new/delete和malloc/free的比较

时间：2022-01-07 18:11:49

从C++角度上说，使用new分配堆空间可以调用类的构造函数，而malloc()函数仅仅是一个函数调用，它不会调用构造函数，它所接受的参数是一个unsigned long类型。同样，delete在释放堆空间之前会调用析构函数，而free函数则不会。

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 details, which is another reason that malloc and new can not be mixed.

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