How can I dereference a pointer created with New without copying?
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How can I dereference a pointer created with New without copying?
In the code snippet below I am creating an object via the new operator and assigning it to a pointer. What I would like to do is add the object that it points to to a vector. What seems to be happening is I am creating the object fine, then when I de-reference it, this creates a reference to the original object. So when I assign this reference to Object to the vector it creates a copy, whereas I would like the actual object. Any help would be really appreciated.
std::vector*Note:The Above Code is a very simplified version of the code I am trying to accomplish.
A note about the Code this interfaces with, This code is interacting with OpenGL and therefore OBJVec must contain Objects.
I have worked on this problem for quite some time, any help would be wonderful.
Answer by pinturic for How can I dereference a pointer created with New without copying?
I think this should be better (this way you are not copying anything):
std::vector OBJVec; Object *O = new Object(); addToVec(O); void addToVec(Object* OParam){ OBJVec.push_back(OParam); } Answer by EfiBN for How can I dereference a pointer created with New without copying?
You'll need to pass a pointer to that pointer
Try this:
addToVec(**O); Answer by WhiZTiM for How can I dereference a pointer created with New without copying?
The behavior is right
This is the signature of std::vector::push_back
void push_back( const T& value ); void push_back( T&& value ); when you dereference your pointer, you now have a reference to the original object. When you pass an lvalue reference to push_back the first overload is picked.
In your second case, its also an lvalue, so the first overload is still picked... You can't dodge copying on push_back except you pass an rvalue or explicitly std::move it
If you want pointers, then let your container be that of pointers. If you want reference, then let your container be of reference_wrapper to your object
But, you have extra memory management work to do if you do those route
Answer by leemes for How can I dereference a pointer created with New without copying?
Since std::vector handles its own memory allocation (as per default)1, you can't allocate an element outside of it and then make vector point to it, unless you make the vector's value_type a pointer by itself, that is std::vector, and pass the pointer itself to addToVec.
std::vector OBJVec; // Make a vector of pointers instead of objects Object *O = new Object(); addToVec(O); // Do not dereference here void addToVec(Object *OParam) { // Use a pointer here as the argument OBJVec.push_back(OParam); } However, then you would need to deallocate the elements once you remove them out of the vector and are not using them anymore. To simplify this, you could instead use a vector of unique_ptrs, that are smart pointers which deallocate the object for you once not needed anymore.
If you need a vector of pointers, you can't apply this workaround. Then you can't use new for every single object outside of the vector. This would necessarily lead to the objects being allocated separatedly, not ensuring contiguous memory layout, as required by any OpenGL function taking an array. You would need to allocate an array with new Object[count] instead, or simply drop the idea of manual memory altogether and just use a vector of objects without allocating the elements outside of it.
1) You could change that behavior, but it would still be impossible to allocate every element by itself outside of the vector, like you currently do.
Answer by Marinos K for How can I dereference a pointer created with New without copying?
In C++98 there is no notion for moving so you will have to use a vector of pointers instead and copy in there a reference to you original object. In C++11 you do have move semantics so that you can literally move an object using std::move. I'm not sure if this is a good idea in your case, however.
In the first place using naked new is not considered a good practice these days. You should use some smart pointer or your own RAII wrapper. In your case I'd simply use std::unique_pointer (C++11 - or boost::unique_pointer) and then simply a std::vector where I could safely move the pointer (you can't copy a std::unique_pointer). There is no real overhead in this approach so if you need heap allocation and unique ownership this is probably the way to go.
If you don't need heap allocation and you're in c++11, then you don't really need to use pointers in the first place. Just create your object and move it inside the vector whenever you want - make sure, however, that you never use the original object again!
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