Efficient Memory Management with Custom Allocators
The Power of C++11
With the improvements introduced in C++11, writing custom allocators has become more efficient than ever. The std::allocator_traits provides reasonable defaults, making it easier to configure and implement allocators. Understanding these traits is crucial to creating effective allocators.
A Minimal Custom Allocator: Mallocator
One famous example of a minimal custom allocator is Mallocator, first published by Stephan T. Lavavej. This allocator uses malloc() and free() to demonstrate how to write a custom allocator. Updated for C++11, Mallocator has become even slimmer. Here’s how it looks:
cpp
template <class T>
struct Mallocator {
//...
};
Understanding Mallocator
Mallocator is a stateless allocator, meaning it doesn’t have any mutable state. Instead, it relies on global functions for allocation and deallocation, namely malloc() and free(). This design ensures that memory allocated with Mallocator can be deallocated with any instance of Mallocator.
The Limitations of Stateless Allocators
While stateless allocators are simple to write, they are limited by their dependence on global state. To overcome this limitation, we need a stateful allocator that can reference a specific arena instance.
Using Arenas for Efficient Memory Management
Arenas are stack-allocated objects that can be used to manage memory efficiently. By creating a custom allocator that uses an arena instance, we can avoid unnecessary heap allocations and improve performance. This approach is particularly useful when working with small amounts of data that fit on the stack.
Alternatives to Heap Allocations
Besides using custom allocators, there are other alternatives to heap allocations. One approach is to build custom containers that use small object optimization, similar to std::string. Another option is to use containers from Boost, such as boost::container::small_vector, which is based on LLVM’s small vector.
Implementing a Stateful Allocator
To create a stateful allocator, we need to hold a reference to the stack-allocated arena object. This custom allocator can then use the arena for allocations. Our implementation is inspired by Howard Hinnant’s short_alloc:
cpp
template <class T, size_t N>
struct ShortAlloc {
//...
};
By using custom allocators and arenas, we can optimize memory management and improve performance in our applications.