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.
Related
Using Valgrind, I see that I have a problem while deleting the memory in the following function:
Obj1 Obj1::operator*(const Obj1& param) const {
int n = param.GetSize(2);
Obj2** s = new Obj2*[n];
for( int i = 0; i < n; ++i) {
s[i] = new Obj2(*this*param.GetColumn(i+1));
}
Obj1 res = foo(s,n);
for(int i=n-1;i>-1;i--) {
s[i]->~Obj2();
}
delete[] s;
return res;
Valgrind tells me that the leak comes from the line
s[i] = new Obj2(*this*param.GetColumn(i+1));
I'm not pretty sure if the problem is when I try to free the memory. Can anyone tell me how to fix this problem?
Here:
s[i] = new Obj2(*this*param.GetColumn(i+1));
you create a dynamic object and assign s[i]to point to it.
In order to delete it, you do this:
delete s[i];
Unless you do that, the allocation will leak.
You must repeat that in a loop for every i just like you repeated the allocations. You of course have to do this before you delete s itself.
s[i]->~Obj2();
Don't do that. Calling the destructor is not appropriate here. delete will call the destructor.
P.S. Don't use raw owning pointers. Use containers or smart pointers instead. std::vector is a standard containers for dynamic arrays.
P.P.S. You should avoid unnecessary dynamic allocation. Your example doesn't demonstrate any need to allocate the pointed objects dynamically. So, in this case you should probably use std::vector<Obj2>.
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.
I create an array like as below
int size = 5;
double my_arr[m_size];
for (int i = 0; i < size; i++)
{
my_arr[i] = (rand()%10+1) + ((double) rand() / (RAND_MAX));;
}
after doing some calculation on array I want to delete the array. So I do this
for (int i = 0; i < size; i++)
{
delete my_arr[i];
}
and I get this error
error: type ‘double’ argument given to ‘delete’, expected pointer
I searched internet and all solutions are related to pointer array. But I am not using any pointer. So how can I delete this array?
The array will be automatically deleted when leaving the scope in which the variable has been declared.
If you really need to free memory fast you can try put your code between embraces:
{ //create new scope
int size = 5;
double my_arr[m_size];
for (int i = 0; i < size; i++)
{
my_arr[i] = (rand()%10+1) + ((double) rand() / (RAND_MAX));;
}
//some stuff
} //all non-pointer objects (or arrays) will be deleted
Or you can use pointers :
double *pMyarr = new double[m_size] ;
First of all, I think you should consider using a memory leak detector in your programms, so you can know by yourself if your code is leaking and if you have to do something about it. In your code you would have seen that there is no need to delete anything. :)
As far as I know, you should worry about memory in only two cases:
You have allocated a C-style array with malloc or calloc. In this case, you need to use the function free to deallocate the array. But generally in C++, you don't want this. You prefer using a container like std::array or std::vector instead.
You have created a new instance of an class with new and you have got a pointer to this instance. In this case, you have to use delete on the pointer when you don't need this instance anymore. But generally in C++11 (or further), you don't want this. If you really have to use a pointer, you prefer creating a smart pointer like std::unique_ptr or std::shared_ptr which will handle the memory for you.
When you define a variable without using new, malloc or calloc, (for example int a = 1 ;), it will be automatically deleted when it goes out of scope. So you don't need to worry.
Here is a simple example of a variable going out of scope:
int a = 1 ;
{
int b = 1 ;
// b is implictly deleted here, just before the bracket.
}
a++; // It works, because a exists in this scope
b++; // It doesn't work, because b is out of scope.
// a is implicitly deleted here, assuming we are at the end of a function
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.
I want to create an array that holds pointers to many object, but I don't know in advance the number of objects I'll hold, which means that I need to dynamically allocate memory for the array. I have thought of the next code:
ants = new *Ant[num_ants];
for (i=1;i<num_ants+1;i++)
{
ants[i-1] = new Ant();
}
where ants is defined as Ant **ants; and Ant is a class.
Will it work?
Will it work?
Yes.
However, if possible, you should use a vector:
#include <vector>
std::vector<Ant*> ants;
for (int i = 0; i < num_ants; ++i) {
ants.push_back(new Ant());
}
If you have to use a dynamically allocated array then I would prefer this syntax:
typedef Ant* AntPtr;
AntPtr * ants = new AntPtr[num_ants];
for (int i = 0; i < num_ants; ++i) {
ants[i] = new Ant();
}
But forget all that. The code still isn't any good since it requires manual memory management. To fix that you could to change your code to:
std::vector<std::unique_ptr<Ant>> ants;
for (auto i = 0; i != num_ants; ++i) {
ants.push_back(std::make_unique<Ant>());
}
And best of all would be simply this:
std::vector<Ant> ants(num_ants);
std::vector<Ant> ants(num_ants);
ants.resize(new_num_ants);
Yes that's the general idea. However, there are alternatives. Are you sure you need an array of pointers? An array of objects of class Ant may be sufficient. The you would only need to allocate the array:
Ant *ants = new Ant[num_ants];
In general, you should prefer using std::vector to using an array. A vector can grow as needed, and it will handle the memory management for you.
In the code you have posted, you would have to delete each element of ants in a loop, and then delete the array itself, delete [] ant. Keep in mind the difference between delete and delete [].
One more point, since array indices in C++ are 0-based, the following convention is used to iterate over the elements:
for (i=0; i<num_ants; i++)
{
ants[i] = new Ant();
}
This makes code much more readable.
Do you really need to hold pointers to the items? If you can use objects by value, a far simpler approach is to use a vector: std::vector<Ant> ants(num_ants);. Then not only do you not have to write looping, but you don't have to worry about memory leaks from raw pointers and other object management items.
If you need object pointers to say satisfy an API you can still use vector for the outer container and allocate the objects manually.
struct CreateAnt
{
Ant* operator()() const { return new Ant; }
};
std::vector<Ant*> ants(num_ants); // Create vector with null pointers.
std::generate(ants.begin(), ants.end(), CreateAnt());
std::vector<Ant*> ants( num_ants );
for ( int i = 0; i != num_ants; ++ i ) {
ants[i] = new Ant;
}
Or if you don't know how many in advance:
std::vector<Ant*> ants;
while ( moreAntsNeeded() ) {
ants.push_back( new Ant );
}
On the other hand, I think you need to ask yourself whether
Ant is an entity type or a value. If it's a value, you'll
probably want to skip the pointers and the dynamic allocation;
if it's an entity type, you'll have to consider the lifetime of
the object, and when and where it will be deleted.