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.
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>.
What's the correct way to free the memory here?
The pointer-array contains duplicates!
class HashTable
{
Bucket<E>** index = new Bucket<E>*[indexSize];
...
}
~ExtHash( )
{
for (size_t i = 0; i < indexSize; ++i)
delete index[i];
delete[] index;
}
I would think hard about whether you want this container to be responsible for deleting the objects; it would be simpler to store them elsewhere, and just use this container to refer to them, not to manage their lifetimes.
Alternatively, you could use std::shared_ptr to manage the objects; then they will be deleted automatically when you've discarded all of them.
If you really want to do it this way, you'll need to remove the duplicates after deleting each one; something like
for (size_t i = 0; i < indexSize; ++i) {
Bucket<E> * victim = index[i];
indexSize = std::remove(index+i+1, index+indexSize, victim) - index;
delete victim;
}
[NOTE: this code may well be wrong; I certainly made a couple of mistakes writing it. If you really want to manage dynamic objects the hard way, then you'll need to test it thoroughly]
Sort index, remove adjacent duplicates with std::unique. O(N log N) and that's as good as it gets.
Use a set to remove duplicates:
std::set<Bucket*> uniqueBuckets(index, index + indexSize);
for(Bucket* bucket: uniqueBuckets)
delete bucket;
In your situation it's dangerous to store raw pointers in such way. Better use std::shared_ptr and just reset shared pointers. And after all pointers will be reseted they will be safely freed.
Perhaps like this:
~ExtHash( )
{
std::sort(index, index + indexSize);
size_t new_end = std::unique(index, index + indexSize) - index;
for (size_t i = 0; i < new_end; ++i)
delete index[i];
delete[] index;
}
Each time you create a pointer, push_back it into a vector. That vector will hold all your created pointers, and will hold each one of them only once.
Later, iterate that vector and delete the pointers.
It's like writing your own simple garbage collector.
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.
I am writing a C++ class that uses some fixed arrays, as well as some dynamically allocated arrays.
I was wondering if anybody can guide me for the proper way to allocate memory for the dynamic arrays , probably in the constructor/deconstructor, and also if I need to explicitly call them to make sure I don't get a seg fault.
Here is a simplified version of the related part of my code:
class Network {
public:
int n_nodes;
int user_index[MAX_USERS]; //a fixed array
int adjacency_matrix[][MAX_ITEMS];
//Network(int n_node, int** adjacency); //I would rather to set the element s in a function other than the constructor
Initializer(int n_node, int** adjacency);
~Netowrk();
}
So here are my specific question for this class:
1 - Can I have the 2D array adjacency_matrix[][] with undecided number of rows and columns until it's set by the user in the initializer function?
2 - where should I delete the 2D array? should I write it in the deconstructor? Should I call the deconstructor explicitly? Is there anything else I need to destroy in the deconstructor?
1 - Can I have the 2D array adjacency_matrix[][] with undecided number of rows and columns until it's set by the user in the initializer function?
Yes. The best way to do this, however, is not to use arrays at all. Instead, use std::vector, which manages the memory for you. There are two ways that you can do this. If you actually want to be able to use the [row][column] syntax to access elements, you'll need to use two dimensions of std::vectors:
std::vector<std::vector<int> > adjacency_matrix;
Once you know the dimensions, you can populate it:
adjacency_matrix.assign(rows, std::vector<int>(columns));
It is often easier to use a single-dimensional array (or a std::vector<int>) containing all of the elements and use row * row_count + column to access the element at index (row, column). This way, there are fewer dynamic allocations. You can wrap up the logic of accessing elements into a couple of helper functions.
2 - where should I delete the 2D array? should I write it in the deconstructor?
You don't have to delete anything if you use a std::vector. It cleans itself up.
Should I call the [destructor] explicitly?
No.
Is there anything else I need to destroy in the [destructor]?
Ideally, no. If you use the Standard Library containers, like std::vector and smart pointers, you shouldn't have to clean anything up. You should avoid trying to manage resources on your own in C++: there are library facilities to do this tedious task for you and you should take advantage of them.
1 - Can I have the 2D array adjacency_matrix[][] with undecided number of rows and columns until it's set by the user in the initializer function?
Yes you can. For example:
int* adjacency_matrix_;
int* getAdjacency(int i, int j)
{
if (!adjacency_matrix_)
return 0;
else
return adjacency_matrix_ + i*n_nodes + j;
}
Network()
: n_nodes(0),
adjacency_matrix_(0)
{}
void Initializer(int n_node, int** adjacency)
{
adjacency_matrix_ = new int[n_nodes * n_nodes];
// Copy over data.
}
As to whether you should, that depends on whether you have a reason for not using std::vector<>.
2 - where should I delete the 2D array? should I write it in the deconstructor?
Should I call the deconstructor explicitly?
Is there anything else I need to destroy in the deconstructor?
Yes, definitely free in the destructor using array operator delete:
~Network()
{
delete [] adjacency_matrix_;
}
No, your destructor will be called whenever the Network object itself goes out of scope. It is (very) rarely necessary to make an explicit destructor call.
No, all a destructor needs to explicitly release is whatever your explicitly acquire.
You may like the example matrix class I wrote in an answer to another question
The question itself was about good C++ design practices, but the chosen example was a multi-dimensional array.
There are several ways to do this.
The easiest way is to use vectors, and if you don't like to manage your own memory, this is perfect for you. However, because I like to manage my own memory, and I have found this method to be slow and cumbersome at times, I have learned of other ways.
The fastest way is to allocated a one dimensional array and treat it as you would a two dimensional array. Here is an example:
int *array = new int[width*height];
int get_array(int column, int row)
{
return array[row*width + column];
}
delete [] array;
This can be generalized to the nth-dimension:
int *array = new int[w1*w2*...*wn];
int get_array(int i1, int i2, ..., int in)
{
return array[in*(w1*w2*...*w(n-1)) + i(n-1)*(w1*w2*...*w(n-2)) + ... + i2*w1 + i1];
}
delete [] array;
If you want to be able to have different widths for each row, then you can make an array of pointers. This solution is slow to initialize and clean up, but flexible, tunable, and has relatively fast execution time. It can also be extremely dangerous if you make a mistake though.
int **array = new int*[height];
for (int i = 0; i < height; i++)
array[i] = new int[width(i)];
at which point, to access it, all you have to do is the customary
array[i][j]
however, to free this array you have to do it row by row
for (int i = 0; i < height; i++)
delete [] array[i];
delete [] array;
This can also generalize to the nth dimension.
int **....*array = new int**...*[w1];
for (int i1 = 0; i1 < w1; i1++)
{
array[i1] = new int**..*[w2];
for (int i2 = 0; i2 < w2; i2++)
{
array[i1][i2] = new int**.*[w3];
...
for (int in = 0; in < wn; in++)
array[i1][i2]...[in] = new int[wn];
}
}
for (int i1 = 0; i1 < w1; i1++)
{
for (int i2 = 0; i2 < w2; i2++)
{
...
for (int in = 0; in < wn; in++)
delete [] array[i1][i2]...[in];
...
delete [] array[i1][i2];
}
delete [] array[i1];
}
delete [] array;
This kind of setup tends to wreak havoc on memory. Just a two dimensional array of these would result in width+1 separate arrays to be malloc-ed. It would be faster to just malloc one big array and figure out the indices yourself.
I'm building a hashmap class that can have string keys and ints, bools, strings or pointers of different types as its values, and I want it to work. For the program I'm using it for I create the pointer and pass it into the hashmap. The problem comes when I need to destruct the map. If the type for the hashmap is a pointer I need to delete it(the value) before I delete it's container.
so the code I have right now goes something like this:
I have a hashNode** drawers, which I use as a two dimensional array to hold pointer to hashNodes in the map. Those same pointers are also held in another hashNode** array, which stores them as they are added to map (for ease/speed of growing and copying the hashmap).
template <typename V>
class str_map {
public:
// ...
virtual ~str_map() {
str_map<V>::~str_map();
}
// ....
};
and then later I have a bunch of methods like these:
one for regular values:
template <>
str_map<int>::~str_map() {
for(int i=0; i < count && array[i] != NULL; i++){
delete array[i];
}
delete array;
delete drawers;
}
and one for pointers:
template <>
str_map<str_map<int>*>::~str_map() {
for(int i=0; i < count && array[i]->val() != NULL; i++)
delete array[i]->val();
for(int i=0; i < count && array[i] != NULL; i++){
delete array[i];
}
delete array;
delete drawers;
}
Is there another better way to deconstruct an instance of str_map class correctly so that all the memory is handled correctly? Or at least a way to make this work?
Your container should handle values. That's it, no more, no less. If someone wants to stick pointers in, let them. Don't take ownership of whatever they may or may not be pointing at.
It's up to the users of your hash map to know how to manage the lifetime of their memory. They should be using smart pointers, so your class just copies them around and the smart pointer manages the memory.
The guideline is manage one resource, or none at all. If you are managing more than one resource, you've set yourself up for failure.
I suspect delete array should be delete [] array;. What this means is you really should be using std::vector. Again, either manage one resource or none at all. std::vector manages one resource, so you don't have to. And so on.