Is there a way to use RAII when creating an array in C++, or do I have to resort to manual memory management via the "new" keyword?
EDIT: By request, some additional details:
First my disclaimer: I'm fairly novice at C++.
Now the details: I simply wanted to iterate through an existing array, SomeClass source[], and parse each element into a storage array, string results[].
So to create that storage string results[], do I need to say string results[] = new string[source.size()] or is there a way to avoid the "new" keyword here via RAII techniques?
It sounds like I'm hearing vector is the way to go. Still, wondered if it's possible with a plain old array.
In C++, especially since we're talking about RAII, we really shouldn't be using C-style arrays at all. The Standard Library provides many containers with different properties that obviate most needs for C-style arrays. By default, you should be using std::vector<> unless you have an understanding of the other containers and a compelling reason not to 1.
Judging from your comments, it looks like what you're trying to do is build an array of string data. This is simple in modern C++ -- just use a std::vector <std::string>.
int main()
{
std::vector <std::string> myStrings;
myStrings.push_back ("hello");
myStrings.push_back ("goodbye");
}
This achieves all your mentioned goals of being RAII and no need to new or delete anything. It also integrates very nicely with other Standard Library facilities, such as find.
I did say "obviate most needs," emphasis mine. There is still occasional need to go old-school. I won't discuss them here since if you actually need it you already know. Which is to say, if you don't know you probably don't need it.
All that being said, std::unique_ptr does support managing C-style arrays with special syntax:
std::unique_ptr <unsigned []> myArray (new unsigned [256]);
the array managed by myArray above will be properly deleted with delete [] at destruction time.
Since there is no unique_ptr analogue to shared_ptr's make_shared, you will either have to use new to actually construct the array, or provide your own (or someone else's) implementation of make_unique
Sidebar: make_unique will be added to C++14.
1 " you should be using std::vector<> unless you have an understanding of the other containers and a compelling reason not to"
This isn't just my opinion. It's also the general (unofficial) opinion of StackOverflow as a whole, C++'s creator Bjarne Stroustup, and the C++ Standard itself (23.1.1).
std::vector<T> is an RAII array type. From the additional information you've given std::vector<string> is appropriate for your use.
string results[] = new string[source.size()]
Just as an aside about arrays, this syntax isn't correct for C++. People teaching C and C++ often teach that arrays and pointers are the same thing, but this is not true and the syntax above shows one of the ways they differ.
The actual relationship between arrays and pointers is
Arrays and pointers are distinct types
In many contexts an implicit conversion from an array to pointer is applied. The conversion results in the address of the array's first element.
pointer arithmetic is defined such that if a pointer points to an element in an array then you can get pointers to other elements of the array by adding and subtracting to the pointer value.
In a some contexts writing an array type will result in a pointer type. In particular, function parameters declared to be of array types are adjusted to be of pointer type. E.g. void foo(int[10]); is effectively identical to void foo(int*);.
So the code you show is declaring an array and initializing it with a pointer. This will result in an error because there's no conversion from pointers to arrays.
Generally you should avoid raw arrays in C++; Arrays just have too many weird rules, such as the weird function parameter rule described above, the rules about when exactly the implicit conversion is performed, prohibitions on assignment and returning arrays from functions, etc. When starting out it's probably best to just stick with the std::array template for arrays with a size known at compile time and the std::vector template for arrays with a size not known at compile time.
You can use std::vector<string> and push_back() each result string in to the vector. You don't have to use new unless you want to avoid vectors and manage memory by yourself
I think a simple template would do the trick in the general case:
template<class T>
class HeapArray {
T* ptr = nullptr;
public:
HeapArray(int size) { ptr = new T[size]; }
~HeapArray() { delete [] ptr; }
operator T*() { return ptr; }
};
You use it as:
HeapArray<std::string> results(source.size());
results[i] = "how cool is this";
basically the HeapArray object is automatically cast'd to its member pointer.
Related
I am converting a C code to C++.
There is matrix pointer:
MATRIX* matrix = NULL;
matrix = new MATRIX[256];
if (matrix == NULL)
return FAIL;
memset(matrix, 0, 256*sizeof(MATRIX));
Then it was filled on a different method:
fillUpMatrix(matrix);
And in fillUpMatrix():
memcpy(&matrix[start], &someOtherMatrix[pos], sizeof(MATRIX));
And later on memset was called for the pointer since it will be filled with a different set of values:
memset(matrix, 0, 256*sizeof(MATRIX));
So what I did was:
auto matrix= std::make_unique<MATRIX[]>(256);
fillUpMatrix(matrix.get());
I skipped the first memset since I believe I do not need it anymore for smart pointers.
But the second memset I believe is needed (since new values will be saved). So how do I write that in C++ and considering that I am using a smart pointer? Is my conversion above correct?
I skipped the first memset since I believe I do not need it anymore for smart pointers.
You're right that it isn't needed, but the reason for not needing it specifically that you used std::make_unique, which value initialises the array; not because you're using smart pointers. That is assuming initialisation is needed in the first place. That doesn't appear to be the case, since the content is about to be filled in a function.
Do take into consideration that std::memset and std::memcpy behave correctly only if the type (Matrix in this case) is trivially copyable. If that is not the case, you must use std::fill_n (or std::fill) and std::copy respectively. Which can be used if the type is trivially copyable too, so you might as well use them in any case.
But the second memset I believe is needed (since new values will be saved).
Similar to the first std::memset, It's unclear why you think that a second std::memset is needed (whether in C or C++). If the new values are going to be written over the array, then what effect does the std::memset have?
So how do I write that in C++ and considering that I am using a smart pointer?
You can std::memset an array pointed by smart pointer like this:
std::memset(matrix.get(), 0, 256*sizeof(decltype(*matrix)));
Or using std::fill_n instead:
std::fill_n(matrix.get(), 256, MATRIX{});
Hmm my thought was since it was MATRIX* matrix = NULL; then I should use a smart pointer.
std::vector is a RAII container that represents a dynamic array. You're dynamically allocating an array. std::vector is appropriate.
Converting C to C++ is not just about replacing bare owning pointers with smart pointers. Another thing to do is to replace custom re-implementations of typical data structures and algorithms with the standard ones provided by the standard library.
Using a vector<MATRIX> would solve many problems.
new doesn't return null, but throws. Hence null check has no effect (unless nothrow is used).
You're mixing raw pointer approach with newer smart-pointer, vector based approach. You shouldn't be using any raw memory manipulation functions in the latest C++ code.
Instead, have a constructor in the MATRIX class which would initialize all members of MATRIX. You can have (or prevent) copy/move constructors/assignment operators. Consider using =default and =delete with special member functions.
memcpy etc may corrupt the state of some non-POD members in MATRIX type (like std::string, std::vector). Such undefined-behaviours are hard to detect, hence don't use any mem* functions.
I am trying to connect two existing codebases — one in C, the other in C++. The C++ code uses std::vector whereas the other one is based on arrays of double. I would like to pass arrays of double from the C code, perform operations on std::vectors in the C++ code, and eventually have these operations reflected in the arrays of double.
Is it possible to create a std::vector that matches the memory occupied by the array of double?
I have tried several options, but they all involve the creation of a new vector and a copy of the array of double into that vector. For instance:
void fcn(double* a, int sizeofa)
{
std::vector<double> vect_a;
vect_a.assign(a, a + sizeofa);
// operations on vect_a
for (int i=0;i<sizeofa;i++) { a[i] = vect_a[i]; }
}
As noted in the comments, std::vector manages its own memory, you can't make it use some other memory as the backing store (it would have no idea what to do if the size changed, among other issues).
But you may not need a vector at all; if you're just using vector for non-dynamic size related features, it's highly likely you could just use the functions from <algorithm> to perform the same work directly on the array that you wanted to use vector's methods to accomplish.
In C++, functions requiring a container often denote the container by Iterator pairs, which are templated. This is very convenient, especially when interfacing with external libraries, because an iterator isn't a type. It is a concept, which is just an interface that defines what a type should look like. Turns out, C style pointers are valid iterators. This means that you can use any C++ function that accepts an iterator with any C array.
So, now to answering your question. In other answers, it was made clear that you cannot make a std::vector control the memory allocated by a C array because a std::vector requires full ownership over the data because it wouldn't know how to deallocate it. You can copy the C array into a vector, but there is no point in using a std::vector unless you want it's resizing capabilities.
In summary: try not to pass std::vectors into functions because iterators are more generic. If you must avoid templates (virtual function, etc) than use a C style array because those are very flexible too, you can turn a std::vector into a C array, but the other way requires a copy.
I know this is hard if you have already made your code interface with std::vectors, in which case a copy is the only possible way. Prefer C style arrays when you don't need to resize the array, and maybe in the future std::array_view
Since std::list and std::vector exist, is there a reason to use traditional C arrays in C++, or should they be avoided, just like malloc?
In C++11 where std::array is available, the answer is "yes, arrays should be avoided". Prior to C++11, you may need to use C arrays to allocate arrays in the automatic storage (i.e. on the stack).
Definitely, although with std::array in C++11, practically only for
static data. C style arrays have three important advantages over
std::vector:
They don't require dynamic allocation. For this reason, C style
arrays are to be preferred where you're likely to have a lot of very
small arrays. Say something like an n-dimension point:
template <typename T, int dims>
class Point
{
T myData[dims];
// ...
};
Typically, one might imagine a that dims will be very small (2 or 3),
T a built-in type (double), and that you might end up with
std::vector<Point> with millions of elements. You definitely don't
want millions of dynamic allocations of 3 double.
The support static initialization. This is only an issue for static
data, where something like:
struct Data { int i; char const* s; };
Data const ourData[] =
{
{ 1, "one" },
{ 2, "two" },
// ...
};
This is often preferable to using a vector (and std::string), since it
avoids all order of initialization issues; the data is pre-loaded,
before any actual code can be executed.
Finally, related to the above, the compiler can calculate the actual
size of the array from the initializers. You don't have to count them.
If you have access to C++11, std::array solves the first two issues,
and should definitely be used in preference to C style arrays in the
first case. It doesn't address the third, however, and having the
compiler dimension the array according to the number of initializers is
still a valid reason to prefer C style arrays.
Never say "never", but I'd agree that their role is greatly diminished by true data structures from STL.
I'd also say that encapsulation inside objects should minimize the impact of choices like this. If the array is a private data member, you can swap it in or out without affecting clients of your class.
I have worked on safety critical systems where you are unable to use dynamic memory allocation. The memory has to always be on the stack. Therefore in this case you would use arrays as the size is fixed at compile time.
array in c++ gives you fixed size fast alternative of dynamic sized std::vector and std::list. std::array is one of the additions in c++11. It provides the benefit of std containers while still providing the aggregate type semantics of C-style arrays.
So in c++11 i'd certainly use std::array, where it is required, over vector. But i'd avoid C style array in C++03.
Most usually, no, I can't think of a reason to use raw arrays over, say, vectors. If the code is new.
You might have to resort to using arrays if your libraries need to be compatible with code that expects arrays and raw pointers.
I know a lot of people are pointing out std::array for allocating arrays on the stack, and std::vector for the heap. But neither seem to support non-native alignment. If you're doing any kind of numeric code that you want use SSE or VPX instructions on (thus requiring 128 or 256 byte alignment respectively), C arrays would still seem to be your best bet.
I would say arrays are still useful, if you are storing a small static amount of data why not.
The only advantage of an array (of course wrapped in something that will manage automatically its deallocation when need) over std::vector I can think about is that vector cannot pass ownership of its data, unless your compiler supports C++11 and move constructors.
C style arrays are a fundamental data structure, so there will be cases when it is better to use it. For the general case, however, use the more advanced data structures that round off the corners of the underlying data. C++ allows you to do some very interesting and useful things with memory, many of which work with simple arrays.
You should use STL containers internally, but you should not pass pointers to such containers between different modules, or you will end up in dependency hell. Example:
std::string foo;
// fill foo with stuff
myExternalOutputProc(foo.c_str());
is a very good solution but not
std::string foo;
// fill foo with stuff
myExternalOutputProc(&foo);
The reason is that std::string can be implemented in many different ways but a c-style string is always a c-style string.
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.
boost::array (or the tr1or std version) offer some nice additional features over a built-in array.
Up to now, our codebase only contains built-in arrays, for example (made up, but the style matches):
WORD m_lastReadFlags[FLAGS_MAX];
...
WORD flagBuffer[FLAGS_MAX];
if (getFlags(flagBuffer)) {
memcpy(m_lastReadFlags, flagBuffer, sizeof(m_lastReadFlags));
...
I think one'll get the idea.
Now, my question is, for those places in the code, where dropping in boost::array would make sense (because of other changes made), is array a 100% semantics preserving drop-in for the built in array? (Possible compiler errors are OK -- only silent behavioral changes are what's bothering me.)
That is, could above code be re-written (for example) to use:
boost::array<WORD, FLAGS_MAX> m_lastReadFlags;
and the memcpy (possibly adapted to use c_array()or data()) and other array-like access would remain the same? Yes, of course I could also replace the local buffer by an array and remove the memcpy or use std::copy or something like that, but the point of this question is about the compatibility of built-in arrays and the array class.
Update: One thing that's bothering me specifically is the places (like in the memcpy case) where the built-in arrays are used as pointers. Will all occurences be caught by the
compiler / handled correctly?
What about assignment?
T arr1[N]; // or array<T, N>
T arr2[N]; // or array<T, N>
T* p1;
...
// Note, not all combinations will compile:
arr1 = arr2;
p1 = arr1;
arr2 = p1;
...
Yes, that should be fine, since the array class is precisely a wrapper for an automatic array. It has the same access syntax with square brackets, and if you need to get at the pointer, you know how to do it. You can even use std::copy everywhere and use iterators; chances are that that will be implemented by memcpy anyway.
The array class is of aggregate type (no non-trivial constructors/destructor/assignment), so you can initialize it with the traditional aggregate (brace) initializer, just like a plain array.