3

i have the following struct:

struct Message {
    Agent *_agent;
    double _val;
};

and the following Ptrs array:

typedef Message* MessageP;
MessageP *_msgArr;
_msgArr = new MessageP[MAX_MESSAGES];

this is the method that inserts a Message to the array:

void Timing::AddMessage(Agent * const agentPtr, double val) {

    MessageP msgPtr = new Message;
    assert(msgPtr != 0);

    //assign values:
    (*msgPtr)._agent = agentPtr;
    (*msgPtr)._val = val;

    //add to messages array:
    assert(_msgArr != 0 && _waitingMsgs<MAX_MESSAGES);
    _msgArr[_waitingMsgs] = msgPtr;
    _waitingMsgs++;

}

My question is about the deletion of this array. I would like to delete the array and all allocated structs. if i write: 

delete [] _msgArr

will this delete also each allocated struct or will free only the allocated memory for the array?

Is the correct way is to go over the entire array with a for loop and write 

delete _msgArr[i]

and at last wite delete [] _msgArr to delete the allocated array ?

thanks!

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14
  • 4
    Have you considered using one of the standard library containers like a std::vector or std::deque? Commented Sep 9, 2010 at 15:53
  • 1
    James has a point - the standard arrays are very useful (and often more resource efficient, if a little slower at times) - but as per the answer below, you still need to delete objects pointed to in the std containers. Commented Sep 9, 2010 at 15:57
  • 1
    Then the course is wrong. It should cover the STL and not pointers or C arrays. Commented Sep 9, 2010 at 16:01
  • 2
    @rob: So the course is teaching it backwards. It should first teach the simple, high-level stuff, and only later teach how this works under the hood. The way you're taught now you first learn to do it manually, and later learn to not to do it manually, but use library facilities instead. Commented Sep 9, 2010 at 16:01
  • 3
    @sbi: I have never heard of a CS course being taught in any other way, and that has always been a source of deep concern for me. People should know how things work, yes, but they should know that there are other solutions. I have encountered a number of long-time "C++" programmers with little to no knowledge of the STL. Not to mention that many C++ teachers are really C teachers in thin disguise, I think this is a serious problem in the industry. Commented Sep 9, 2010 at 16:19

4 Answers 4

Reset to default
4

Yes, you need to loop over all elements and delete them manually before calling delete[] on the array.

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4 Comments

So from what i understand, delete [] _msgArr doesn't call delete on each cell. what is the meaning of delete _msgArr without the []?
i mean, does it also deletes the allocated array?
@rob: If _msgArr points to an array created by new T[N] (the array form of new), then delete _msgArr yields undefined behavior. You must delete it using the array form of delete.
It calls the destructor on each cell, but since the cells are pointers and not objects there is no destructor action. Look into using a smart pointer type for array elements if you want auto deletion of the allocated structs.
2

The delete [] will call the destructors on the struct pointers, which doesn't dispose of the structs or the _agent members, which itself points to memory. You could call delete _msgArr[i]._agent and then delete _msgArr[i] in a loop, which will dispose of the Agent and then the Message.

First, though, you need to know who should get rid of the Agents, and when. If these are owned by another data structure, you shouldn't get rid of them when getting rid of _msgArr, and looping over _delete _msgArr[i] followed by delete [] _msgarr; is all you need.

If you do need to delete the Agents also, you have three reasonable choices.

First, you can give Message a destructor that will delete its _agent. It should also have a copy constructor and assignment operator defined then, either to pass ownership or to copy, or else define them as private so any attempt to use them will be a compile-time error.

Second, you could change the Agent * to a smart pointer, so that the extra memory will be deleted when the Message goes away.

Third, you could go through the loop I suggested above when getting rid of the array.

Unless you have good reasons to keep the code C-compatible, I'd suggest that you use a std::vector<boost::shared_ptr<Message> >, and have Message contain a boost::shared_ptr<Agent> rather than an Agent * (if you don't have to dispose of the Agents, Agent * is fine). At that point, you don't need to worry: when _msgArr goes out of scope, all the memory is cleaned up properly.

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1 Comment

Can u explain your second option - the use of a smart pointer that will delete all memory when Message is deleted?
0

unfortunately yes. But you can simplify the thing with a couple of lines if you are going to do this a lot.

#include <algorithm>
// might already exists in a boost header (i don't remember)
template <T>
void delete_func(T* e) {
    delete e;
}

// then, to delete your array :
std::for_each(_msgArr,_msgArr+MAX_MESSAGES, delete_func);
delete[] _msgArr;

but i suggest not to do so. You could use a std::vector with auto_ptr or shared_ptr (depending on your use case). So you won't have to delete each element yourself because the auto_ptr will do so. You will just have to delete the std::vector

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0

Except for the fact that it is within a class member function and uses new and delete, this is C code. You don't need to check the result of new (allocation errors will throw), you shouldn't do your own dynamic arrays (that's what std::vector is for), and you shouldn't manage dynamically allocated using dumb pointers.

What you want is std::vector< std::shared_ptr<Message> > or one of boost's pointer containers.

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