Regarding the differences between malloc and new in terms of their respective mechanisms of handling memory allocation?

Question

What are the differences between malloc and new in terms of their respective mechanisms of handling memory allocation?

`malloc` doesn't throw `bad_alloc` exepetion as `new`, as I think — , Sep 30 '11 at 14:20
`malloc` and `free` do not call object constructors and destructors. — Alan, Sep 30 '11 at 14:20
http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.4 — Lundin, Sep 30 '11 at 14:24
Please see http://stackoverflow.com/questions/7443782/does-dynamic-memory-allocation-differ-in-c-and-c-in-popular-implementations/7444032#7444032 — NPE, Sep 30 '11 at 14:33

score 7 · Accepted Answer · edited May 23 '17 at 12:01

7

malloc doesn't throw bad_alloc exception as new does.
- Because malloc doesn't throw exceptions, you have to check its result against NULL (or nullptr in c++11 and above), which isn't necessary with new. However, new can be used in a way it won't throw expections, as when function set_new_handler is set
malloc and free do not call object constructors and destructors, since there is no objects in C.
see this question and this post.

edited May 23 '17 at 12:01

Community

1
1

answered Sep 30 '11 at 14:21

Robᵩ

163,533
20
239
308

Also, because malloc doesn't throw exceptions, you have to check its result against NULL, which isn't necessary with new. – Lundin Sep 30 '11 at 14:25
Two more points can be added: (1) placement `new` (2) Compatibility of `malloc/realloc`. – iammilind Sep 30 '11 at 14:41
@iammilind Ok add them by editing the answer – Sep 30 '11 at 14:45

score 2 · Answer 2 · answered Sep 30 '11 at 14:23

Well, malloc() is a more low-level primitive. It just gives you a pointer to n bytes of heap memory. The C++ new operator is more "intelligent" in that it "knows" about the type of the object(s) being allocated, and can do stuff like call constructors to make sure the newly allocated objects are all properly initialized.

Implementations of new often end up calling malloc() to get the raw memory, then do things on top of that memory to initalize the objecs(s) being constructed.

score 2 · Answer 3 · answered Sep 30 '11 at 14:25

Do you mean how they are implemented?

They can be implemented as anything, just malloc may not call new, and all standard news must call the global operator new(void*). Often new is even implemented as calling malloc but there is no requirement on how that is implemented. There are even dozens of allocators out there, each with some strengths and some weeknesses.

Or do you mean how they differ on the language level?

new throws (unless it is called with std::nothrow) on allocation error. new expressions (not the operator new) calls the ctor.
malloc returns 0 on allocation failures.

score 1 · Answer 4 · answered Sep 30 '11 at 14:24

If the call fails, new will throw an exception whereas malloc will return NULL.

For malloc, the caller has to specify the amount of memory to be allocated, while new automatically determines it.

These differences are concerning allocation, there are tons of others - new will call constructor, new can be overloaded, new is an operator whereas malloc is a function...

Regarding the differences between malloc and new in terms of their respective mechanisms of handling memory allocation?

4 Answers4