packed_any.h

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#ifndef STDEXT_PACKEDANY_H
#define STDEXT_PACKEDANY_H
 
#include <algorithm>
#include <cassert>
#include <type_traits>
#include <typeinfo>
 
namespace stdext {
 
// disable memory alignment
#pragma pack(push,1)
 
template<typename T>
struct can_pack_in_any : std::integral_constant<bool,
    (sizeof(T) <= sizeof(void*) && std::is_trivial<T>::value)> {};
 
// improved to use less memory
class packed_any {
public:
    struct placeholder {
        virtual ~placeholder()  { }
        virtual const std::type_info& type() const = 0;
        virtual placeholder* clone() const = 0;
    };
 
    template<typename T>
    struct holder : public placeholder {
        holder(const T& value) : held(value) { }
        const std::type_info& type() const { return typeid(T); }
        placeholder* clone() const { return new holder(held); }
        T held;
    private:
        holder& operator=(const holder &);
    };
 
    placeholder* content;
    bool scalar;
 
    packed_any() :
        content(nullptr), scalar(false) { }
    packed_any(const packed_any& other) :
        content(!other.scalar && other.content ? other.content->clone() : other.content),
        scalar(other.scalar) { }
    template<typename T>
    packed_any(const T& value, typename std::enable_if<(can_pack_in_any<T>::value)>::type* = nullptr) :
        content(reinterpret_cast<placeholder*>(static_cast<std::size_t>(value))), scalar(true) { }
    template<typename T>
    packed_any(const T& value, typename std::enable_if<!(can_pack_in_any<T>::value)>::type* = nullptr) :
        content(new holder<T>(value)), scalar(false) { }
    ~packed_any()
        { if(!scalar && content) delete content; }
 
    packed_any& swap(packed_any& rhs) { std::swap(content, rhs.content); std::swap(scalar, rhs.scalar); return *this; }
 
    template<typename T> packed_any& operator=(const T& rhs) { packed_any(rhs).swap(*this); return *this; }
    packed_any& operator=(packed_any rhs) { rhs.swap(*this); return *this; }
 
    bool empty() const { return !scalar && !content; }
    template<typename T> T cast() const;
    const std::type_info& type() const {
        if(!scalar)
            return content ? content->type() : typeid(void);
        else
            return typeid(std::size_t);
    }
};
 
template<typename T>
typename std::enable_if<can_pack_in_any<T>::value, T>::type
packed_any_cast(const packed_any& operand) {
    assert(operand.scalar);
    union {
        T v;
        packed_any::placeholder* content;
    };
    content = operand.content;
    return v;
}
 
template<typename T>
typename std::enable_if<!can_pack_in_any<T>::value, T>::type
packed_any_cast(const packed_any& operand) {
    assert(operand.type() == typeid(T));
    return static_cast<packed_any::holder<T>*>(operand.content)->held;
}
 
template<typename T> T packed_any::cast() const { return packed_any_cast<T>(*this); }
 
// restore memory alignment
#pragma pack(pop)
 
}
 
#endif