Are there STL implementations that use operator new[] as an allocator? On my compiler, making Foo::operator new[] private did not prevent me from creating a vector<Foo>... is that behavior guaranteed by anything?
C++ Standard, section 20.4.1.1. The default allocator allocate() function uses global operator new:
pointer allocate(size_type n, allocator<void>::const_pointerhint=0);
3 Notes: Uses ::operator new(size_t) (18.4.1).
std library implementations won't use T::operator new[] for std::allocator. Most of them use their own memory pooling infrastructure behind the scenes.
In general, if you want to stop Foo objects being dynamically allocated, you'll have to have make all the constructors private and provide a function that creates Foo objects. Of course, you won't be able to create them as auto variables either though.
std::vector uses an Allocator that's passed as a template argument, which defaults to std::allocate. The allocator doesn't work like new[] though -- it just allocates raw memory, and placement new is used to actually create the objects in that memory when you tell it to add the objects (e.g. with push_back() or resize()).
About the only way you could use new[] in an allocator would be if you abused things a bit, and allocated raw space using something like new char[size];. As abuses go, that one's fairly harmless, but it's still unrelated to your overload of new[] for the class.
If you want to prohibit the creation of your object make private constructor rather than operator new.
In addition to the other answers here, if you want to prevent anyone from creating a STL container for your type Foo, then simply make the copy-constructor for Foo private (also the move-constructor if you're working with C++11). All STL-container objects must have a valid copy or move constructor for the container's allocator to properly call placement new and construct a copy of the object in the allocated memory block for the container.
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I know of storage classes in both C and C++ (static, extern, auto, register, C++ also adds mutable and some compiler-specific ones) but I can't figure out what a storage allocator is. I don't think it's referred to memory allocators implementable on STL, what is it in simple terms?
It's whatever is behind operator new and operator delete (not to be confused with the new operator and the delete operator). operator new allocates memory from the free store, and operator delete releases memory previously allocated by operator new for possible reuse. When code does foo *ptr = new foo (new operator), the compiler generates code that calls operator new to get the right number of bytes of storage, then calls the constructor for foo. When code does delete ptr (delete operator) the compiler calls the destructor for foo, then calls operator delete to release the memory.
Note that this is how the term is used in the C++03 standard. In the C++11 standard it is also used to refer to standard allocators.
In the C++ standard, that term is used to refer to the allocator class used by STL-style containers - either std::allocator, or a user-defined custom allocator that meets the requirements given by C++11 17.6.3.5.
However, it's not a formally defined term, and also appears once referring to the implementation of the free store - that is, the dynamic storage allocated by new.
[NOTE: I'm referring to the current (2011) language specification. As noted in the comments, historical versions of the specification apparently only used the term (informally) to refer to the free store]
I understand that there are 3 general ways to modify the behaviour of new and delete in C++:
Replacing the default new/delete and new[]/delete[]
Overriding or overloading the placement versions (overriding the one with a memory location passed to it, overloading when creating versions which pass other types or numbers of arguments)
Overloading class specific versions.
What are the restrictions for performing these modifications to the behaviour of new/delete?
In particular are there limitations on the signatures that new and delete can be used with?
It makes sense if any replacement versions must have the same signature (otherwise they wouldn't be replacement or would break other code, like the STL for example), but is it permissible to have global placement or class specific versions return smart pointers or some custom handle for example?
First off, don't confuse the new/delete expression with the operator new() function.
The expression is a language construct that performs construction and destruction. The operator is an ordinary function that performs memory (de)allocation.
Only the default operators (operator new(size_t) and operator delete(void *) can be used with the default new and delete expressions. All other forms are summarily called "placement" forms, and for those you can only use new, but you have to destroy objects manually by invoking the destructor. Placement forms are of rather limited and specialised need. By far the most useful placement form is global placement-new, ::new (addr) T, but the behavior of that cannot even be changed (which is presumably why it's the only popular one).
All new operators must return void *. These allocation functions are far more low-level than you might appreciate, so basically you "will know when you need to mess with them".
To repeat: C++ separates the notions of object construction and memory allocation. All you can do is provide alternative implementations for the latter.
When you overload new and delete within a class you are effectively modifying the way the memory is allocated and released for the class, asking for it to give you this control.
This may be done when a class wants to use some kind of pool to allocate its instances, either for optimisation or for tracking purposes.
Restrictions, as with pretty much any operator overload, is the parameter list you may pass, and the behaviour it is expected to adhere to.
Why can't it just be regular function calls? New is essentially:
malloc(sizeof(Foo));
Foo::Foo();
While delete is
Foo:~Foo();
free(...);
So why does new/delete end up having it's own syntax rather than being regular functions?
Here's a stab at it:
The new operator calls the operator new() function. Similarly, the delete operator calls the operator delete() function (and similarly for the array versions).
So why is this? Because the user is allowed to override operator new() but not the new operator (which is a keyword). You override operator new() (and delete) to define your own allocator, however, you are not responsible (or allowed to for that matter) for calling appropriate constructors and destructors. These function are called automatically by the compiler when it sees the new keyword.
Without this dichotomy, a user could override the operator new() function, but the compiler would still have to treat this as a special function and call the appropriate constructor(s) for the object(s) being created.
You can overload operator new and operator delete to provide your own allocation semantics. This is useful when you want to bypass the default heap allocator's behavior. For example if you allocate and deallocate a lot of instances of a small, fixed-size object, you may want to use a pool allocator for its memory management.
Having new and delete as explicit operators like other operators makes this flexibility easier to express using C++'s operator overloading mechanism.
For auto objects on the stack, allocation/constructor call and deallocation/destructor calls basically are transparent as you request. :)
'Cause there is no way to provide complie-time type safety with a function (malloc() returns void*, remember). Additionally, C++ tries to eliminate even a slightest chance of allocated but uninitialized objects floating around. And there are objects out there without a default constructor - for these, how would you feed constructor arguments to a function? A function like this would require too much of a special-case handling; easier to promote it to a language feature. Thus operator new.
'new/delete' are keywords in the C++ language (like 'for' and 'while'), whereas malloc/calloc are function calls in the standard C library (like 'printf' and 'sleep'). Very different beasts, more than their similar syntax may let on.
The primary difference is that 'new' and 'delete' trigger additional user code - specifically, constructors and destructors. All malloc does is set aside some memory for you to use. When setting aside memory for a simple plain old data (floats or ints, for example), 'new' and 'malloc' behave very similarly. But when you ask for space for a class, the 'new' keyword sets aside memory and then calls a constructor to initialize that class. Big difference.
Why does C++ have separate syntax for greater-than? Why can't it just be a regular function call?
greaterThan(foo, bar);
Looks like operator new and operator new[] have exactly the same signature:
void* operator new( size_t size );
void* operator new[]( size_t size );
and do exactly the same: either return a pointer to a big enough block of raw (not initialized in any way) memory or throw an exception.
Also operator new is called internally when I create an object with new and operator new[] - when I create an array of objects with new[]. Still the above two special functions are called by C++ internally in exactly the same manner and I don't see how the two calls can have different meanings.
What's the purpose of having two different functions with exactly the same signatures and exactly the same behavior?
In Design and Evolution of C++ (section 10.3), Stroustrup mentions that if the new operator for object X was itself used for allocating an array of object X, then the writer of X::operator new() would have to deal with array allocation too, which is not the common usage for new() and add complexity. So, it was not considered to use new() for array allocation. Then, there was no easy way to allocate different storage areas for dynamic arrays. The solution was to provide separate allocator and deallocator methods for arrays: new[] and delete[].
The operators can be overridden (for a specific class, or within a namespace, or globally), and this allows you to provide separate versions if you want to treat object allocations differently from array allocations. For example, you might want to allocate from different memory pools.
I've had a reasonably good look at this, and to be blunt there's no reason from an interface standpoint.
The only possible reason that I can think of is to allow an optimization hint for the implementation, operator new[] is likely to be called upon to allocate larger blocks of memory; but that is a really, really tenuous supposition as you could new a very large structure or new char[2] which doesn't really count as large.
Note that operator new[] doesn't add any magic extra storage for the array count or anything. It is the job of the new[] operator to work out how much overhead (if any) is needed and to pass the correct byte count to operator new[].
[A test with gcc indicates that no extra storage is needed by new[] unless the type of the array members being constructed have a non-trivial desctructor.]
From an interface and contract standpoint (other than require the use of the correct corresponding deallocation function) operator new and operator new[] are identical.
One purpose is that they can be separately defined by the user. So if I want to initialize memory in single heap-allocated objects to 0xFEFEFEFE and memory in heap-allocated arrays to 0xEFEFEFEF, because I think it will help me with debugging, then I can.
Whether that's worth it is another matter. I guess if your particular program mostly uses quite small objects, and quite large arrays, then you could allocate off different heaps in the hope that this will reduce fragmentation. But equally you could identify the classes which you allocate large arrays of, and just override operator new[] for those classes. Or operator new could switch between different heaps based on the size.
There is actually a difference in the wording of the requirements. One allocates memory aligned for any object of the specified size, the other allocates memory aligned for any array of the specified size. I don't think there's any difference - an array of size 1 surely has the same alignment as an object - but I could be mistaken. The fact that by default the array version returns the same as the object version strongly suggests there is no difference. Or at least that the alignment requirements on an object are stricter than those on an array, which I can't make any sense of...
Standard says that new T calls operator new( ) and new T[ ] results in a call of operator new[]( ). You could overload them if you want. I believe that there is no difference between them by default. Standard says that they are replaceable (3.7.3/2):
The library provides default definitions for the global allocation and deallocation functions. Some global
allocation and deallocation functions are replaceable (18.4.1). A C + + program shall provide at most one
definition of a replaceable allocation or deallocation function. Any such function definition replaces the
default version provided in the library (17.4.3.4). The following allocation and deallocation functions
(18.4) are implicitly declared in global scope in each translation unit of a program
void* operator new(std::size_t) throw(std::bad_alloc);
void* operator new[](std::size_t) throw(std::bad_alloc);
void operator delete(void*) throw();
void operator delete[](void*) throw();
For a project of mine, I am writing some STL containers from scratch (I have my reasons). Since I am mimicking the functionality and interfaces of the STL so closely I am doing my best to keep with the policy "if it has the same name as a standard construct, it will conform to the standard as much as possible."
So, of course my containers take allocators as a template parameter, which is very nice as it allows for some custom allocation schemes. On to my question.
The std::allocator interface separates memory allocation from object construction. Likewise it separates deallocation from destruction. This makes sense since where you get memory from is more or less irrelevant to properly constructing an object in c++.
So there are two construction/deallocation functions which look like this for the default implementation (lifted straight from a book):
void construct(pointer p, const T& val) { new(p) T(val); }
void destroy(pointer p) { p->~T(); }
as you can see construct simply calls placement new and destroy simply calls the destructor.
Is there any reason to use these over just using placement new and destructor syntax? can a "correct" allocator implement these in another way? Or am I guaranteed that all allocator implementations which conform to the standard will have there construct/destroy methods implemented in this way?
More to the point, is it safe to say that I can always use the std::uninitialized_copy and std::uninitialized_fill for constructing the elements of my containers?
Thanks.
The allocator could add logging statements before and after construction/destruction, or any other side effects it cared to do.
Of course the actual construction has to occur by calling placement new and the destructor, but it doesn't say in the rulebook that nothing else must happen in the construct/destroy functions
This is just to hide the details of allocation in a method. i.e., we are providing the APIs for construction and destruction, in the future we can change the implementation. Now we are using placement new to allocate the memory, in the future if we want to change the allocation we just have to change these two methods.