STLAllocators.h

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/* Copyright (C) 2021 Wildfire Games.
 *
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 * a copy of this software and associated documentation files (the
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 * permit persons to whom the Software is furnished to do so, subject to
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 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
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#ifndef INCLUDED_STL_ALLOCATORS
#define INCLUDED_STL_ALLOCATORS

#include <memory>
#include <type_traits>

/**
 * Adapt a 0 A.D.-style allocator for usage in STL containers.
 * Use 'Backend' as an underlying allocator.
 */
template<typename T, class Backend>
class STLAllocator
{
    template<typename A, class B>
    friend class STLAllocator;
public:
    using value_type = T;
    using pointer = T*;
    using is_always_equal = std::false_type;

    STLAllocator() : allocator(std::make_shared<Backend>())
    {
    }

    template<typename V>
    STLAllocator(const STLAllocator<V, Backend>& proxy) : allocator(proxy.allocator)
    {
    }

    template<typename V>
    struct rebind
    {
        using other = STLAllocator<V, Backend>;
    };

    T* allocate(size_t n)
    {
        return static_cast<T*>(allocator->allocate(n * sizeof(T), nullptr, alignof(T)));
    }

    void deallocate(T* ptr, const size_t n)
    {
        return allocator->deallocate(static_cast<void*>(ptr), n * sizeof(T));
    }

private:
    std::shared_ptr<Backend> allocator;
};


/**
 * Proxies allocation to another allocator.
 * This allows a single allocator to serve multiple STL containers.
 */
template<typename T, class Backend>
class ProxyAllocator
{
    template<typename A, class B>
    friend class ProxyAllocator;
public:
    using value_type = T;
    using pointer = T*;
    using is_always_equal = std::false_type;

    ProxyAllocator(Backend& alloc) : allocator(alloc)
    {
    }

    template<typename V>
    ProxyAllocator(const ProxyAllocator<V, Backend>& proxy) : allocator(proxy.allocator)
    {
    }

    template<typename V>
    struct rebind
    {
        using other = ProxyAllocator<V, Backend>;
    };

    T* allocate(size_t n)
    {
        return static_cast<T*>(allocator.allocate(n * sizeof(T), nullptr, alignof(T)));
    }

    void deallocate(T* ptr, const size_t n)
    {
        return allocator.deallocate(static_cast<void*>(ptr), n * sizeof(T));
    }

private:
    Backend& allocator;
};

#endif  // INCLUDED_STL_ALLOCATORS