第23课 可变参数模板（4）_Optional和Lazy类的实现

template<unsigned size, unsigned alignment>
struct aligned_storage
{
  using type = struct { alignas(alignment) unsigned char data[size]; };
};

#ifndef _OPTIONAL_H_
#define _OPTIONAL_H_

#include <type_traits>
#include <utility>  //for std::forward
#include <stdexcept>

template <typename T>
class Optional
{
    //std::alignment_of<T>::value获取T的内存对齐大小，std::aligned_storage将T重定义为对齐后的类型
    using data_t = typename std::aligned_storage<sizeof(T), std::alignment_of<T>::value>::type;
private:
    data_t m_data;  //内存对齐缓冲区
    bool m_hasInit; //是否己经初始化
private:
    //调用placement_new来创建T对象
    template<class... Args>
    void create(Args... args) //可以接收左右值
    {
        new (&m_data) T(std::forward<Args>(args)...); //调用T的构造函数来初始化m_data空间
        m_hasInit = true;
    }

    //销毁缓冲区的对象
    void destroy()
    {
        if(m_hasInit){
            m_hasInit = false;
            ((T*)(&m_data))->~T(); //调用T的析构函数
        }
    }

    //缓冲区的拷贝
    void copy(const data_t& val)
    {
        destroy();
        new (&m_data) T(*((T*)(&val)));
    }

    //缓冲区的移动
    void move(data_t&& val)
    {
        destroy();

        //调用T的移动构造函数进行移动
        new (&m_data) T(std::move(*((T*)(&val))));
    }

    //Optional赋值操作（左值版本）
    void assign(const Optional& other)
    {
        if(other.isInit()){
            copy(other.m_data);
            m_hasInit = true;
        }else{
            destroy();
        }
    }

    //Optional赋值操作（右值版本）
    void assign(Optional&& other)
    {
        if(other.isInit()){
            move(std::move(other.m_data));
            m_hasInit = true;

            other.destroy(); //other失去资源控制权
        }else{
            destroy();
        }
    }

public:
    Optional():m_hasInit(false){};
    Optional(const T& v)
    {
        create(v);
    }

    Optional(T&& v) : m_hasInit(false)
    {
        create(std::move(v));
    }

    Optional(const Optional& other) : m_hasInit(false)
    {
        if(other.isInit()){
            assign(other);
        }
    }

    Optional(Optional&& other) : m_hasInit(false)
    {
        if(other.isInit()){
            assign(std::move(other));
            other.destroy();
        }
    }

    //根据参数创建对象,如emplace(1,2);
    template<class ...Args>
    void emplace(Args&& ...args)
    {
        destroy();
        create(std::forward<Args>(args)...);
    }

    Optional& operator=(const Optional& other)
    {
        assign(other);
        return *this;
    }

    Optional& operator=(Optional&& other)
    {
        assign(std::move(other));
        return *this;
    }

    explicit operator bool() const //类型转换函数，如if(op)
    {
        return isInit();
    }

    T& operator*()
    {
        if(isInit()){
            return *((T*)(&m_data));
        }

        throw std::logic_error{"try to get data in a Optional which is not initialized"};
    }

    const T& operator*() const
    {
        if(isInit()){
            return *((T*)(&m_data));
        }

        throw std::logic_error{"try to get data in a Optional which is not initialized"};
    }

    T* operator->()
    {
        return &operator*();
    }

    const T* operator->() const
    {
        return &operator*();
    }

    bool operator==(const Optional<T>& rhs) const
    {
        bool bInit = bool(*this);
        return ( !bInit != (!rhs) ? //*this和rhs中一个初始化，一个未初始化
                 false :
                 !bInit ? true : (*(*this) == (*rhs)) //两者都未初始化，返回true
                                                      //两者都初始化时，比较两个T对象是否相等
               );
    }

    bool operator<(const Optional<T>& rhs) const
    {
        bool bInit = bool(*this);
        return !rhs ? false : (!bInit ? true : (*(*this) < (*rhs)));
    }

    bool operator!=(const Optional<T>& rhs) const
    {
        return !(*this == rhs);
    }

    bool isInit() const {return m_hasInit;}

    ~Optional()
    {
        destroy();
    }

};

#endif

#include "Optional.hpp"
#include <iostream>

using namespace std;

struct Test
{
    Test() : m_a(0), m_b(0){}
    Test(int a, int b) : m_a(a), m_b(b){}

    int m_a;
    int m_b;
    void show()
    {
        cout << "a = "<< m_a << ", b = " << m_b << endl;
    }
};

void TestOptional()
{
    const Optional<string> a("ok");
    Optional<string> b("ok");
    Optional<string> c("aa");
    Optional<string> d = b;
    Optional<string> e;

    cout << (e<b) << endl;  //true
    cout << (b==d) << endl; //true
    cout << *c << endl;
    //cout << *e << endl; //error

    Optional<Test> op;
    op.emplace(1, 2);
    (*op).show();

    Test t;
    if(op)     //判断op是否被初始化
       t = *op;
    t.show();

    op.emplace(3, 4);
    t = *op;
    t.show();
}

int main()
{
    TestOptional();
    return 0;
}

/*输出结果：
e:\Study\C++11\23>g++ -std=c++11 test.cpp
e:\Study\C++11\23>a.exe
1
1
aa
a = 1, b = 2
a = 1, b = 2
a = 3, b = 4
*/

#ifndef _LAZY_H_
#define _LAZY_H_

#include "Optional.hpp"
#include <functional>
#include <type_traits>
#include <utility>  //for std::forward

template<typename T>
struct Lazy
{
private:
    Optional<T> m_value;
    std::function<T()> m_func;
public:
    Lazy(){}

    //保存需要延迟执行的函数及其参数
    template<typename Func, typename ...Args>
    Lazy(Func&& f, Args&&... args)
    {
        m_func = [&f, &args...]{return f(args...);};
    }

    //延迟执行，将结果放到Optional中缓存起来，下次不用重新计算就可以直接返回结果
    T& value()
    {
        if(! m_value.isInit()){
            m_value = m_func();
        }

        return *m_value;
    }

    bool isValueCreated() const
    {
        return m_value.isInit();
    }
};

//辅助函数，简化Lazy的调用
template<class Func, typename... Args>
Lazy<typename std::result_of<Func(Args...)>::type>   //返回值类型Lazy<T>
lazy(Func&& fun, Args&&... args)
{
    using ret_type_t = typename std::result_of<Func(Args...)>::type;
    return Lazy<ret_type_t>(std::forward<Func>(fun), std::forward<Args>(args)...);
}


#endif // _LAZY_H_

#include "Lazy.hpp"
#include <iostream>
#include <memory>  //for std::shared_ptr

using namespace std;

struct BigObject
{
    BigObject()
    {
        cout << "lazy load big object..." << endl;
    }
};

struct Test
{
private:
    Lazy<std::shared_ptr<BigObject>> m_obj;
public:
    Test()
    {
        m_obj = lazy([]{return std::make_shared<BigObject>();});
    }

    void load()
    {
        m_obj.value();
    }
};

int Foo(int x)
{
    return x * 2;
}

void TestLazy()
{
    //带参数的普通函数
    int a = 4;
    auto lazy1 = lazy(Foo, a);
    cout << lazy1.value() << endl;  //8

    //不带参数的lambda表达式
    Lazy<int> lazy2 = lazy([]{return 12;});
    cout << lazy2.value() << endl;   //12

    //带参的function
    std::function<int(int)> f = [](int x){return x + 3;};
    auto lazy3 = lazy(f, a);
    cout << lazy3.value() << endl;  //7

    //延迟加载大对象
    Test t;
    t.load();  //lazy load big object...
}

int main()
{
    TestLazy();
    return 0;
}