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I'm allocating memory that will later be used for constructing objects with placement new. Should I be using operator new(n), or should I be using new unsigned char[n]? Why?
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Placement new is a variation new operator in C++. Normal new operator does two things : (1) Allocates memory (2) Constructs an object in allocated memory. Placement new allows us to separate above two things. In placement new, we can pass a preallocated memory and construct an object in the passed memory.
The operator new (or better the void* operator new(size_t) variant) just allocate memory, but does not do any object construction. The new keyword calls the operator new function, but then calls the object constructor.
¶ Δ Probably not. Unless you used placement new , you should simply delete the object rather than explicitly calling the destructor.
When new is used to allocate memory for a C++ class object, the object's constructor is called after the memory is allocated. Use the delete operator to deallocate the memory allocated by the new operator.
Factors:
new[] must be matched with delete[] / new() with delete
operator new(n) is a request for memory for unspecified purposes, whereas new unsigned char[n] loosely implies intent to store characters there.The array form may be slightly worse performance / efficiency wise - exact details depending on your implementation:
5.3.4/12 new T[5] results in a call of operator new where x is a non-neagtive unspecified value representing array allocation overhead: the result of the new-expression will be offset by this amount from the value returned by
operator new[]....
BTW - neither are initialised:
operator new() returns a void* to uninitialised memory: see 3.7.4.1/2 "There are no constraints on the contents of the allocated storage on return from the allocation function", whereas 5.3.4/15 says "a new-expression that creates an object of type T initializes that object as follows: if the new-initializer is ommitted, the object is default-initialized (8.5)"; 8.5/6 says only class types default constructors provide initialisation.
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The former returns a pointer to some storage area. The latter returns a pointer to the first element of an array with some objects in it. If you need storage, use the one that gives storage and not objects.
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