在预先分配的内存中创建对象

Question

We can use placement new to create an object in pre-allocated memory.

Let's consider the following example:

char *buf  = new char[1000];   //pre-allocated buffer
string *p = new (buf) MyObject();  //placement new 
string *q = new (buf) MyObject();  //placement new

I've created two objects in the pre-allocated buffer. Are the two objects created randomly within the buffer or created in contiguous memory blocks? If we keep creating more objects in the buffer and want them to be stored in contiguous memory blocks, what should we do? Create an array in the buffer first and then create each object in the element slots of the array?

Answer 1

The two objects are both created at the same memory location, namely buf . This is undefined behaviour (unless the objects are POD).

If you want to allocate several objects, you have to increment the pointer, eg buf + n * sizeof(MyObject) , but beware of alignment issues

Also don't forget to call destructors when you're done.

Answer 2

The following line of code:

string *p = new(adr) MyObject();

will create a MyObject object at the address adr. Then next time you create another object, you will know that the memory at adr is being used by the first object, so your next object will have to be created at adr + sizeof(MyObject) :

string *q = new(adr + sizeof(MyObject)) MyObject();

The point of pre-allocated memory is so that you aren't allocating at runtime, which is pretty slow. You do one big allocation at the beginning of the loop/program and then you just have to use chunks of that allocation. The down side is that you have to manage your own memory which means you have to figure out where to put your objects which gets tricky when your memory pool gets fragmented!