C++ Design Pattern for allocator type arguments

问题

The C++03 standard library uses simple template type arguments when passing a type to a class which is meant to be an allocator. This is possible because of how templates work in C++. However, it isn't very straightforward and you might don't know what exactly the type definition should look like - especially in case of non standard types.

I thought it might be a good idea to use adaptor classes instread. I've created an example to show you what I mean:

#ifndef HPP_ALLOCATOR_ADAPTOR_INCLUDED
#define HPP_ALLOCATOR_ADAPTOR_INCLUDED


#include <memory>


template<typename T>
struct allocator_traits;

template<typename T, class allocator_type = std::allocator<T>>
class allocator_adaptor;


template<>
struct allocator_traits<void>
{
    typedef std::allocator<void>::const_pointer const_pointer;
    typedef std::allocator<void>::pointer       pointer;
    typedef std::allocator<void>::value_type    value_type;
};

template<typename T>
struct allocator_traits
{
    typedef typename std::allocator<T>::const_pointer   const_pointer;
    typedef typename std::allocator<T>::const_reference const_reference;
    typedef typename std::allocator<T>::difference_type difference_type;
    typedef typename std::allocator<T>::pointer         pointer;
    typedef typename std::allocator<T>::reference       reference;
    typedef typename std::allocator<T>::size_type       size_type;
    typedef typename std::allocator<T>::value_type      value_type;
};


template<class allocator_type>
class allocator_adaptor<void, allocator_type>
    : public allocator_traits<void>
{
public:
    template<typename U> struct rebind { typedef allocator_adaptor<U, allocator_type> other; };
};

template<typename T, class allocator_type>
class allocator_adaptor
    : public allocator_traits<T>
{
private:
    allocator_type m_impl;

public:
    template<typename U> struct rebind { typedef allocator_adaptor<U, allocator_type> other; };

    allocator_adaptor() throw() /*noexcept*/;
    allocator_adaptor(allocator_adaptor const&) throw() /*noexcept*/;
    allocator_adaptor(allocator_type const&) throw() /*noexcept*/;
    template<typename U> allocator_adaptor(allocator_adaptor<U, allocator_type> const&) throw() /*noexcept*/;
    ~allocator_adaptor() throw();

    pointer       address(reference x) const /*noexcept*/;
    const_pointer address(const_reference x) const /*noexcept*/;

    pointer   allocate  (size_type, allocator_traits<void>::const_pointer hint = 0);
    void      deallocate(pointer p, size_type n) /*noexcept*/;
    size_type max_size  () const throw() /*noexcept*/;

    template<class U, typename... argument_types> void construct(U* p, argument_types&&... args);
    template<class U> void destroy(U* p);
};


#endif /* HPP_ALLOCATOR_ADAPTOR_INCLUDED */

The implemention should be obvious. Here's some usage example.

template<class allocator_type>
int max_size(allocator_type const& alloc)
{
    // we don't know what kind of max_szie function will be called.
    return alloc.max_size();
}

template<typename T>
int max_size(allocator_adaptor<T> const& alloc)
{
    // we know what kind of max_size function will be called.
    return alloc.max_size();
}

is this an improvement compared to the usual way?

回答1:

Actually your point here is to introduce a construct member which is based on variadic arguments and allows you to write instead of:

typedef std::allocator<T> my_alloc;
my_alloc alloc;
my_alloc::pointer p = alloc.allocate(10);
alloc::construct(p, T(param1, param2, param3));
alloc::construct(p+1, T(param1, param2, param3));
//...

some easier form:

alloc::construct(p, param1, param2, param3);
alloc::construct(p+1, param1, param2, param3);

This seems to be a nice feature. On the other hand, you move all the initialization parameters, which will prohibit the proper initialization of the p+1 object. What if I want to repeat the initialization for the same parameters for multiple objects. I think your current approach will fail (and unfortunately not at compile-time).

回答2:

looks good to me... it would be nice to have an explanation of when your design will have an edge on the usual way. an example will be great to help me

"However, it isn't very straightforward and you might don't know what exactly the type definition should look like"

how does your implementation improves upon the -type definition- part as compared to the std::allocator, please explain

来源：https://stackoverflow.com/questions/6334864/c-design-pattern-for-allocator-type-arguments

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c++

design-patterns

stl

allocator