delete vs delete[] operators in C++
The operators delete
and delete []
are used respectively to destroy the objects created with new
and new[]
, returning to the allocated memory left available to the compiler's memory manager.
Objects created with new
must necessarily be destroyed with delete
, and that the arrays created with new[]
should be deleted with delete[]
.
This the basic usage of allocate/DE-allocate pattern in c++
malloc
/free
, new
/delete
, new[]
/delete[]
We need to use them correspondingly. But I would like to add this particular understanding for the difference between delete
and delete[]
1) delete
is used to de-allocate memory allocated for single object
2) delete[]
is used to de-allocate memory allocated for array of objects
class ABC{}
ABC *ptr = new ABC[100]
when we say new ABC[100]
, compiler can get the information about how many objects that needs to be allocated(here it is 100) and will call the constructor for each of the objects created
but correspondingly if we simply use delete ptr
for this case, compiler will not know how many objects that ptr
is pointing to and will end up calling of destructor and deleting memory for only 1 object(leaving the invocation of destructors and deallocation of remaining 99 objects). Hence there will be a memory leak.
so we need to use delete [] ptr
in this case.
The delete[]
operator is used to delete arrays. The delete
operator is used to delete non-array objects. It calls operator delete[]
and operator delete
function respectively to delete the memory that the array or non-array object occupied after (eventually) calling the destructors for the array's elements or the non-array object.
The following shows the relations:
typedef int array_type[1];
// create and destroy a int[1]
array_type *a = new array_type;
delete [] a;
// create and destroy an int
int *b = new int;
delete b;
// create and destroy an int[1]
int *c = new int[1];
delete[] c;
// create and destroy an int[1][2]
int (*d)[2] = new int[1][2];
delete [] d;
For the new
that creates an array (so, either the new type[]
or new
applied to an array type construct), the Standard looks for an operator new[]
in the array's element type class or in the global scope, and passes the amount of memory requested. It may request more than N * sizeof(ElementType)
if it wants (for instance to store the number of elements, so it later when deleting knows how many destructor calls to done). If the class declares an operator new[]
that additional to the amount of memory accepts another size_t
, that second parameter will receive the number of elements allocated - it may use this for any purpose it wants (debugging, etc...).
For the new
that creates a non-array object, it will look for an operator new
in the element's class or in the global scope. It passes the amount of memory requested (exactly sizeof(T)
always).
For the delete[]
, it looks into the arrays' element class type and calls their destructors. The operator delete[]
function used is the one in the element type's class, or if there is none then in the global scope.
For the delete
, if the pointer passed is a base class of the actual object's type, the base class must have a virtual destructor (otherwise, behavior is undefined). If it is not a base class, then the destructor of that class is called, and an operator delete
in that class or the global operator delete
is used. If a base class was passed, then the actual object type's destructor is called, and the operator delete
found in that class is used, or if there is none, a global operator delete
is called. If the operator delete
in the class has a second parameter of type size_t
, it will receive the number of elements to deallocate.
The delete
operator deallocates memory and calls the destructor for a single object created with new
.
The delete []
operator deallocates memory and calls destructors for an array of objects created with new []
.
Using delete
on a pointer returned by new []
or delete []
on a pointer returned by new
results in undefined behavior.