!")", 32);
screen ->addChild(label);

LUA

screen= Screen()
label= ScreenLabel ("Hello, Polycode!", 32)
screen:addChild(label)

LOL wow....i hate c++ 🙁.

 

[mv2devnull]

http://thbecker.net/articles/rvalue_references/section_04.html

As we all know, the First Amendment to the C++ Standard states: "The committee shall make no rule that prevents C++ programmers from shooting themselves in the foot." Speaking less facetiously, when it comes to choosing between giving programmers more control and saving them from their own carelessness, C++ tends to err on the side of giving more control.

A man is happy when he has the illusion of being in control.

 

[JumBie]

http://thbecker.net/articles/rvalue_references/section_04.html

A man is happy when he has the illusion of being in control.

Good quote lol.

Should i be learning c++11 standards?

 

[soccerballtux]

Good quote lol.

Should i be learning c++11 standards?

no for myself but I'm not sure.

 

[mv2devnull]

C++11 is the current standard. http://en.wikipedia.org/wiki/C++#Standardization

 

[Cogman]

Good quote lol.

Should i be learning c++11 standards?

I see no reason why not. C++11 adds a boatload of features over C++03.

Just realize that there is a good chance that an employer won't be using C++11 yet. (however, for personal projects and new projects, there is no good reason not to use C++11)