I have a structure called scene. Within the structure called scene, I need to make an array of other scene objects. Is this possible?
asked Dec 3, 2011 at 18:27
4 Answers
No, because before scene is completely defined, the compiler doesn't know how big it is, and doesn't know what size to make the array.
However, you can have an array of pointers to scene, because pointers (not counting pointers to members and other oddities - thanks Nawaz) are all the same size:
class scene {
scene* array[20];
};
Alternatively, you can store a pointer that will point to a dynamic array allocated with new[] and deallocated with delete[]:
class scene {
scene() : array(new scene[20]) { }
~scene() { delete[] array; }
scene* array;
};
or even more alternatively, store a vector<scene>, a vector of scenes:
class scene {
vector<scene> array;
};
and with vector, you get a resizable array with no manual memory management.
answered Dec 3, 2011 at 18:30
7 Comments
Michael Krelin - hacker
Don't know, I've just upvoted two answers, one starting with "yes", the other with "no". I feel stupid, but both are correct :)
Seth Carnegie
@MichaelKrelin-hacker I just took the word "array" more literally than Nawaz did :)
Sarfaraz Nawaz
@SethCarnegie:
because pointers are all the same size. Not all pointers are necessarily of same size. He can use pointers, not because "pointers are all the same size.", rarher because the compiler already knows the size (whatever it is) of pointer of particular type.Seth Carnegie
@Nawaz wow really? I thought that for any type
T, sizeof(T*) == C where C is a constant integer. How does the compiler know how big the pointer will be to that particular type if it's not defined yet? Or what would make one pointer bigger and one smaller than another? And if that's true, you can't cast any pointer to any other type of pointer without possible loss of information, right?
Kerrek SB
@MichaelKrelin-hacker: Because the "yes" answers all say "yes, you can do that, if by 'that' you mean something entirely different, which is in fact possible".
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Yes. You can do that. But you've to declare the member as pointer as:
struct scene
{
//other members
scene *children; //this is what you need.
//you cannot make it like : scene children[100];
};
Then create the dynamic array as:
scene parent;
parent.chidren = new scene[100]; //100 children!
Just remember that you've to allocate and deallocate the memory yourself.
Alternatively, you can use std::vector<scene*>, or boost::ptr_vector<scene>.
answered Dec 3, 2011 at 18:30
Comments
You can do it if you use std::vector. This is from some code I wrote yesterday:
#include <vector>
struct ChangeList // Tree of changes in a tree of values
{
int index ;
std::vector<ChangeList> Changes ;
} ;
Comments
Sure it is possible.
Pseudocode:
struct Scene {
int id;
Scene* scenes;
};
PS. you could easily test this - do not be so lazy. ;)
struct X { X a };?