Say I want to declare an array of pointers to a specific object like
Object *objects = new Object[size];
and then add some new objects to my dynamically allocated array
for (int i = 0; i < size; i++){
Object* obj = new obj;
objects[i] = *obj;
}
is it enough to deallocate all memory by just calling delete[] on the array, or do I have to loop through the array first and call delete on each object? Or is this stupid to do in practice?
You always have to delete everything you new in some way. That means, you would need to delete obj in every iteration to avoid leaks. Note that you never really store the Object obj points to, but a copy of it.
The way you do it right know is quite unusual and not very handy anyway: The loop you showed does nothing useful since the new Objects[size] already default constructed your Objects. In particular, it does not add any elements, they are already there. You could just leave that out. If you want to change the content of your array, e.g. do more initialization, your loop would usually look more like this
for (int i = 0; i < size; i++){
objects[i] = newValue;
}
Remember, the Objects are already there after new Objects[size]!
In practice, you would be much better of with a
std::vector<Object> objects(size);
(or maybe, if the above does not fit your usecase, a
std::vector<std::unique_ptr<Object>> objects(size);
, but I feel like that is unlikely to be the case). For a good overview of what a std::vector can do and how to use it, read the documentation. Probably most importantly for you: You can index it just like an array.
First, you have to follow the deallocation is reverse order of allocation.
So number of times new that many times delete , similarly number of times new [] that many times delete []
// Assuming objects as array of pointers [ Object **objects ]
for (int i = 0; i < size; i++)
{
delete objects[i] ;
}
delete [] objects;
And your correct allocation should go as:
Object **objects= new Object*[size];
for (int i = 0; i < size; i++) {
objects[i] = new Object;
}
Also , you should use smart pointers std::unique_ptr for hassle free memory management
std::vector< std::unique_ptr<Object> > objects ;
You are storing an array of Objects and not pointers to Object. This means that they won't be deleted when you delete the array and the memory will leak.
If you want to store pointers to Objects in your array then you need to declare it as Object **objects = new Object *[size]; and then you will have to delete all the pointers in the array.
So the better code will look like:
//Allocation:
Object **objects = new Object *[size];
for (int i = 0; i < size; i++){
objects[i] = new Object;
}
//Deletion
for (int i = 0; i < size; i++){
delete objects[i];
}
delete [] objects;
An even better solution would be to use vectors and smart pointers and then you don't need the deletion code at all.
//Allocation:
vector<unique_ptr<Object>> objects;
for (int i = 0; i < size; i++){
objects.push_back(make_unique(new Object));
}
//Deletion
//Hey where did all the code go.
Related
I think it may probably an assignment error of shared_ptr. I wrote some code regarding my error about shared_ptr of vector containing int pointer. The error occurs the 2-th loop of j-loop. Please let me know what's the mistake in the code. And I wonder whether 'delete vec.get()' is correct for free the memory of the vector.
int i,j;
shared_ptr<vector<int*>> vec = NULL;
for (j = 0; j < 2; j++)
{
vec = shared_ptr<vector<int*>>(new vector<int*>());
for (i = 0; i < 5; i++)
{
int* ia = new int[10];
vec->push_back(ia);
}
delete vec.get();
}
From what you are showing, there is no need for the smart pointer or the raw pointers. The following code has the same effect as what you seem to intend, except that it also properly initializes all the int elements to zero, which your current code does not do:
vector<vector<int>> vec;
for (int j = 0; j < 2; j++)
{
vec = {5, vector<int>(10)};
// Do something with vec
}
The concrete problem with your current code is that you are trying to delete the raw pointer owned by the shared_ptr. The shared_ptr will delete the owned pointer when its own lifetime ends and no other shared_ptr instance referring to the raw pointer exists anymore. That is its purpose.
If you want to delete the int array you allocated for the int* pointers in the vector, then you need to decide which of the pointers at which index you want to delete:
delete[] (*vec)[index];
vec is the shared_ptr, *vec is a reference to the owned vector<int*>, (*vec)[index] is a reference to the int* stored in the vector<int*> at index index. You need to use delete[] instead of delete, because you allocated with the array form of new.
Given the way your code is structured, you would need to call delete[] for each index of the vector once to avoid any memory leak. Since doing that manually before the vector is destroyed violates the RAII principle, one would use std::unique_ptr for the inner int* instead of raw new. That being said, I already mentioned above that I don't see any reason for pointers of any kind at all.
Below code should work for you:
for (int j = 0; j < 2; j++)
{
auto vec = std::make_shared<vector<int*>>();
for (int i = 0; i < 5; i++)
{
int* ia = new int[10];
vec->push_back(ia);
}
for (int i = 0; i < 5; i++)
{
delete[] vec.get()->at(i);
}
}
I do not understand requirement of this type of strange code. If you are using smart pointer, Why Share_ptr instead unique_ptr? why to call delete? Why you allocate the memory to int* when can be handled by vector of integer. Think on it.
No need to declare and then allocate the memory.
Hope this will help you.
When we call
delete[] array; // array is a pointer to an array
or
delete[] vector; // vector is a pointer to a vector
Assuming these are array/vector of linked list head pointers: Do these 2 statements call the destructors for every item in them? Or should we loop through them, delete each item and then delete[] the array or vector itself?
Let me be more specific,
If the array holds a head pointer with its destructor is called it deletes its next node or that it points to.
Your second code snippet is incorrect. delete and delete[] are two different operators. delete is for delete an instance created by new and delete[] is for delete an array of elements created by new[].
In C++, arrays and vectors does not care about what they store. For both array and vector, you need to delete each item (if they were allocated using new) before deleting the array and the vector.
If you create the array like this
Sample* arr[100];
for (int i = 0; i < 100; ++i) {
arr[i] = new Sample();
}
you need to delete them like this
for (int i = 0; i < 100; ++i) {
delete arr[i];
}
If you create the vector like this
vector<Sample*> arr;
for (int i = 0; i < 100; ++i) {
arr.push_back(new Sample());
}
you need to delete like this
for (int i = 0; i < 100; ++i) {
delete arr[i];
}
Deleting an array of pointers delete[] array is suitable, but for an vector of pointers i would use a construct like this:
std::vector<Data*> m_data;
while(!m_data.empty()) delete m_data.back(), m_data.pop_back();
Calling delete[] on a std::vector is not correct C++. The vector is destroyed when it goes out of scope or when you delete a pointer to it if allocated on the heap with new. In both cases, however, destroying the array/vector does not call delete on the pointers it contains - you need to iterate through and do this yourself if you need to free that memory.
I have an array of pointers (that I created by calling new ptr*[size]). All of these pointers point to an object that was also put on the heap.
What is the proper way to delete the array and all new'd ptr's?
This is what I do now:
for (int i = 0; i < size; i++) delete array[i];
delete[] array; // Not sure since this double deletes array[0]
Does this do what I think it should?
Thanks
Every pointer allocated with new gets a corresponding delete. Every pointer allocated with new [] gets a corresponding delete []. That's really all you need to know. Of course, when you have a dynamically allocated array which contains dynamically allocated pointers the deallocation must occur in reverse order.
So it follows that the correct idiom would be...
int main()
{
int **container = new int*[n];
for(int i = 0; i < n; ++i)
container[i] = new int[size];
// ... and to deallocate...
for(int i = 0; i < n; ++i)
delete [] container[i];
delete [] container;
}
And then of course I say "stop doing that" and recommend you use a std::array or std::vector (and the template type would be unique_ptr<int>).
Yes, that does what you think it should. Since you did new for each element, you have to delete each element. And since you did new[] for the entire array, you need to delete[] the entire array.
As #djechlin rightly says in the comments, there's not really enough information to go on, but I'm presuming your prior code is something like this:
int** array = new int*[5];
for (int i = 0; i < 5; i++) {
array[i] = new int;
}
Note that array is not actually an array type. It is a "pointer to pointer to int" and the array of pointers it points to was allocated with new[]. That's why you need to delete[] it.
Yes. First you have to free the object each pointer in the array points to, then you have to free the array itself. In that order. If you reverse the order you'll have no reference to the objects and will leak a lot of memory.
Yes, first you delete each object to which elements of array point, and then you delete array of pointers itself. If you want to check your memory management, you can use tools like valgrind, they will be able to spot most errors.
I am creating a program that uses an array of objects declared with
Element * elements = new Element[number];
where an element is a class that has/needs a its own destructor.
when I go to delete this would I use just use array delete, and have the program worry about calling the destructor:
delete [] elements;
or do I call the destructor for each item explicitly with keyword delete:
for(int ii = 0; ii< ArraySize; ii++)
delete elements[ii];
delete [] elements;
Note: I understand that I could probably use something like boost::ptr_vector, but I wanted similar to hashTable functionality (so the for loop would need additional information, but that is outside of the direct scope of this question) which is why I am using a traditional array. I would still like to know which behavior is needed to avoid memory leaks.
The first one. You should
delete [] elements;
The second one is incorrect and should give you errors if you try to compile it.
Yes, delete [] elements; should be sufficient.
You'd use your second piece of code with something like:
Element **elements;
elements = new Element *[rows];
for (int i=0; i<rows; i++)
elements[i] = new Element;
This combination rarely makes much sense though. To make at least some sense, each row would itself be a dynamically allocated array as well:
elements = new Element *[rows];
for (int i=0; i<rows; i++)
elements[i] = new Element[row_len];
In this case, your deletion would look something like:
for (int i=0; i<rows; i++)
delete [] elements[i];
delete [] elements;
As you're doing it right now, however, none of what you've said really justifies home-rolled dynamic allocation at all. std::vector<Element> elements(number); would work perfectly well.
In C++, Lets say I'm creating an array of pointers and each element should point to a data type MyType. I want to fill this array in a function fillArPtr(MyType *arPtr[]). Lets also say I can create MyType objects with a function createObject(int x). It works the following way:
MyType *arptr[10]; // Before there was a mistake, it was written: "int *arptr[10]"
void fillArPtr(MyType *arptr[])
{
for (int i = 0; i < 10; i++)
{
MyType myObject = createObject(i);
arptr[i] = new MyType(myobject);
}
}
Is it the best way to do it? In this program how should I use delete to delete objects created by "new" (or should I use delete at all?)
Since you asked "What is the best way", let me go out on a limb here and suggest a more C++-like alternative. Since your createObject is already returning objects by value, the following should work:
#include <vector>
std::vector<MyType> fillArray()
{
std::vector<MyType> res;
for (size_t i = 0; i != 10; ++i)
res.push_back(createObject(i));
return res;
}
Now you don't need to do any memory management at all, as allocation and clean-up is done by the vector class. Use it like this:
std::vector<MyType> myArr = fillArray();
someOtherFunction(myArr[2]); // etc.
someLegacyFunction(&myArr[4]); // suppose it's "void someLegacyFunction(MyType*)"
Do say if you have a genuine requirement for manual memory management and for pointers, though, but preferably with a usage example.
Your method places the array of pointers on the stack, which is fine. Just thought I'd point out that it's also possible to store your array of pointers on the heap like so. Youd do this if you want your array to persist beyond the current scope
MyType **arptr = new MyType[10];
void fillArPtr(MyType *arptr[])
{
for (int i = 0; i < 10; i++)
{
MyType myObject = createObject(i);
arptr[i] = new MyType(myobject);
}
}
If you do this, don't forget to delete the array itself from the heap
for ( int i = 0 ; i < 10 ; i++ ) {
delete arptr[i];
}
delete [] arptr;
If you're going to use vector, and you know the size of the array beforehand, you should pre-size the array. You'll get much better performance.
vector<MyType*> arr(10);
for (int i = 0; i < 10; i++)
{
delete arptr[i];
arptr[i] = 0;
}
I suggest you look into boost shared_ptr (also in TR1 library)
Much better already:
std::vector<MyType*> vec;
for (int i=0; i<10; i++)
vec.push_back(new MyType(createObject(i));
// do stuff
// cleanup:
while (!vec.empty())
{
delete (vec.back());
vec.pop_back();
}
Shooting for the stars:
typedef boost::shared_ptr<MyType> ptr_t;
std::vector<ptr_t> vec;
for (int i=0; i<10; i++)
vec.push_back(ptr_t(new MyType(createObject(i)));
You would basically go through each element of the array and call delete on it, then set the element to 0 or null.
for (int i = 0; i < 10; i++)
{
delete arptr[i];
arptr[i] = 0;
}
Another way to do this is with an std::vector.
Use an array of auto_ptrs if you don't have to return the array anywhere. As long as you don't make copies of the auto_ptrs, they won't change ownership and they will deallocate their resources upon exiting of the function since its RAII based. It's also part of the standard already, so don't need boost to use it :) They're not useful in most places but this sounds like a good one.
You can delete the allocated objects using delete objPtr. In your case,
for (int i = 0; i < 10; i++)
{
delete arptr[i];
arptr[i] = 0;
}
The rule of thumb to remember is, if you allocate an object using new, you should delete it. If you allocate an array of objects using new[N], then you must delete[] it.
Instead of sticking pointers into a raw array, have a look at std::array or std::vector. If you also use a smart pointer, like std::unique_ptr to hold the objects within an std::array you don't need to worry about deleting them.
typedef std::array<std::unique_ptr<MyType>, 10> MyTypeArray;
MyTypeArray arptr;
for( MyTypeArray::iterator it = arptr.begin(), int i = 0; it != arptr.end(); ++it ) {
it->reset( new MyType( createObject(i++) ) );
}
You don't need to worry about deleting those when you're done using them.
Is the createObject(int x) function using new to create objects and returning a pointer to this?. In that case, you need to delete that as well because in this statement
new MyType( createObject(i++) )
you're making a copy of the object returned by createObject, but the original is then leaked. If you change createObject also to return an std::unique_ptr<MyType> instead of a raw pointer, you can prevent the leak.
If createObject is creating objects on the stack and returning them by value, the above should work correctly.
If createObject is not using new to create objects, but is creating them on the stack and returning pointers to these, your program is not going to work as you want it to, because the stack object will be destroyed when createObject exits.