[C++] Classes and dynamic memory

Ephracis · 12-25-2006, 06:46 PM

I have a class (MyClass) in which I have an array of N elements of type MyItem.
MyClass has a public method called MyClass::addItem() which would allocate memory and create a new instance of MyItem. I use the private variable numPos to keep track of how many instances of MyItem I have within the array.

psuedo code:

Code:

class MyItem {
  ...
};
class MyClass {
  private:
    int numPos;
    MyItem *items;
  public:
    void addItem();
};

MyClass::addItem() {
  items[numPos] = new MyItem();
}

I hope you understood the above, cause here comes the question:
the function addItem allocates memory when called. Will this code do it or will I have to call malloc too?

Regards,

tuxdev · 12-25-2006, 07:27 PM

This is wrong for C++. This is even wrong for C. Use std::vector instead of arrays. push_back() will do what you want.

graemef · 12-25-2006, 08:20 PM

Just to clarify what tuxdev has said, memory is being allocated in two different ways. First the allocation of memory for your new object myItem which is achieved by the new keyword. The other is the storage of the memory location of the new object into the member variable item. item has been declared as a pointer to a MyItem object and in addItem you are using it as an array of a pointer to MyItem objects.

How many elements are in this array? Answer one.
How many do you want? Unknown, so you require a data structure that will dynamically resize an array.

So as tuxdev said the use of std::vector will solve your problem because it will dynamically allocate sufficient storage to hold the details that you require.

Ephracis · 12-25-2006, 08:37 PM

Thanks for your fast replies. I will check std::vector out right away.

Ephracis · 12-25-2006, 11:39 PM

std::vector did the trick, although I tried to use it in another case, where I have a globally declared instance of a class and I tried to declare a bunch of those using vector:

std::vector<MyClass> my_instance;
my_instance.push_back(MyClass);

but it results in the error:

error: expected constructor, destructor, or type conversion before ‘.’ token

Although, it works if I put the declaration and push_back in a function, but I need it to be globally. :/

Any ideas?

nadroj · 12-26-2006, 01:08 AM

the declaration should work globally but not the push_back call. this will need to be inside another function.

Ephracis · 12-28-2006, 07:10 PM

Is there a way I can modify a certain index in a vector? I have a file that I parse and I want to insert number from a row at a certain position, but the problem is that the file may specify the position 10 before 1.

So I figured that I should use vector::reserve() since it will grow the vector if it's needed. But then, how will I set vector::at(N) to a value? Can I just do something like vector::at(N) = MyClass?

nadroj · 12-28-2006, 07:32 PM

have you tested this? looking at the documentation (here http://www.cppreference.com/cppvector/at.html), it says it returns a reference (pointer) to the object at that index. with that reference you should be able to reassign it to something else of the same type. sorry i havent done any programming in months and almost forget it!

Ephracis · 12-29-2006, 12:49 PM

Yes I have tested. And I end up with

551: error: expected primary-expression before ‘;’ token

This error message was brought to you by:

Code:

550: animations.reserve(st+1);
551: animations.at(st) = MyClass;

nadroj · 12-29-2006, 01:11 PM

dont really know sorry. is MyClass an object? does it help if you put & in front of MyClass?

Ephracis · 12-29-2006, 01:51 PM

Quote:

Originally Posted by nadroj

dont really know sorry. is MyClass an object? does it help if you put & in front of MyClass?

My class is a class, and I want a vector of instances.

dmail · 12-29-2006, 03:20 PM

Have you overloaded the equal operator? otherwise you are trying to reset a reference which is not allowed. There are many more questions I could ask but I think it maybe better to see your code.

tuxdev · 12-29-2006, 03:51 PM

dmail: Operator = is provided by the compiler if you don't declare it.

If you want an instance, you have to use MyClass(), not just MyClass. I don't think a vector is really supposed to be used that way. It sounds more like how a map is used.

dmail · 12-29-2006, 04:43 PM

tuxdev I can't believe I actually wrote that. Every day I learn n new things and forget n^2, I think it's a age thing.

tuxdev · 12-29-2006, 04:45 PM

As long as n=1, you're safe. :-)