Allocating object to predefined address in c++ without placement new method

Question

Is it possible in c++ to allocate an object at a specific place in memory? I'm implementing my hobby os kernel memory manager which gives void* addresses to store my stuff and I would like to know how can I use that pointer to allocate my object there. I have tried this:

string* s = (string*)235987532//Whatever address is given.
*s = string("Hello from string\n\0");//My own string class
//This seems to call the strings destructor here even thought I am using methods from s after this in my code...

The only problem with this is that it calls string objects destructor which it shouldn't do. Any help is appreciated.

EDIT: I cannot use placement new because I'm developing in kernel level.

`I cannot use placement new because I'm developing in kernel level.` Eh? — Lightness Races in Orbit, Feb 03 '14 at 12:46
Related: http://stackoverflow.com/questions/21288582/can-we-do-assign-or-access-a-value-explicitly-to-or-from-specific-address-locati — herohuyongtao, Feb 03 '14 at 12:46
You've created a temporary `string`, its destructor _should_ be called. Why do you think it shouldn't? — CB Bailey, Feb 03 '14 at 12:48
You should really explain *why* you cannot use placement `new` - it's *the* feature designed for this exact use. — Angew is no longer proud of SO, Feb 03 '14 at 13:05
@Angew Because I the only libraries I can use right now are stdint and stddef. I can't even use new method because I currently have only stack available and I'm trying to implement the new method. — The amateur programmer, Feb 04 '14 at 14:56

score 5 · Answer 1 · answered Feb 03 '14 at 12:44

5

Assignment only works if there's a valid object there already. To create an object at an arbitrary memory location, use placement new:

new (s) string("Hello");

Once you've finished with it, you should destroy it with an explicit destructor call

s->~string();

UPDATE: I just noticed that your question stipulates "without placement new". In that case, the answer is "you can't".

answered Feb 03 '14 at 12:44

Mike Seymour

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Didnt even know that it is possible to call the destructor explicitily :) – Sebastian Hoffmann Feb 03 '14 at 12:57

score 4 · Answer 2 · answered Feb 03 '14 at 12:43

4

You need to use placement new. There is not an alternative, because this is precisely what placement new is for.

answered Feb 03 '14 at 12:43

John Zwinck

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score 2 · Accepted Answer · answered Feb 04 '14 at 15:21

2

I think you should be able to first implement the operator new used by placement new:

void* operator new (std::size_t size, void* ptr) noexcept
{
  return ptr;
}

(See [new.delete.placement] in C++11)

Then, you can use placement new the way it's intended to.

answered Feb 04 '14 at 15:21

Angew is no longer proud of SO

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score 0 · Answer 4 · edited May 23 '17 at 12:32

0

you can use placement new allocation

void* memory = malloc(sizeof(string));
string* myString = new (memory) string();

source: What is an in-place constructor in C++?

edited May 23 '17 at 12:32

Community

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answered Feb 03 '14 at 12:44

Creris

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But bizarrely the OP stipulated "without placement new." – John Zwinck Feb 03 '14 at 12:45
`malloc` is likely an over-simplification of requirements here. Either he needs a more hand-crafted memory allocation method, or none at all. – Lightness Races in Orbit Feb 03 '14 at 12:46
@John he edited it after I post, now what Mike said applies, you cant(what he does is Undefined Behvaiour); Lightness yes, I just "copied" the solution from the link – Creris Feb 03 '14 at 12:47

Allocating object to predefined address in c++ without placement new method

4 Answers4