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C++ class member functions when using an array of classes

Posted on 2013-12-10
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Hi,

Let's say I create an array of a dog class. That dog class has member functions, such as eat, sleep etc. Those functions are the same for every instance in the array. To me that seems like a huge waste of memory. How can this be improved?

dog dogs[1000];

dogs[0].eat();
dogs[0].sleep();

It seems like a pointer to the eat and sleep functions would be better, but I'm coming from a C background so I would rather ask the experts rather than reinvent the wheel.

Thanks,
Sean
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Question by:sean-keys
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by:imladris
imladris earned 100 total points
ID: 39709332
Even though there are a thousand objects there aren't actually a thousand copies of the code. The code exists in a central place. What there will be a thousand of is the data and various necessary pointers to keep things separate.
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by:sarabande
sarabande earned 100 total points
ID: 39709348
the member functions are defined once for the class. there is no repetition of the code when an array or multiple instances of the class were defined.

when a c++ member function was called it is exactly the same as when a c function was called (as long as it is not a virtual call). the 'this' pointer of the object the member function was associated with, was passed as a (hidden) argument to the function.

Sara
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by:
jkr earned 300 total points
ID: 39709352
>>Those functions are the same for every instance in the array. To me that seems like a huge
>>waste of memory. How can this be improved? [...]
>>It seems like a pointer to the eat and sleep functions would be better, but I'm coming from
>>a C background so I would rather ask the experts rather than reinvent the wheel.

Well, these member functions exist in memory only once, not on a per-object basis. Each of these members is passed a hidden 'this' pointer as their 1st argument, so they know which object they are invoked for.

See also http://www.learncpp.com/cpp-tutorial/87-the-hidden-this-pointer/ ("The hidden “this” pointer") which is a quite thourough explanation of the above.
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Author Closing Comment

by:sean-keys
ID: 39709473
Makes sense. Thanks.
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