Which to use - "operator new" or "operator new[]" - to allocate a block of raw memory in C++?

Question

My C++ program needs a block of uninitialized memory and a void* pointer to that block so that I can give it to a third party library. I want to pass control of the block lifetime to the library, so I don't want to use std::vector. When the library is done with the block it will call a callback that I have to supply and that will deallocate the block. In C I would use malloc() and later free().

In C++ I can either call ::operator new or ::operator new[] and ::operator delete or operator delete[] respectively later:

void* newBlock = ::operator new( sizeOfBlock );
// then, later
::operator delete( newBlock );

Looks like both ::operator new and ::operator new[] have exactly the same signature and exactly the same behavior. The same for ::operator delete and ::operator delete[]. The only thing I shouldn't do is pairing operator new with operator delete[] and vice versa - undefined behavior. Other than that which pair do I choose and why?

Answer 1

Use new with a single object and new[] with an array of objects. So, for example:

int* x = new int; // Allocates single int
int* y = new int[5]; // Allocates an array of integers

*x = 10; // Assignment to single value
y[0] = 8; // Assignment to element of the array

If all you are doing is allocating a memory buffer, then allocate an array of char as in:

int bufferlen = /* choose a buffer size somehow */
char* buffer = new char[bufferlen];
// now can refer to buffer[0] ... buffer[bufferlen-1]

However, in C++, you should really use std::vector for arbitrary arrays, and you should use std::string for character arrays that are to be interpreted or used as strings.

There is no reason to invoke ::operator new or ::operator new[] explicitly rather than using the ordinary syntax for these calls. For POD and primitive types (e.g. char) no initialization will take place. If you need to get a void* buffer, then simply use static_cast to convert char* to void*.

Answer 2

The advantage of the C++ new operators over C's malloc() and free() is that the former throws an exception if there is not enough memory, rather than returning NULL.

Regarding choosing new(size) and new[] for character buffers, I'd advocate the latter since it is less likely to surprise people maintaining the code later i.e. char* buf = new char[size] and delete[] buf.

The values in the buffer will not be initialised, and there is no range-checking - you have to build a nice C++ object to do that for you, or use an existing object such as std::vector or std::string.

Answer 3

The question cannot be answered sensibly.

Firstly, it is said that the program 'needs' a block of uninitialized memory but, from the code sample given, it seems that the program 'needs' a block of uninitialized and UNTYPED memory which seems not very C++ or OO.

Secondly, a std::vector gives sole and automatic control over a block of typed memory that may or may not change in size according to its use. You can lose this control if an instance of std::vector is created on the heap and tracked with raw pointers just as for any other C or C++ object such as a void* memory block.

Thirdly, what is the intended use of this memory block? The answer to this may or may not dictate the use of operator new or operator new[]. In the design of this program, is there a single interpretation of this memory block? What ownership semantics do you require, if any? Etc, etc.