I'm looking for a way to automatically deallocate an array of wchar_ts – kind of like an autopointer (I'm not really aquainted with std::auto_ptr, but I think it cannot be used for arrays).
The code I have right now is this:
/* volume is of type wstring,
* hr is of type HRESULT,
* VSS_PWSZ equals wchar_t*
*/
VSS_PWSZ pwszVolume = new wchar_t[volume.size() + 1];
std::copy(volume.begin(), volume.end(), &pwszVolume);
pwszVolume[volume.size()] = 0;
hr = pDiffMgmt->QueryDiffAreasOnVolume(pwszVolume, &pEnumMgmt);
delete[] pwszVolume;
pwszVolume = NULL;
I don't really get why this stupid function cannot take a const wchar_t*, otherwise I could just pass volume.c_str().
So far so good, I think my code solves this problem, but now the memory management is getting more complicated: I would have to duplicate the delete[] code to account for exceptions which might be thrown (and which I do not want to catch at this point.)
Is there a way I can get pwszVolume to be deallocated automatically when the current scope is left?
Use std::vector<wchar_t> it is your basic C++ array (or std::wstring if you want to manipulate it like a string).
std::vector<wchar_t> pwszVolume(volume.begin(), volume.end());
pwszVolume.push_back(0);
hr = pDiffMgmt->QueryDiffAreasOnVolume(&pwszVolume[0], &pEnumMgmt);
The question may be. What does QueryDiffAreasOnVolume() do with the data?
Maybe you do not need to copy it out.
std::unique_ptr can be used with arrays as follows:
std::unique_ptr<wchar_t[]> pwszVolume(new wchar_t[volume.size() + 1]);
Another option is std::array.
But I agree with Martin's answer that you should just use an std::vector unless you really cannot afford to have the couple of extra pointers that the vector class holds.
As others have said, std::vector is the preferred solution, by far.
Otherwise (if e.g. you originally get the pointer from third party
software which you cannot modify), there's boost::scoped_array or
boost::shared_array.
If you don't want the overhead from std::vector, use boost::array. It is your basic C++ array with static size.
You can wrap wchar_t* inside a class, deallocate memory on destruct-or and you have an object that will be automatically deallocated when it loses scope.
Related
I'm new to C++. I've heard that using unique_ptr / shared_ptr is the "way to go" for references to data allocated on the heap. Does it make sense, therefore, to use unique_ptrs instead of std::strings?
Why would you want to do that?
An std::string object manages the life time of the "contained" string (memory bytes) by itself.
Since you are new to C++. Inside your function / class method, I will advice you create your objects on the stack:
Like so:
std::string s;
as opposed to using the heap:
std::string* s = new std::string();
Objects created on the stack will be destroyed when your object goes out of scope. So there is no need for smart pointers.
You can following this link to know more: http://www.learncpp.com/cpp-tutorial/79-the-stack-and-the-heap/
There's no need to use a std::unique_ptr or std::shared_ptr for a std::string.
You do not have to allocate memory for a simple std::string.
std::string str = "abc";
As simple as that. No need for memory allocation, as the std::string manages the 'real' string by itself.
There are situations which may lead to usage of a pointer, though it is likely a class/struct instance.
For instance consider using
std::unique_ptr<MyClass> p;
instead of
MyClass *p;
if possible.
You normally don't need pointers to strings, just like you generally don't need pointers to integers. You can just store string values, you can pass string values, etc.
But if you are in the exceptional situation where you'd need a pointer to a string, then yes, std::unique_ptr<std::string> or std::shared_ptr<std::string> are better than std::string*.
An std::unique_ptr ensures that the object pointed is not accidentally copied and properly deleted.
As you should avoid dynamic allocation as much as you can, you can simply keep the std::string as a member of your class. If it is a returned value, as already pointed out, the string class is smart enough to properly move resources in a safe way.
You don't need to guarantee that a string is unique, I mean, no copies exist, so the unique ptr is just a too strong constraint.
KIS rule: Keep it simple.
Usually, and by default the answer is no, as others have suggested. However - sometimes, the answer may paradoxically be "Possibly Yes"! How come?
With std::string, you don't control how, when and by whom your buffer is allocated. While this is somewhat mitigated if you use a different allocator (std::basic_string<char, MyAllocatorType>) - the resulting class is not std::string; and will generally not be accepted by functions which taken std::string's. And it may not make sense to get into allocators just for this purpose.
More specifically, you can allow your unique-pointer-based string class to be created as an owning wrapper for an existing buffer.
Now that we can use string_view's - and they are even in the standard in C++20 (std::string_view) - you don't have to rewrite the whole string class for unique-pointer-based strings; all you need is to create a string view over it, using the raw pointer and the size in bytes (or size - 1 if you want better null-termination safety.) And if you do want the std::string methods still, they'll be one-liners, e.g.
std::string_view view() const {
return std::string_view{uptr_.get(), size_};
}
substr(size_type pos = 0, size_type count = npos) const {
return view().substr(pos, count);
}
If you want to update a string in-place, while maintaining its size - an std::string won't work for you: Either its completely constant, or mutable both in size and in contents.
I've been looking into this for the past few days, and so far I haven't really found anything convincing other than dogmatic arguments or appeals to tradition (i.e. "it's the C++ way!").
If I'm creating an array of objects, what is the compelling reason (other than ease) for using:
#define MY_ARRAY_SIZE 10
// ...
my_object * my_array=new my_object [MY_ARRAY_SIZE];
for (int i=0;i<MY_ARRAY_SIZE;++i) my_array[i]=my_object(i);
over
#define MEMORY_ERROR -1
#define MY_ARRAY_SIZE 10
// ...
my_object * my_array=(my_object *)malloc(sizeof(my_object)*MY_ARRAY_SIZE);
if (my_object==NULL) throw MEMORY_ERROR;
for (int i=0;i<MY_ARRAY_SIZE;++i) new (my_array+i) my_object (i);
As far as I can tell the latter is much more efficient than the former (since you don't initialize memory to some non-random value/call default constructors unnecessarily), and the only difference really is the fact that one you clean up with:
delete [] my_array;
and the other you clean up with:
for (int i=0;i<MY_ARRAY_SIZE;++i) my_array[i].~T();
free(my_array);
I'm out for a compelling reason. Appeals to the fact that it's C++ (not C) and therefore malloc and free shouldn't be used isn't -- as far as I can tell -- compelling as much as it is dogmatic. Is there something I'm missing that makes new [] superior to malloc?
I mean, as best I can tell, you can't even use new [] -- at all -- to make an array of things that don't have a default, parameterless constructor, whereas the malloc method can thusly be used.
I'm out for a compelling reason.
It depends on how you define "compelling". Many of the arguments you have thus far rejected are certainly compelling to most C++ programmers, as your suggestion is not the standard way to allocate naked arrays in C++.
The simple fact is this: yes, you absolutely can do things the way you describe. There is no reason that what you are describing will not function.
But then again, you can have virtual functions in C. You can implement classes and inheritance in plain C, if you put the time and effort into it. Those are entirely functional as well.
Therefore, what matters is not whether something can work. But more on what the costs are. It's much more error prone to implement inheritance and virtual functions in C than C++. There are multiple ways to implement it in C, which leads to incompatible implementations. Whereas, because they're first-class language features of C++, it's highly unlikely that someone would manually implement what the language offers. Thus, everyone's inheritance and virtual functions can cooperate with the rules of C++.
The same goes for this. So what are the gains and the losses from manual malloc/free array management?
I can't say that any of what I'm about to say constitutes a "compelling reason" for you. I rather doubt it will, since you seem to have made up your mind. But for the record:
Performance
You claim the following:
As far as I can tell the latter is much more efficient than the former (since you don't initialize memory to some non-random value/call default constructors unnecessarily), and the only difference really is the fact that one you clean up with:
This statement suggests that the efficiency gain is primarily in the construction of the objects in question. That is, which constructors are called. The statement presupposes that you don't want to call the default constructor; that you use a default constructor just to create the array, then use the real initialization function to put the actual data into the object.
Well... what if that's not what you want to do? What if what you want to do is create an empty array, one that is default constructed? In this case, this advantage disappears entirely.
Fragility
Let's assume that each object in the array needs to have a specialized constructor or something called on it, such that initializing the array requires this sort of thing. But consider your destruction code:
for (int i=0;i<MY_ARRAY_SIZE;++i) my_array[i].~T();
For a simple case, this is fine. You have a macro or const variable that says how many objects you have. And you loop over each element to destroy the data. That's great for a simple example.
Now consider a real application, not an example. How many different places will you be creating an array in? Dozens? Hundreds? Each and every one will need to have its own for loop for initializing the array. Each and every one will need to have its own for loop for destroying the array.
Mis-type this even once, and you can corrupt memory. Or not delete something. Or any number of other horrible things.
And here's an important question: for a given array, where do you keep the size? Do you know how many items you allocated for every array that you create? Each array will probably have its own way of knowing how many items it stores. So each destructor loop will need to fetch this data properly. If it gets it wrong... boom.
And then we have exception safety, which is a whole new can of worms. If one of the constructors throws an exception, the previously constructed objects need to be destructed. Your code doesn't do that; it's not exception-safe.
Now, consider the alternative:
delete[] my_array;
This can't fail. It will always destroy every element. It tracks the size of the array, and it's exception-safe. So it is guaranteed to work. It can't not work (as long as you allocated it with new[]).
Of course, you could say that you could wrap the array in an object. That makes sense. You might even template the object on the type elements of the array. That way, all the desturctor code is the same. The size is contained in the object. And maybe, just maybe, you realize that the user should have some control over the particular way the memory is allocated, so that it's not just malloc/free.
Congratulations: you just re-invented std::vector.
Which is why many C++ programmers don't even type new[] anymore.
Flexibility
Your code uses malloc/free. But let's say I'm doing some profiling. And I realize that malloc/free for certain frequently created types is just too expensive. I create a special memory manager for them. But how to hook all of the array allocations to them?
Well, I have to search the codebase for any location where you create/destroy arrays of these types. And then I have to change their memory allocators accordingly. And then I have to continuously watch the codebase so that someone else doesn't change those allocators back or introduce new array code that uses different allocators.
If I were instead using new[]/delete[], I could use operator overloading. I simply provide an overload for operators new[] and delete[] for those types. No code has to change. It's much more difficult for someone to circumvent these overloads; they have to actively try to. And so forth.
So I get greater flexibility and reasonable assurance that my allocators will be used where they should be used.
Readability
Consider this:
my_object *my_array = new my_object[10];
for (int i=0; i<MY_ARRAY_SIZE; ++i)
my_array[i]=my_object(i);
//... Do stuff with the array
delete [] my_array;
Compare it to this:
my_object *my_array = (my_object *)malloc(sizeof(my_object) * MY_ARRAY_SIZE);
if(my_object==NULL)
throw MEMORY_ERROR;
int i;
try
{
for(i=0; i<MY_ARRAY_SIZE; ++i)
new(my_array+i) my_object(i);
}
catch(...) //Exception safety.
{
for(i; i>0; --i) //The i-th object was not successfully constructed
my_array[i-1].~T();
throw;
}
//... Do stuff with the array
for(int i=MY_ARRAY_SIZE; i>=0; --i)
my_array[i].~T();
free(my_array);
Objectively speaking, which one of these is easier to read and understand what's going on?
Just look at this statement: (my_object *)malloc(sizeof(my_object) * MY_ARRAY_SIZE). This is a very low level thing. You're not allocating an array of anything; you're allocating a hunk of memory. You have to manually compute the size of the hunk of memory to match the size of the object * the number of objects you want. It even features a cast.
By contrast, new my_object[10] tells the story. new is the C++ keyword for "create instances of types". my_object[10] is a 10 element array of my_object type. It's simple, obvious, and intuitive. There's no casting, no computing of byte sizes, nothing.
The malloc method requires learning how to use malloc idiomatically. The new method requires just understanding how new works. It's much less verbose and much more obvious what's going on.
Furthermore, after the malloc statement, you do not in fact have an array of objects. malloc simply returns a block of memory that you have told the C++ compiler to pretend is a pointer to an object (with a cast). It isn't an array of objects, because objects in C++ have lifetimes. And an object's lifetime does not begin until it is constructed. Nothing in that memory has had a constructor called on it yet, and therefore there are no living objects in it.
my_array at that point is not an array; it's just a block of memory. It doesn't become an array of my_objects until you construct them in the next step. This is incredibly unintuitive to a new programmer; it takes a seasoned C++ hand (one who probably learned from C) to know that those aren't live objects and should be treated with care. The pointer does not yet behave like a proper my_object*, because it doesn't point to any my_objects yet.
By contrast, you do have living objects in the new[] case. The objects have been constructed; they are live and fully-formed. You can use this pointer just like any other my_object*.
Fin
None of the above says that this mechanism isn't potentially useful in the right circumstances. But it's one thing to acknowledge the utility of something in certain circumstances. It's quite another to say that it should be the default way of doing things.
If you do not want to get your memory initialized by implicit constructor calls, and just need an assured memory allocation for placement new then it is perfectly fine to use malloc and free instead of new[] and delete[].
The compelling reasons of using new over malloc is that new provides implicit initialization through constructor calls, saving you additional memset or related function calls post an malloc And that for new you do not need to check for NULL after every allocation, just enclosing exception handlers will do the job saving you redundant error checking unlike malloc.
These both compelling reasons do not apply to your usage.
which one is performance efficient can only be determined by profiling, there is nothing wrong in the approach you have now. On a side note I don't see a compelling reason as to why use malloc over new[] either.
I would say neither.
The best way to do it would be:
std::vector<my_object> my_array;
my_array.reserve(MY_ARRAY_SIZE);
for (int i=0;i<MY_ARRAY_SIZE;++i)
{ my_array.push_back(my_object(i));
}
This is because internally vector is probably doing the placement new for you. It also managing all the other problems associated with memory management that you are not taking into account.
You've reimplemented new[]/delete[] here, and what you have written is pretty common in developing specialized allocators.
The overhead of calling simple constructors will take little time compared the allocation. It's not necessarily 'much more efficient' -- it depends on the complexity of the default constructor, and of operator=.
One nice thing that has not been mentioned yet is that the array's size is known by new[]/delete[]. delete[] just does the right and destructs all elements when asked. Dragging an additional variable (or three) around so you exactly how to destroy the array is a pain. A dedicated collection type would be a fine alternative, however.
new[]/delete[] are preferable for convenience. They introduce little overhead, and could save you from a lot of silly errors. Are you compelled enough to take away this functionality and use a collection/container everywhere to support your custom construction? I've implemented this allocator -- the real mess is creating functors for all the construction variations you need in practice. At any rate, you often have a more exact execution at the expense of a program which is often more difficult to maintain than the idioms everybody knows.
IMHO there both ugly, it's better to use vectors. Just make sure to allocate the space in advance for performance.
Either:
std::vector<my_object> my_array(MY_ARRAY_SIZE);
If you want to initialize with a default value for all entries.
my_object basic;
std::vector<my_object> my_array(MY_ARRAY_SIZE, basic);
Or if you don't want to construct the objects but do want to reserve the space:
std::vector<my_object> my_array;
my_array.reserve(MY_ARRAY_SIZE);
Then if you need to access it as a C-Style pointer array just (just make sure you don't add stuff while keeping the old pointer but you couldn't do that with regular c-style arrays anyway.)
my_object* carray = &my_array[0];
my_object* carray = &my_array.front(); // Or the C++ way
Access individual elements:
my_object value = my_array[i]; // The non-safe c-like faster way
my_object value = my_array.at(i); // With bounds checking, throws range exception
Typedef for pretty:
typedef std::vector<my_object> object_vect;
Pass them around functions with references:
void some_function(const object_vect& my_array);
EDIT:
IN C++11 there is also std::array. The problem with it though is it's size is done via a template so you can't make different sized ones at runtime and you cant pass it into functions unless they are expecting that exact same size (or are template functions themselves). But it can be useful for things like buffers.
std::array<int, 1024> my_array;
EDIT2:
Also in C++11 there is a new emplace_back as an alternative to push_back. This basically allows you to 'move' your object (or construct your object directly in the vector) and saves you a copy.
std::vector<SomeClass> v;
SomeClass bob {"Bob", "Ross", 10.34f};
v.emplace_back(bob);
v.emplace_back("Another", "One", 111.0f); // <- Note this doesn't work with initialization lists ☹
Oh well, I was thinking that given the number of answers there would be no reason to step in... but I guess I am drawn in as the others. Let's go
Why your solution is broken
C++11 new facilities for handling raw memory
Simpler way to get this done
Advices
1. Why your solution is broken
First, the two snippets you presented are not equivalent. new[] just works, yours fails horribly in the presence of Exceptions.
What new[] does under the cover is that it keeps track of the number of objects that were constructed, so that if an exception occurs during say the 3rd constructor call it properly calls the destructor for the 2 already constructed objects.
Your solution however fails horribly:
either you don't handle exceptions at all (and leak horribly)
or you just try to call the destructors on the whole array even though it's half built (likely crashing, but who knows with undefined behavior)
So the two are clearly not equivalent. Yours is broken
2. C++11 new facilities for handling raw memory
In C++11, the comittee members have realized how much we liked fiddling with raw memory and they have introduced facilities to help us doing so more efficiently, and more safely.
Check cppreference's <memory> brief. This example shows off the new goodies (*):
#include <iostream>
#include <string>
#include <memory>
#include <algorithm>
int main()
{
const std::string s[] = {"This", "is", "a", "test", "."};
std::string* p = std::get_temporary_buffer<std::string>(5).first;
std::copy(std::begin(s), std::end(s),
std::raw_storage_iterator<std::string*, std::string>(p));
for(std::string* i = p; i!=p+5; ++i) {
std::cout << *i << '\n';
i->~basic_string<char>();
}
std::return_temporary_buffer(p);
}
Note that get_temporary_buffer is no-throw, it returns the number of elements for which memory has actually been allocated as a second member of the pair (thus the .first to get the pointer).
(*) Or perhaps not so new as MooingDuck remarked.
3. Simpler way to get this done
As far as I am concered, what you really seem to be asking for is a kind of typed memory pool, where some emplacements could not have been initialized.
Do you know about boost::optional ?
It is basically an area of raw memory that can fit one item of a given type (template parameter) but defaults with having nothing in instead. It has a similar interface to a pointer and let you query whether or not the memory is actually occupied. Finally, using the In-Place Factories you can safely use it without copying objects if it is a concern.
Well, your use case really looks like a std::vector< boost::optional<T> > to me (or perhaps a deque?)
4. Advices
Finally, in case you really want to do it on your own, whether for learning or because no STL container really suits you, I do suggest you wrap this up in an object to avoid the code sprawling all over the place.
Don't forget: Don't Repeat Yourself!
With an object (templated) you can capture the essence of your design in one single place, and then reuse it everywhere.
And of course, why not take advantage of the new C++11 facilities while doing so :) ?
You should use vectors.
Dogmatic or not, that is exactly what ALL the STL container do to allocate and initialize.
They use an allocator then allocates uninitialized space and initialize it by means of the container constructors.
If this (like many people use to say) "is not c++" how can be the standard library just be implemented like that?
If you just don't want to use malloc / free, you can allocate "bytes" with just new char[]
myobjet* pvext = reinterpret_cast<myobject*>(new char[sizeof(myobject)*vectsize]);
for(int i=0; i<vectsize; ++i) new(myobject+i)myobject(params);
...
for(int i=vectsize-1; i!=0u-1; --i) (myobject+i)->~myobject();
delete[] reinterpret_cast<char*>(myobject);
This lets you take advantage of the separation between initialization and allocation, still taking adwantage of the new allocation exception mechanism.
Note that, putting my first and last line into an myallocator<myobject> class and the second ands second-last into a myvector<myobject> class, we have ... just reimplemented std::vector<myobject, std::allocator<myobject> >
What you have shown here is actually the way to go when using a memory allocator different than the system general allocator - in that case you would allocate your memory using the allocator (alloc->malloc(sizeof(my_object))) and then use the placement new operator to initialize it. This has many advantages in efficient memory management and quite common in the standard template library.
If you are writing a class that mimics functionality of std::vector or needs control over memory allocation/object creation (insertion in array / deletion etc.) - that's the way to go. In this case, it's not a question of "not calling default constructor". It becomes a question of being able to "allocate raw memory, memmove old objects there and then create new objects at the olds' addresses", question of being able to use some form of realloc and so on. Unquestionably, custom allocation + placement new are way more flexible... I know, I'm a bit drunk, but std::vector is for sissies... About efficiency - one can write their own version of std::vector that will be AT LEAST as fast ( and most likely smaller, in terms of sizeof() ) with most used 80% of std::vector functionality in, probably, less than 3 hours.
my_object * my_array=new my_object [10];
This will be an array with objects.
my_object * my_array=(my_object *)malloc(sizeof(my_object)*MY_ARRAY_SIZE);
This will be an array the size of your objects, but they may be "broken". If your class has virtual funcitons for instance, then you won't be able to call those. Note that it's not just your member data that may be inconsistent, but the entire object is actully "broken" (in lack of a better word)
I'm not saying it's wrong to do the second one, just as long as you know this.
I'd like to use a std::vector to control a given piece of memory. First of all I'm pretty sure this isn't good practice, but curiosity has the better of me and I'd like to know how to do this anyway.
The problem I have is a method like this:
vector<float> getRow(unsigned long rowIndex)
{
float* row = _m->getRow(rowIndex); // row is now a piece of memory (of a known size) that I control
vector<float> returnValue(row, row+_m->cols()); // construct a new vec from this data
delete [] row; // delete the original memory
return returnValue; // return the new vector
}
_m is a DLL interface class which returns an array of float which is the callers responsibility to delete. So I'd like to wrap this in a vector and return that to the user.... but this implementation allocates new memory for the vector, copies it, and then deletes the returned memory, then returns the vector.
What I'd like to do is to straight up tell the new vector that it has full control over this block of memory so when it gets deleted that memory gets cleaned up.
UPDATE: The original motivation for this (memory returned from a DLL) has been fairly firmly squashed by a number of responders :) However, I'd love to know the answer to the question anyway... Is there a way to construct a std::vector using a given chunk of pre-allocated memory T* array, and the size of this memory?
The obvious answer is to use a custom allocator, however you might find that is really quite a heavyweight solution for what you need. If you want to do it, the simplest way is to take the allocator defined (as the default scond template argument to vector<>) by the implementation, copy that and make it work as required.
Another solution might be to define a template specialisation of vector, define as much of the interface as you actually need and implement the memory customisation.
Finally, how about defining your own container with a conforming STL interface, defining random access iterators etc. This might be quite easy given that underlying array will map nicely to vector<>, and pointers into it will map to iterators.
Comment on UPDATE: "Is there a way to construct a std::vector using a given chunk of pre-allocated memory T* array, and the size of this memory?"
Surely the simple answer here is "No". Provided you want the result to be a vector<>, then it has to support growing as required, such as through the reserve() method, and that will not be possible for a given fixed allocation. So the real question is really: what exactly do you want to achieve? Something that can be used like vector<>, or something that really does have to in some sense be a vector, and if so, what is that sense?
Vector's default allocator doesn't provide this type of access to its internals. You could do it with your own allocator (vector's second template parameter), but that would change the type of the vector.
It would be much easier if you could write directly into the vector:
vector<float> getRow(unsigned long rowIndex) {
vector<float> row (_m->cols());
_m->getRow(rowIndex, &row[0]); // writes _m->cols() values into &row[0]
return row;
}
Note that &row[0] is a float* and it is guaranteed for vector to store items contiguously.
The most important thing to know here is that different DLL/Modules have different Heaps. This means that any memory that is allocated from a DLL needs to be deleted from that DLL (it's not just a matter of compiler version or delete vs delete[] or whatever). DO NOT PASS MEMORY MANAGEMENT RESPONSIBILITY ACROSS A DLL BOUNDARY. This includes creating a std::vector in a dll and returning it. But it also includes passing a std::vector to the DLL to be filled by the DLL; such an operation is unsafe since you don't know for sure that the std::vector will not try a resize of some kind while it is being filled with values.
There are two options:
Define your own allocator for the std::vector class that uses an allocation function that is guaranteed to reside in the DLL/Module from which the vector was created. This can easily be done with dynamic binding (that is, make the allocator class call some virtual function). Since dynamic binding will look-up in the vtable for the function call, it is guaranteed that it will fall in the code from the DLL/Module that originally created it.
Don't pass the vector object to or from the DLL. You can use, for example, a function getRowBegin() and getRowEnd() that return iterators (i.e. pointers) in the row array (if it is contiguous), and let the user std::copy that into its own, local std::vector object. You could also do it the other way around, pass the iterators begin() and end() to a function like fillRowInto(begin, end).
This problem is very real, although many people neglect it without knowing. Don't underestimate it. I have personally suffered silent bugs related to this issue and it wasn't pretty! It took me months to resolve it.
I have checked in the source code, and boost::shared_ptr and boost::shared_array use dynamic binding (first option above) to deal with this.. however, they are not guaranteed to be binary compatible. Still, this could be a slightly better option (usually binary compatibility is a much lesser problem than memory management across modules).
Your best bet is probably a std::vector<shared_ptr<MatrixCelType>>.
Lots more details in this thread.
If you're trying to change where/how the vector allocates/reallocates/deallocates memory, the allocator template parameter of the vector class is what you're looking for.
If you're simply trying to avoid the overhead of construction, copy construction, assignment, and destruction, then allow the user to instantiate the vector, then pass it to your function by reference. The user is then responsible for construction and destruction.
It sounds like what you're looking for is a form of smart pointer. One that deletes what it points to when it's destroyed. Look into the Boost libraries or roll your own in that case.
The Boost.SmartPtr library contains a whole lot of interesting classes, some of which are dedicated to handle arrays.
For example, behold scoped_array:
int main(int argc, char* argv[])
{
boost::scoped_array<float> array(_m->getRow(atoi(argv[1])));
return 0;
}
The issue, of course, is that scoped_array cannot be copied, so if you really want a std::vector<float>, #Fred Nurk's is probably the best you can get.
In the ideal case you'd want the equivalent to unique_ptr but in array form, however I don't think it's part of the standard.
In short, I am wondering if there is an auto_ptr like type for arrays. I know I could roll my own, I'm just making sure that there isn't already something out there.
I know about vectors as well. however I don't think I can use them. I am using several of the Windows APIs/SDKs such as the Windows Media SDK, Direct Show API which in order to get back some structures to call a function which takes a pointer and a size twice. The first time passing NULL as the pointer to get back the size of the structure that I have to allocated in order to receive the data I am looking for. For example:
CComQIPtr<IWMMediaProps> pProps(m_pStreamConfig);
DWORD cbType = 0;
WM_MEDIA_TYPE *pType = NULL;
hr = pProps->GetMediaType(NULL, &cbType);
CHECK_HR(hr);
pType = (WM_MEDIA_TYPE*)new BYTE[cbType]; // Would like to use auto_ptr instread
hr = pProps->GetMediaType(pType, &cbType);
CHECK_HR(hr);
// ... do some stuff
delete[] pType;
Since cbType typically comes back bigger than sizeof(WM_MEDIA_TYPE) due to the fact is has a pointer to another structure in it, I can't just allocate WM_MEDIA_TYPE objects. Is there anything like this out there?
Use
std::vector<BYTE> buffer(cbType);
pType = (WM_MEDIA_TYPE*)&buffer[0];
or since C++11
std::vector<BYTE> buffer(cbType);
pType = (WM_MEDIA_TYPE*)buffer.data();
instead.
Additional:
If someone is asking if the Vectors are guaranteed to be contiguous the answer is Yes since C++ 03 standard. There is another thread that already discussed it.
If C++11 is supported by your compiler, unique_ptr can be used for arrays.
unique_ptr<BYTE[]> buffer(new BYTE[cbType]);
pType = (WM_MEDIA_TYPE*)buffer.get();
boost scoped_array or you can use boost scoped_ptr with a custom deleter
There is nothing for this in the current std library. However, the future standard C++0x has an unique_ptr, which comes in replacement of auto_ptr, and which works with arrays.
A first implementation can be found here:
unique_ptr
Not in STL. Boost has some smart pointers with a similar idea. Check out scoped_array and shared_array
We are using the CString class throughout most of our code. However sometimes we need to convert to a char *. at the moment we have been doing this using variable.GetBuffer(0) and this seems to work ( this mainly happens when passing the Csting into a function where the function requires a char *). The function accepts this and we keep going.
However we have lately become worried about how this works, and whether there is a better way to do it.
The way i understand it to work is it passes a char pointer into the function that points at the first character in the CString and all works well.
I Guess we are just worried about memory leaks or any unforseen circumstances where this might not be a good idea.
If your functions only require reading the string and not modifying it, change them to accept const char * instead of char *. The CString will automatically convert for you, this is how most of the MFC functions work and it's really handy. (Actually MFC uses LPCTSTR, which is a synonym for const TCHAR * - works for both MBC and Unicode builds).
If you need to modify the string, GetBuffer(0) is very dangerous - it won't necessarily allocate enough memory for the resulting string, and you could get some buffer overrun errors.
As has been mentioned by others, you need to use ReleaseBuffer after GetBuffer. You don't need to do that for the conversion to const char *.
# the OP:
>>> I Guess we are just worried about memory leaks or any ...
Hi, calling the GetBuffer method won't lead to any memory leaks. Because the destructor is going to deallocate the buffer anyway. However, others have already warned you about the potential issues with calling this method.
#Can >>> when you call the getbuffer function it allocates memory for you.
This statement is not completely true. GetBuffer(0) does NOT allocate any memory. It merely returns a pointer to the internal string buffer that can be used to manipulate the string directly from "outside" the CString class.
However, if you pass a number, say N to it like GetBuffer(N), and if N is larger than the current length of the buffer, then the function ensures that the returned buffer is at least as large as N by allocating more memory.
Cheers,
Rajesh.
MVP, Visual ++.
when you call the getbuffer function it allocates memory for you.
when you have done with it, you need to call releasebuffer to deallocate it
try the documentation at http://msdn.microsoft.com/en-us/library/awkwbzyc.aspx for help on that.