假设我有一个使用结构数组 (AoS) 内存布局的大代码。我想用 C++ 构建一个零成本的抽象，它允许我以尽可能少的重构工作在 AoS 和 SoA 之间切换。
例如，使用具有访问成员函数的类

 struct Item{
   auto& myDouble(){ return mDouble; }
   auto& myChar(){ return mChar; }
   auto& myString(){ return mString; }
 private:
   double mDouble;
   char mChar;
   std::string mString;
 };

std::vector<Item> vec_(1000);
for (auto& i : vec_)
  i.myDouble()=5.;

MyContainer<Item, SoA> vec_(1000)
for (auto& i : vec_)
  i.myDouble()=5.;

最佳答案

我实现了一个通用的解决方案，我将在下面解释它(这将是一个很长的帖子)。这当然不是唯一可能的答案，收集反馈会很棒。我把这个解决方案的完整代码放在这里 https://github.com/crosetto/SoAvsAoS

我们创建了两个助手类，它们根据标签模板参数将容器类型生成为元组 vector 或 vector 元组。我们称这个类为 DataLayoutPolicy，我们将使用它，例如通过这种方式:

DataLayoutPolicy<std::vector, SoA, char, double, std::string>

enum class DataLayout { SoA, //structure of arrays
                        AoS //array of structures
};
template <template <typename...> class Container, DataLayout TDataLayout, typename TItem>
struct DataLayoutPolicy;

template <template <typename...> class Container, template<typename...> class TItem, typename... Types>
struct DataLayoutPolicy<Container, DataLayout::AoS, TItem<Types...>> {
    using type = Container<TItem<Types...>>;
    using value_type = TItem<Types...>&;

    constexpr static value_type get( type& c_, std::size_t position_ ){ return value_type(*static_cast<TItem<Types...>*>(&c_[ position_ ])); }

    constexpr static void resize( type& c_, std::size_t size_ ) { c_.resize( size_ ); }

    template <typename TValue>
    constexpr static void push_back( type& c_, TValue&& val_ ){ c_.push_back( val_ ); }
    static constexpr std::size_t size(type& c_){ return  c_.size(); }
};

template <typename T>
struct ref_wrap : public std::reference_wrapper<T>{
    operator T&() const noexcept { return this->get(); }
    ref_wrap(T& other_) : std::reference_wrapper<T>(other_){}
    void operator =(T && other_) {this->get()=other_;}
};

template <template <typename...> class Container, template<typename...> class TItem, typename... Types>
struct DataLayoutPolicy<Container, DataLayout::SoA, TItem<Types...>> {
    using type = std::tuple<Container<Types>...>;
    using value_type = TItem<ref_wrap<Types>...>;

    constexpr static value_type get( type& c_, std::size_t position_ )
    {
        return doGet( c_, position_, std::make_integer_sequence<unsigned, sizeof...( Types )>() ); // unrolling parameter pack
    }

    constexpr static void resize( type& c_, std::size_t size_ ) {
        doResize( c_, size_, std::make_integer_sequence<unsigned, sizeof...( Types )>() ); // unrolling parameter pack
    }

    template <typename TValue>
    constexpr static void push_back( type& c_, TValue&& val_ ){
        doPushBack( c_, std::forward<TValue>(val_), std::make_integer_sequence<unsigned, sizeof...( Types )>() ); // unrolling parameter pack
    }

    static constexpr std::size_t size(type& c_){ return std::get<0>( c_ ).size(); }

    private:

    template <unsigned... Ids>
    constexpr static auto doGet( type& c_, std::size_t position_, std::integer_sequence<unsigned, Ids...> )
    {
        return value_type{ ref_wrap( std::get<Ids>( c_ )[ position_ ] )... }; // guaranteed copy elision
    }

    template <unsigned... Ids>
    constexpr static void doResize( type& c_, unsigned size_, std::integer_sequence<unsigned, Ids...> )
    {
        ( std::get<Ids>( c_ ).resize( size_ ), ... ); //fold expressions
    }

    template <typename TValue, unsigned... Ids>
    constexpr static void doPushBack( type& c_, TValue&& val_, std::integer_sequence<unsigned, Ids...> )
    {
        ( std::get<Ids>( c_ ).push_back( std::get<Ids>( std::forward<TValue>( val_ ) ) ), ... ); // fold expressions
    }
};

template <template <typename T> class TContainer, DataLayout TDataLayout, typename TItem>
using policy_t = DataLayoutPolicy<TContainer, TDataLayout, TItem>;

template <template <typename ValueType> class TContainer, DataLayout TDataLayout, typename TItem>
struct BaseContainer
{
    /*member functions like puhs_back, resize,...*/
    value_type operator[]( std::size_t position_ )
    {
            return policy_t::get( mValues, position_ );
    }

    iterator       begin() { return iterator( this, 0 ); }
    iterator       end() { return iterator( this, size() ); }

    private:

    typename policy_t::type mValues;

};

template <typename  TContainer>
class Iterator
{

private:
    using container_t = TContainer;
public:

    /* ... usual iterator member functions and type definitions ...*/

    template<typename TTContainer>
    Iterator( TTContainer* container_, std::size_t position_ = 0 ):
        mContainer( container_ )
        , mIterPosition( position_ )
    {
    }

    value_type operator*() {
        return (*mContainer)[ mIterPosition ];
    }

    private:
    container_t*        mContainer = nullptr;
    std::size_t         mIterPosition = std::numeric_limits<std::size_t>::infinity();
};

template<typename ... T>
struct Item : public std::tuple<T ...>{
    using std::tuple<T...>::tuple;
    auto & myDouble(){return std::get<0>(*this);}
    auto & myChar()  {return std::get<1>(*this);}
    auto & myString(){return std::get<2>(*this);}
};

template<typename T>
using MyVector = std::vector<T, std::allocator<T>>;

int main(int argc, char** argv){
using container_t = BaseContainer<MyVector, DataLayout::SoA, Item<double, char, std::string, Pad> >;
container_t container_(1000);

 for(auto&& i : container_){
    i.myDouble()=static_cast<double>(argc);
}

> clang++ -std=c++1z -O3 -g main.cpp -o test

> gdb test
(gdb) break main.cpp : 10 # set breakpoint inside the loop
(gdb) run # execute until the breakpoint
(gdb) layout split # show assembly and source code in 2 separate frames
(gdb) stepi # execute one instruction

0x400b00 <main(int, char**)+192>        movsd  %xmm0,(%rsi)
0x400b04 <main(int, char**)+196>        add    $0x610,%rsi
0x400b0b <main(int, char**)+203>        add    $0xffffffffffffffff,%rcx
0x400b0f <main(int, char**)+207>        jne    0x400b00 <main(int, char**)+192>

0x400b60 <main(int, char**)+224>        movups %xmm1,(%rdi,%rsi,8)
0x400b64 <main(int, char**)+228>        movups %xmm1,0x10(%rdi,%rsi,8)
0x400b69 <main(int, char**)+233>        movups %xmm1,0x20(%rdi,%rsi,8)
0x400b6e <main(int, char**)+238>        movups %xmm1,0x30(%rdi,%rsi,8)
0x400b73 <main(int, char**)+243>        movups %xmm1,0x40(%rdi,%rsi,8)
0x400b78 <main(int, char**)+248>        movups %xmm1,0x50(%rdi,%rsi,8)
0x400b7d <main(int, char**)+253>        movups %xmm1,0x60(%rdi,%rsi,8)
0x400b82 <main(int, char**)+258>        movups %xmm1,0x70(%rdi,%rsi,8)
0x400b87 <main(int, char**)+263>        movups %xmm1,0x80(%rdi,%rsi,8)
0x400b8f <main(int, char**)+271>        movups %xmm1,0x90(%rdi,%rsi,8)
0x400b97 <main(int, char**)+279>        movups %xmm1,0xa0(%rdi,%rsi,8)
0x400b9f <main(int, char**)+287>        movups %xmm1,0xb0(%rdi,%rsi,8)
0x400ba7 <main(int, char**)+295>        movups %xmm1,0xc0(%rdi,%rsi,8)
0x400baf <main(int, char**)+303>        movups %xmm1,0xd0(%rdi,%rsi,8)
0x400bb7 <main(int, char**)+311>        movups %xmm1,0xe0(%rdi,%rsi,8)
0x400bbf <main(int, char**)+319>        movups %xmm1,0xf0(%rdi,%rsi,8)
0x400bc7 <main(int, char**)+327>        add    $0x20,%rsi
0x400bcb <main(int, char**)+331>        add    $0x8,%rbx
0x400bcf <main(int, char**)+335>        jne    0x400b60 <main(int, char**)+224>

关于SoA/AoS 内存布局的 C++ 零成本抽象，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50574639/