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


Question

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

Related (duplicate?): In what cases do I use malloc vs new?

1
275
5/23/2017 11:33:24 AM

Accepted Answer

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 (set_new_handler)
    9. operator new/delete can be overridden legally
    10. constructor/destructor used to initialize/destroy the object

malloc/free

  • Allocates/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 (con-)/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 can not be mixed.

456
8/23/2019 8:55:17 PM

The most relevant difference is that the new operator allocates memory then calls the constructor, and delete calls the destructor then deallocates the memory.


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