What is a allocater?

[peak144]

What is it and what is it used for?

I'm a beginner so try to have some easy to understand info while being informative at the same time, i've google'd it but can't really understand it.

[kbw]

An allocator is an object that deals with memory model matters. It describes how to allocate/release storage and how to point to/reference an object in that storage.

[peak144]

is a memory model like a template for how memory is stored?

[Cubbi]

Allocator is a class that can do four main things:
1) obtain storage
2) construct objects in allocated storage
3) destruct objects
4) release storage

For example:

#include <memory>
#include <string>
#include <iostream>

int main()
{
    std::allocator<std::string> a;

    std::string* data = a.allocate(2); // allocate storage for 2 strings
    a.construct(data, "Hello"); // constructs a string
    a.construct(data+1, "world"); // constructs another string

    std::cout << data[0] << ", " << data[1] << '\n';

    a.destroy(data + 1); // destructs a string
    a.destroy(data); // destructs another string
    a.deallocate(data, 2); // frees storage
}

Whenever you're using a vector, a string, or another C++ container, or a shared pointer, or anything else that needs to allocate memory at runtime, you are using std::allocator and you're provided with an opportunity to substitute your own.

This std::allocator is not very interesting, it just calls new and delete, but user-defined allocators can be a whole lot more interesting - they can obtain storage from a memory pool (which can be very fast for fixed-size objects), from shared memory (so that many programs can access the same vector), from stack (again, fast), from thread-local storage, from other sources.

At my work, for example, there are special memory areas that are bound to user id - if I create a vector with an allocator that uses that area, the user can log out, log back in, restart my program, and the vector is already there.