dynamically allocating a c++ object without using the new operator

Question

(C++/Win32)

consider the following call:

Object obj = new Object(a,b);

other than allocating the virtual memory needed for an instance of an Object, what else is going on under the hood up there? does the compiler places an explicit call to the constructor of Object?

is there any way to initialize a c++ object dynamically without the use of the keyword new?

Define "dynamic". `{ Object obj{a,b}; }` allocates an object as well, and destroys it, without any usage of `new` or `delete`. — WhozCraig, Sep 24 '13 at 15:44
And you really should explain why you ask! Notice that many standard containers are constructing objects.... — Basile Starynkevitch, Sep 24 '13 at 16:35

score 4 · Accepted Answer · edited May 23 '17 at 10:25

If you want to initialize an object in some given memory zone, consider the placement new (see this)

BTW, the ordinary Object* n = new Object(123) expression is nearly equivalent to (see operator ::new)

 void* p = malloc(sizeof(Object));
 if (!p) throw std::bad_alloc; 
 Object* n = new (p) Object(123); // placement new at p, 
                                  // so invokes the constructor

^{But the implementation could use some non-malloc compatible allocator, so don't mix new and free!}

score 1 · Answer 2 · edited May 23 '17 at 11:56

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You can always use malloc instead of new, but don't forget to always couple it with free and not delete.

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

edited May 23 '17 at 11:56

Community

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answered Sep 24 '13 at 15:43

Oragon Efreet

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This does not invoke the constructor. – Dark Falcon Sep 24 '13 at 15:45
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... or the destructor with `free()` – WhozCraig Sep 24 '13 at 15:45

dynamically allocating a c++ object without using the new operator

2 Answers2