I've got a lightweight templated class that contains a couple of member objects that are very rarely used, and so I'd like to avoid calling their constructors and destructors except in the rare cases when I actually use them.
To do that, I "declare" them in my class like this:
template <class K, class V> class MyClass
{
public:
MyClass() : wereConstructorsCalled(false) {/* empty */}
~MyClass() {if (wereConstructorsCalled) MyCallPlacementDestructorsFunc();}
[...]
private:
bool wereConstructorsCalled;
mutable char keyBuf[sizeof(K)];
mutable char valBuf[sizeof(V)];
};
... and then I use placement new and placement delete to set up and tear down the objects only when I actually need to do so.
Reading the C++ FAQ it said that when using placement new, I need to be careful that the placement is properly aligned, or I would run into trouble.
My question is, will the keyBuf and valBuf arrays be properly aligned in all cases, or is there some extra step I need to take to make sure they will be aligned properly? (if so, a non-platform-dependent step would be preferable)
There's no guarantee that you'll get the appropriate alignment. Arrays are in general only guaranteed to be aligned for the member type. A char array is aligned for storage of char.
The one exception is that char and unsigned char arrays allocated with new are given maximum alignment, so that you can store arbitrary types into them. But this guarantee doesn't apply in your case as you're avoiding heap allocation.
TR1 and C++0x add some very helpful types though:
std::alignment_of and std::aligned_storage together give you a portable (and functioning) answer.
std::alignment_of<T>::value gives you the alignment required for a type T. std::aligned_storage<A, S>::type gives you a POD type with alignment A and size S. That means that you can safely write your object into a variable of type std::aligned_storage<A, S>::type.
(In TR1, the namespace is std::tr1, rather than just std)
May I ask why you want to place them into a char buffer? Why not just create pointer objects of K and V then instantiate it when you need it.
Maybe I didn't understand your question, but can't you just do char *keyBuf[..size..];, set it initially to NULL (not allocated) and allocate it the first time you need it?
What you're trying to do with placement new seems risky business and bad coding style.
Anyway, code alignment is implementation dependent.
If you want to change code alignment use pragma pack
#pragma pack(push,x)
// class code here
#pragma pack(pop) // to restore original pack value
if x is 1, there will be no padding between your elements.
Heres a link to read
http://www.cplusplus.com/forum/general/14659/
I found this answer posted by SiCrane at http://www.gamedev.net/community/forums/topic.asp?topic_id=455233 :
However, for static allocations, it's less wasteful to declare the memory block in a union with other types. Then the memory block will be guaranteed to be aligned to the alignment of the most restrictive type in the union. It's still pretty ugly either way.
Sounds like a union might do the trick!
I recommend that you look at the boost::optional template. It does what you need, even if you can't use it you should probably look at its implementation.
It uses alignment_of and type_with_alignment for its alignment calculations and guarantees.
To make a very very long story very very short this isn't going to help your performance any and will cause lots of headaches and it won't be long before you get sucked into writing your own memory managemer.
Placement new is fine for a POD (but won't save you anything) but if you have a constructor at all then it's not going to work at all.
You also can't depend on the value of your boolean variable if you use placement new.
Placement new has uses but not really for this.
Related
I often encounter some parts of phobos that are classes, but usually I do not need polymorphism.
Like for example Fiber
I would like to write
Array!Fiber fibers;
But then the fibers should be allocated directly in the array.
I have looked at scoped but it then disables the copy constructor and I am not allowed to move the types, which makes it pretty useless to me.
it's illegal to move a class reference even if you are sure there are no pointers to it. As such, it is illegal to move a scoped object.
Edit:
Is this the correct way of allocating a class inside a struct?
struct Struct(T)
if(is(T == class)){
static immutable size = __traits( classInstanceSize, T);
void[size] buffer;
T t;
alias t this;
this(Args...)(auto ref Args args){
import std.conv: emplace;
t = emplace!T(buffer, args);
}
}
Short answer: https://dlang.org/library/std/conv/emplace.html (make it array of void chunks and emplace into array elements).
Correct answer: you don't really want to do it in this case and it doesn't make much sense :) Most of fiber memory taken comes from stack allocation and that isn't part of Fiber object but allocated separately: https://github.com/D-Programming-Language/druntime/blob/master/src/core/thread.d#L4320
That means you don't benefit much from having Fiber instances in contiguous memory chunk as fragmentation remains. At the same time you risk a lot by hacking around class inherent polymorphic semantics and add a point of risk in your code that will need extra care/attention all the time.
In my C++ project, there is a class which needs to create an array of objects.
Between different instances of the class, the size of the array will be different, which is why I chose to use an array.
If I do :
int numberOfPlayers; // This is determined at run time.
int *players;
//In constructor
players= new int[numberOfPlayers]; // This works
But if I do:
Character *players;
players = new Character[numberOfPlayers]; // Compiler complains
The Compiler complains "No matching constructor for initialisation of Character"
How do I dynamically declare an array of type "Character".
Note: Character has nothing to do with char. Character is a name of an class I created myself.
EDIT: Character does not have a default constructor, since it needs to be passed several arguments so it can be initialised with the proper state. The only constructor is has takes several arguments.
EDIT: I chose a dynamically created array, over a vector since I know during the lifetime of the instance, the size of the array will be constant, though between different instances the size will be different. I thought this would make sense for performance reasons (memory / speed).
The "proper" way is to use std::vector. It is a fast, safe, more robust alternative to horrible new.
std::vector<Character> vec;
vec.push_back(Character(params));
vec.push_back(Character(other_params));
If you know the size ahead, you can avoid reallocation overhead by using std::vector::reserve
std::vector<Character> vec;
vec.reserve(50);
vec.push_back(Character(params));
vec.push_back(Character(other_params));
The overhead of std::vector is practically non-existent.
Now, the reason why you can't do this your way, it's because by default new uses default constructor, and it doesn't exist.
The problem is that your type Character does not define a default constructor of the form:
Character::Character()
{
// etc.
}
Your type needs a default constructor. Unlike C's malloc, operator new constructs instances for you at the time of allocation. It then follows that it requires a parameterless (default) constructor as it provides no way to pass arguments. So...
class Character
{
public:
Character(){}
};
"Character does not have a default constructor, since it needs to be passed several arguments so it can be initialised with the proper state. The only constructor is has takes several arguments"
Then an array is the wrong type to use, because an array will always default-construct its members.
Use:
std::vector<Character> players;
The size can vary, as you wanted, and you can call players.push_back( character ) when each character has been constructed
The short answer is that you can't do this because the standard doesn't allow it. There's no technical reason it couldn't be allowed--it just isn't.
Some compilers (e.g., gcc) have supported it as an extension to C++ for years. It's also been supported in C by various compilers for long enough that C99 standardized it, so all (reasonably current) C compilers now support it.
There was a proposal to add an array-like class with a size determined when it was created, and remaining constant after that, but the committee decided against accepting it1. That leaves only std::array, which requires a size determined at compile time, and std::vector, with a size that can vary dynamically at run time.
In fairness, however, if you do know the size of a vector when you create it, you can specify the size at creation time. While it's certainly still capable of resizing itself, that functionality is mostly in resize and push_back. If you simply don't use those, the overhead from using std::vector compared to a native array is generally quite minimal anyway, so you're unlikely to see significant gains from other techniques (except, perhaps, under rather obscure circumstances).
1. At least IMO, this was the right decision--although I can see the basic reasoning behind the idea, I think the proposal was sufficiently flawed that we're better off without it.
Because allocating an array of Character means that the array will contain an amount of Character instances. When you allocate the array, each instance contained must be initialized somehow and the default constructor is required.
You have to declare Character::Character() { } to let the compiler invoke it. If you can't provide a default constructor then you should consider using a Character** so that you can init them as you wish, eg:
Character **array = new Character*[amount];
array[0] = new Character(...);
Mind that this requires to delete each instance, so instead of having delete[] array you will need
for (int i = 0; i < amount; ++i)
delete array[i];
delete [] array;
An additional way would be to forget about arrays and use a std::vector:
vector<Character> character;
character.push_back(Character(...));
Which would also relief the need to manage memory by yourself.
Say, if I want to create a vector type of only for holding POD structures and regular data types. Can I do the following? It looks very unsafe but it works. If it is, what sort of issues might arise?
template <size_t N>
struct Bytes {
char data[N];
};
std::vector<Bytes<sizeof(double)> > d_byte_vector;
std::vector<double>* d_vectorP = reinterpret_cast<std::vector<double>*>(&d_byte_vector);
for (int i=0;i<50;i++) {
d_vectorP->push_back(rand()/RAND_MAX);
}
std::cout << d_vectorP->size() << ":" << d_byte_vector.size() << std::endl;
No, this is not safe. Specific compilers may make some guarantee that is particular to that dialect of C++, but according to the Standard you evoke Undefined Behavior by instantiating an array of char and pretending it's actually something else completely unrelated.
Usually when I see code like this, the author was going for one of three things and missed one of the Right Ways to go about it:
Maybe the author wanted an abstract data structure. A better way to go about that is to use an abstract base class, and move the implementation elsewhere.
Maybe the author wanted an opaque data structure. In that case I would employ some variant of the pimpl idiom, where the void* (or char*) presented to the user actually points to some real data type.
Maybe the author wanted some kind of memory pool. The best way to accomplish that is to allocate a large buffer of char, and then use placement-new to construct real objects within it.
No, this is not safe and generally not recommended because compilers aren't required to operate in a method that allows it. With that said, I've found exactly one reason to ever do this (very recently as well) and that is a variant of the pimpl idiom where I wanted to avoid pointers so that all of my data access could avoid the need to allocate memory for, deallocate memory for, and dereference the extra pointer. That code isn't in production yet and I'm still keeping an eye on that section of code to make sure that it doesn't start causing any other problems.
Unless you're generating code that has to be Extremely optimized, I would recommend finding some other way of doing whatever it is you need to do.
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.