What is the difference between new/delete and malloc/free?

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new / delete

  • Allocate / release memory
    1. Memory allocated from ‘Free Store’.
    2. Returns a fully typed pointer.
    3. new (standard version) never returns a NULL (will throw on failure).
    4. Are called with Type-ID (compiler calculates the size).
    5. Has a version explicitly to handle arrays.
    6. Reallocating (to get more space) not handled intuitively (because of copy constructor).
    7. Whether they call malloc / free is implementation defined.
    8. Can add a new memory allocator to deal with low memory (std::set_new_handler).
    9. operator new / operator delete can be overridden legally.
    10. Constructor / destructor used to initialize / destroy the object.

malloc / free

  • Allocate / release memory
    1. Memory allocated from ‘Heap’.
    2. Returns a void*.
    3. Returns NULL on failure.
    4. Must specify the size required in bytes.
    5. Allocating array requires manual calculation of space.
    6. Reallocating larger chunk of memory simple (no copy constructor to worry about).
    7. They will NOT call new / delete.
    8. No way to splice user code into the allocation sequence to help with low memory.
    9. malloc / free can NOT be overridden legally.

Table comparison of the features:

Feature new / delete malloc / free
Memory allocated from ‘Free Store’ ‘Heap’
Returns Fully typed pointer void*
On failure Throws (never returns NULL) Returns NULL
Required size Calculated by compiler Must be specified in bytes
Handling arrays Has an explicit version Requires manual calculations
Reallocating Not handled intuitively Simple (no copy constructor)
Call of reverse Implementation defined No
Low memory cases Can add a new memory allocator Not handled by user code
Overridable Yes No
Use of constructor / destructor Yes No

Technically, memory allocated by new comes from the ‘Free Store’ while memory allocated by malloc comes from the ‘Heap’. Whether these two areas are the same is an implementation detail, which is another reason that malloc and new cannot be mixed.

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