Thread: 'new' keyword

  1. #1
    Registered User
    Join Date
    Nov 2004

    'new' keyword

    I feel bound to explain first off that my entire knowledge of the use of the 'new' keyword comes from Java

    everytime I come to a place in my c++ apps where in java I would have used the word new, I try to put new and my compiler yells and screams at me

    how do I use the new keyword in c++?

  2. #2
    Slave MadCow257's Avatar
    Join Date
    Jan 2005
    show some code of an attempt to use new or something

    or read the tutorial,
    Last edited by MadCow257; 02-28-2005 at 06:32 PM.

  3. #3
    Registered User
    Join Date
    Sep 2004
    new is used to Dynamicly allocate memory. Unlike java not every thing is created this way. you can just do
    MyClass var1 = MyClass(constructorarg);
    Now what the new keyword does is dynamicly allocate memory for an object on the heap, and then return the ADDRESS to that location. Now what you need to do if you want to do this, is use a pointer (something you didn't deal with dirrectly in Java) here is an example

    MyClass *var1 = new MyClass(constructorarg);
    Notice the "*" in there, that specifies that its a pointer, which simply holds the address to the location. if you wish to use the object you have allocated, you need to dereference the pointer, using the *, or their are some other operators you can use to do this as well. Read up on pointers and dynamic memory allocation and you should be able to find this information and more.

    OH and don't forget to "delete" the variable when your done, becuase if you allocate more memory, and use that pointer again without using delete first, you get what is some times called a memory leak, nastly little problem that can really screw you up.
    If any part of my post is incorrect, please correct me.

    This post is not guarantied to be correct, and is not to be taken as a matter of fact, but of opinion or a guess, unless otherwise noted.

  4. #4
    Registered User
    Join Date
    Apr 2003
    If you use 'new', then you have to have a pointer variable on the left side of the equals sign. Also, in C++ when you use 'new' to create an object, the object stays in scope until it is deleted, which is much different than a normal object, which goes out of scope as soon as the block it was declared in ends.

  5. #5
    Registered User
    Join Date
    Apr 2003
    Try running this code:
    using namespace std;
    class Apple
    	int num1;
    	int num2;
    	int num3;
    int main()
    	Apple A;
    	Apple* p = &A;
    	cout<<sizeof a<<endl;
    	cout<<sizeof p<<endl;
    	return 0;
    On my computer that outputs 12 and 4. That shows that the variable A is an object in memory that takes up 12 bytes(4 bytes per int variable). On the other hand p is a variable in memory that stores an address of A, which only requires 4 bytes. So, they are different.

    I think that references to objects in Java are like pointers to objects in C++. In Java, a statement like this:

    Apple A = new Apple();

    creates the variable A and 'new' returns a 'reference' to a newly created object in memory. If there was a way to look at the sizeof A in Java, I think it would be something small like 4 bytes--not 12 bytes. The reason you have 'references' or 'pointers' is so that you can pass something that is 4 bytes back and forth between functions rather than something that could be 1000's of bytes in size for a large array or large object.
    Last edited by 7stud; 02-28-2005 at 09:37 PM.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. keyword replacement
    By JackBear in forum C Programming
    Replies: 11
    Last Post: 05-18-2007, 10:54 PM
  2. Virtual Keyword
    By Shal in forum C++ Programming
    Replies: 6
    Last Post: 05-18-2006, 11:37 AM
  3. extern keyword
    By trekker in forum C Programming
    Replies: 7
    Last Post: 06-01-2005, 01:31 PM
  4. how to search a keyword in C?
    By kreyes in forum C Programming
    Replies: 2
    Last Post: 03-01-2002, 08:22 PM
  5. keyword searching in C
    By Unregistered in forum C Programming
    Replies: 2
    Last Post: 02-08-2002, 02:26 AM