When new has an object, space for the object is not only allocated but the object's constructor is called. And similarly when delete as an object, the object's destructor is called before the memory is released. If malloc and free are used, the destructor and constructor do not get called respectively and obviously, this simply won't do in C++ except in certain very rare situations where classes are present without any specific destructor/constructors.
Operator new automatically calculates the size of the object that it constructs. Conversely, with malloc(), the programmer has to specify explicitly the number of bytes that have to be allocated. In addition, malloc() returns void *, which has to be explicitly cast to the desired type. This is both tedious and dangerous. Operator new returns a pointer to the desired type, so no explicit typecast is required.
new and delete automatically construct and destroy objects. malloc() and free(), on the other hand, merely allocate and deallocate raw memory from the heap. delete can be overloaded, free cannot. delete invokes the destructor of the object to be deallocated, free does not do this.
The difference could be qouted as -
1. Operator new constructs an object (calls constructor of object), malloc does not.
2. Operator new is an operator, malloc is a function.
3. Operator new can be overloaded, malloc cannot be overloaded.
4. Operator new throws an exception if there is not enough memory, malloc returns a NULL.
5. Operator new[] requires to specify the number of objects to allocate, malloc requires to specify the total number of bytes to allocate.
6. malloc() returns void *, which has to be explicitly cast to the desired type but new returns the proper type.
7. Operator new/new[] must be matched with operator delete/delete[] to deallocate memory, malloc() must be matched with free() to deallocate memory.
8. The new/delete couple does not have a realloc alternative that is available when malloc/free pair is used. realloc is used to resize the length of an array or a memory block dynamically.