使用 boost::any 的替代方案在 C++ 中实现高性能动态类型

引言

Boost 库提供了一个非常有用的类，名为 boost::any，它可以包含几乎任何类型的值，只要该值支持拷贝构造和赋值。这个 boost::any 类型允许动态查询包含的类型，并进行安全的类型转换。尽管它很有用，但 boost::any 的效率并不像它应该的那样高，所以我重写了自己的版本，我自豪地称之为 cdiggins::any。

2011年更新：经过五年，我从头开始重写了这个类，以修复一些非常严重的错误并简化代码。

使用 cdiggins::any

cdiggins::any 类型可以用来保存普通值类型，并提供了一种安全且显式地转换回适当类型的机制。

any a = 42;
cout << a.cast<int>() << endl;
a = 13;
cout << a.cast<int>() << endl;
a = "hello";
cout << a.cast<const char*>() << endl;
a = std::string("1234567890");
cout << a.cast<std::string>() << endl;
int n = 42;
a = &n;
cout << *a.cast<int*>() << endl;
any b = true;
cout << b.cast<bool>() << endl;
swap(a, b);        
cout << a.cast<bool>() << endl;
a.cast<bool>() = false;
cout << a.cast<bool>() << endl;

cdiggins::any 的设计

cdiggins::any 类包含两个指针：一个指向策略类 (any::policy)，另一个是指向数据的指针 (any::data)，在某些情况下，该指针可能用于直接包含数据本身（例如，如果存储的原始数据类型小于或等于指针的大小）。

策略类用于执行分配、释放、拷贝等操作，并确定是使用 any::data 来保存数据还是指向数据。

最后，不再赘述，以下是 cdiggins::any 类的实现，供您欣赏

#pragma once
/*
 * (C) Copyright Christopher Diggins 2005-2011
 * (C) Copyright Pablo Aguilar 2005
 * (C) Copyright Kevlin Henney 2001
 *
 * Distributed under the Boost Software License, Version 1.0. (See
 * accompanying file LICENSE_1_0.txt or copy at
 * https://boost.ac.cn/LICENSE_1_0.txt
 */

#include <stdexcept>

namespace cdiggins
{
namespace anyimpl
{
    struct bad_any_cast 
    {
    };

    struct empty_any 
    {
    };

    struct base_any_policy 
    {
        virtual void static_delete(void** x) = 0;
        virtual void copy_from_value(void const* src, void** dest) = 0;
        virtual void clone(void* const* src, void** dest) = 0;
        virtual void move(void* const* src, void** dest) = 0;
        virtual void* get_value(void** src) = 0;
        virtual size_t get_size() = 0;
    };

    template<typename T>
    struct typed_base_any_policy : base_any_policy
    {
        virtual size_t get_size() { return sizeof(T); } 
    };

    template<typename T>
    struct small_any_policy : typed_base_any_policy<T>
    {
        virtual void static_delete(void** x) { }
        virtual void copy_from_value(void const* src, void** dest)
            { new(dest) T(*reinterpret_cast<T const*>(src)); }
        virtual void clone(void* const* src, void** dest) { *dest = *src; }
        virtual void move(void* const* src, void** dest) { *dest = *src; }
        virtual void* get_value(void** src) { return reinterpret_cast<void*>(src); }
    };

    template<typename T>
    struct big_any_policy : typed_base_any_policy<T>
    {
        virtual void static_delete(void** x) { if (*x) 
            delete(*reinterpret_cast<T**>(x)); *x = NULL; }
        virtual void copy_from_value(void const* src, void** dest) { 
           *dest = new T(*reinterpret_cast<T const*>(src)); }
        virtual void clone(void* const* src, void** dest) { 
           *dest = new T(**reinterpret_cast<T* const*>(src)); }
        virtual void move(void* const* src, void** dest) { 
          (*reinterpret_cast<T**>(dest))->~T(); 
          **reinterpret_cast<T**>(dest) = **reinterpret_cast<T* const*>(src); }
        virtual void* get_value(void** src) { return *src; }
    };

    template<typename T>
    struct choose_policy 
    {
        typedef big_any_policy<T> type;
    };

    template<typename T> 
    struct choose_policy<T*> 
    { 
        typedef small_any_policy<T*> type; 
    };
    
    struct any;

    /// Choosing the policy for an any type is illegal, but should never happen.
    /// This is designed to throw a compiler error.
    template<>
    struct choose_policy<any>
    {
        typedef void type;
    };

    /// Specializations for small types.
    #define SMALL_POLICY(TYPE) template<> struct 
       choose_policy<TYPE> { typedef small_any_policy<TYPE> type; };

    SMALL_POLICY(signed char);
    SMALL_POLICY(unsigned char);
    SMALL_POLICY(signed short);
    SMALL_POLICY(unsigned short);
    SMALL_POLICY(signed int);
    SMALL_POLICY(unsigned int);
    SMALL_POLICY(signed long);
    SMALL_POLICY(unsigned long);
    SMALL_POLICY(float);
    SMALL_POLICY(bool);

    #undef SMALL_POLICY

    /// This function will return a different policy for each type. 
    template<typename T>
    base_any_policy* get_policy()
    {
        static typename choose_policy<T>::type policy;
        return &policy;
    };
}

struct any
{
private:
    // fields
    anyimpl::base_any_policy* policy;
    void* object;

public:
    /// Initializing constructor.
    template <typename T>
    any(const T& x) 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL) 
    {
        assign(x);
    }       

    /// Empty constructor. 
    any() 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL) 
    { }

    /// Special initializing constructor for string literals. 
    any(const char* x) 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL) 
    { 
        assign(x);
    }

    /// Copy constructor. 
    any(const any& x) 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL)         
    {            
        assign(x);
    }

    /// Destructor. 
    ~any() {
        policy->static_delete(&object);
    }

    /// Assignment function from another any. 
    any& assign(const any& x) {
        reset();
        policy = x.policy;
        policy->clone(&x.object, &object);
        return *this;
    }

    /// Assignment function. 
    template <typename T>
    any& assign(const T& x) {
        reset();
        policy = anyimpl::get_policy<T>();
        policy->copy_from_value(&x, &object);
        return *this;
    }

    /// Assignment operator.
    template<typename T>
    any& operator=(const T& x) {
        return assign(x);
    }

    /// Assignment operator, specialed for literal strings. 
    /// They have types like const char [6] which don't work as expected. 
    any& operator=(const char* x) {
        return assign(x);
    }

    /// Utility functions
    any& swap(any& x) {
        std::swap(policy, x.policy);
        std::swap(object, x.object);
        return *this;
    }

    /// Cast operator. You can only cast to the original type.
    template<typename T>
    T& cast() {
        if (policy != anyimpl::get_policy<T>()) 
            throw anyimpl::bad_any_cast();
        T* r = reinterpret_cast<T*>(policy->get_value(&object)); 
        return *r;
    }

    /// Returns true if the any contains no value. 
    bool empty() const {
        return policy == anyimpl::get_policy<anyimpl::empty_any>();
    }

    /// Frees any allocated memory, and sets the value to NULL.
    void reset() {
        policy->static_delete(&object);
        policy = anyimpl::get_policy<anyimpl::empty_any>();
    }

    /// Returns true if the two types are the same. 
    bool compatible(const any& x) const {
        return policy == x.policy;
    }
};
}

结束语

感谢 Pablo Aguilar 帮助完成了 cdiggins::any 类的早期版本。感谢 Raute 帮助我识别了 cdiggins::any 原始版本中的关键问题。