More C++ Idioms/Fast Pimpl

Increase performance of Handle Body idiom.

Regular PIMPL idiom achieves "Compilation Firewall" by sacrificing performance. Fast PIMPL attempts to reduce the overhead of heap allocation and non-local memory access by composing the implementation object within the original interface object.

// Wrapper.hpp
struct Wrapper {
    Wrapper();
    ~Wrapper();
    
    // deprecated in C++23
    std::aligned_storage_t<32, alignof(std::max_align_t)> storage;
    
    struct Wrapped; // forward declaration
    Wrapped* handle;
};

// Wrapper.cpp
struct Wrapper::Wrapped {
};

Wrapper::Wrapper() {
    static_assert(sizeof(Wrapped) <= sizeof(this->storage) , "Object can't fit into local storage");
    this->handle = new (&this->storage) Wrapped();
}

Wrapper::~Wrapper() {
    handle->~Wrapped();
}

Note that handle to instance of Wrapped class is not required. To reduce memory footprint Wrapped class can be accessed by a helper function instead .

static Wrapper::Wrapped* get_wrapped(Wrapper* wrapper) {
    // c++17 compatible
    return std::launder(reinterpret_cast<Wrapper::Wrapped*>(&wrapper->storage));
}

This pattern is frequently used in high performance or memory constrained environments when implementation is desired to be unseen or decoupled.

• Handle Body (aka pimpl)

The Fast Pimpl Idiom