move C++ object to memory location

Question

I probably will need to do something similar to what std::vector doing:

T *mem = malloc(...); // notice this is just memory allocation.
T t;
move... t to mem
mem->doSomething();

How I can move t over allocated memory?

How can I move object from allocated memory onto new variable.

How can I delete object from allocated memory - call d-tor manually?

Must I use placement new and assignment operators?

refactoring something from C. It is something very similar to std::vector. Currently have pointers to Objects, but I want to "put" objects inside the "array". Suppose you need to insert in the middle, at the moment I memmove lots of pointers and insert new pointer in the "hole". — Nick, Jul 24 '15 at 13:48
Are you looking for [placement new](http://stackoverflow.com/questions/222557/what-uses-are-there-for-placement-new)? — Michael, Jul 24 '15 at 13:48
I am exploring possibilities for optimisation. Pls don't tell me to use std::vector :) — Nick, Jul 24 '15 at 13:49
Allocate your objects using `new`, then put the pointer inside the array (if you know what you are doing and casting always correct, you can even use void pointers to store whatever object you want) — Nidhoegger, Jul 24 '15 at 13:50
@Michael - well not sure how it will be faster. I could do a new object with placement new and copy c-tor. but is there other better way? — Nick, Jul 24 '15 at 13:51
@Nidhoegger - this is what I already have. But I ant to remove pointers and put actual objects into the array. — Nick, Jul 24 '15 at 13:52
@Nick: Is `T` a class of your own? If so, you could define a custom allocator for it. — Michael, Jul 24 '15 at 13:56

score 1 · Accepted Answer · answered Jul 24 '15 at 13:49

1

Like this, but if you don't know what you're doing then I would avoid doing this:

#include <new>
#include <stdlib.h>

void* mem = malloc(sizeof(T));
T t;
T* tt = new(mem) T( std::move(t) );
tt->doSomething();
tt->~T();
free(mem);

You can't literally "move" t to that memory, t will always be where it was created, but you can create another object at that location, as a copy of t (using a move constructor to make the copy if it has one, but that still leaves t where it was and still existing).

answered Jul 24 '15 at 13:49

Jonathan Wakely

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Still existing, but possibly empty. – nwp Jul 24 '15 at 13:53
Excellent, can you just include the code how to move tt over t , e.g. memory over normal variable. – Nick Jul 24 '15 at 13:57
@Nick, `t = tt`. if you mean something else you'll have to explain what you mean. I have no idea what "move memory over a variable" means. – Jonathan Wakely Jul 24 '15 at 13:58
@nwp yeah for some value of empty :) – Jonathan Wakely Jul 24 '15 at 14:00
@JonathanWakely - move object pointed by tt to local variable, so I do not need to call destructor manually. does T t2 = std::move(*tt) will work ? – Nick Jul 24 '15 at 14:10
1

@Nick No. You should still destroy `tt` if you don't like leaks. You can skip it if you can guarantee that the destructor is a noop after `T` has been moved from, but that is very fragile code. – nwp Jul 24 '15 at 14:16
Let me summarize if I understood correctly - if I `std::move`, it will probably `swap`, And I still need to call the d-tor or if I don't, there will be a leak. Is that correct? – Nick Jul 24 '15 at 14:21
1

@Nick, close enough. `std::move` does not destroy anything, it just casts an object to an rvalue reference, which (depending on what you do with it) might allow it to be the argument to a move constructor or move assignment operator. But that still doesn't destroy anything, the object still exists. – Jonathan Wakely Jul 24 '15 at 14:34

score 1 · Answer 2 · answered Jul 24 '15 at 13:55

You can't move a live object, but you can move-construct another from it.

int main() {
    std::string str = "Hello World !";

    // Allocate some storage
    void *storage = std::malloc(sizeof str);

    std::cout << str << '\n';

    // Move-contruct the new string inside the storage
    std::string *str2 = new (storage) std::string(std::move(str));

    std::cout << str << '|' << *str2 << '\n';

    // Destruct the string and free the memory
    str2->~basic_string();
    free(storage);
}

Output:

Hello World !
|Hello World !

move C++ object to memory location

2 Answers2