delete vs delete[] operators in C++

What is the difference between delete and delete[] operators in C++?

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.

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.

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 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[].

When I asked this question, my real question was, "is there a difference between the two? Doesn't the runtime have to keep information about the array size, and so will it not be able to tell which one we mean?" This question does not appear in "related questions", so just to help out those like me, here is the answer to that: "why do we even need the delete[] operator?"

C++ delete[] operator ensures that Destructor for all object allocated with new[] is called. The following example demonstrates the same. Also, delete[] must be preferred (if new[] used previously) when the class has a non-default destructor to release the acquired resources. Otherwise, it might result in memory leaks.

Common Code:-

#include <iostream>
using namespace std;

class memTest{
    public:
    static int num;
    memTest(){
cout<<"Constructor from object " << num++ << endl;
    }
    ~memTest(){
cout<<"Destructor from object " << --num << endl;        
    }
};
int memTest::num=0;

Example 1:- use of new[] and delete may result in undefined behavior.

int main() {
    memTest* Test1=new memTest[3];
    
    delete Test1; //<-----
    return 0;
} 

Output 1:-

Constructor from object 0
Constructor from object 1
Constructor from object 2
Destructor from object 2 //<-----

Example 2: The correct behavior is using new[] and delete[].

int main() {
    memTest* Test1=new memTest[3];
    
    delete[] Test1; //<-----
    return 0;
} 

Output 2:-

Constructor from object 0
Constructor from object 1
Constructor from object 2
Destructor from object 2
Destructor from object 1 //<-----
Destructor from object 0 //<-----

delete is used for one single pointer and delete[] is used for deleting an array through a pointer. This might help you to understand better.