c++ - safe_ptr 实现

Question

我正在尝试实现一个安全版本的 std::shared_ptr，称为“safe_ptr”，它保证“非空性”。

编辑:删除问题。如果有兴趣，请参阅编辑。将最终解决方案发布给任何感兴趣的人:

此代码现在托管在 google code 上.

#pragma once

#include <memory>
#include <type_traits>
#include <exception>

template<typename T>
class safe_ptr
{   
    template <typename> friend class safe_ptr;
public:
    typedef T element_type;

    safe_ptr() : impl_(std::make_shared<T>()){} 

    safe_ptr(const safe_ptr<T>& other) : impl_(other.impl_){}

    template<typename U>
    safe_ptr(const safe_ptr<U>& other, typename std::enable_if<std::is_convertible<U*, T*>::value, void*>::type = 0) : impl_(other.impl_){}

    template<typename U>    
    safe_ptr(const U& impl, typename std::enable_if<std::is_convertible<typename std::add_pointer<U>::type, T*>::value, void>::type* = 0)
        : impl_(std::make_shared<U>(impl)) {}

    template<typename U, typename D>        
    safe_ptr(const U& impl, D dtor, typename std::enable_if<std::is_convertible<typename std::add_pointer<U>::type, T*>::value, void>::type* = 0)
        : impl_(new U(impl), dtor) {}

    template<typename U>    
    safe_ptr(U&& impl, typename std::enable_if<std::is_convertible<typename std::add_pointer<U>::type, T*>::value, void>::type* = 0)
        : impl_(std::make_shared<U>(std::forward<U>(impl))) {}

    template<typename U, typename D>    
    safe_ptr(U&& impl, D dtor, typename std::enable_if<std::is_convertible<typename std::add_pointer<U>::type, T*>::value, void>::type* = 0)
        : impl_(new U(std::forward<U>(impl)), dtor) {}

    template<typename U>    
    explicit safe_ptr(const std::shared_ptr<U>& impl, typename std::enable_if<std::is_convertible<U*, T*>::value, void*>::type = 0) : impl_(impl)
    {
        if(!impl_)
            throw std::invalid_argument("impl");
    }

    template<typename U>    
    explicit safe_ptr(std::shared_ptr<U>&& impl, typename std::enable_if<std::is_convertible<U*, T*>::value, void*>::type = 0) : impl_(std::move(impl))
    {
        if(!impl_)
            throw std::invalid_argument("impl");
    }

    template<typename U>    
    explicit safe_ptr(U* impl, typename std::enable_if<std::is_convertible<U*, T*>::value, void*>::type = 0) : impl_(impl)
    {
        if(!impl_)
            throw std::invalid_argument("impl");
    }

    template<typename U, typename D>    
    explicit safe_ptr(U* impl, D dtor, typename std::enable_if<std::is_convertible<U*, T*>::value, void*>::type = 0) : impl_(impl, dtor)
    {
        if(!impl_)
            throw std::invalid_argument("impl");
    }

    template<typename U>
    typename std::enable_if<std::is_convertible<U*, T*>::value, safe_ptr<T>&>::type
    operator=(const safe_ptr<U>& other)
    {
        safe_ptr<T> temp(other);
        temp.swap(*this);
        return *this;
    }

    template <typename U>
    typename std::enable_if<std::is_convertible<typename std::add_pointer<U>::type, T*>::value, safe_ptr<T>&>::type
    operator=(U&& impl)
    {
        safe_ptr<T> temp(std::forward<T>(impl));
        temp.swap(*this);
        return *this;
    }

    T& operator*() const { return *impl_.get();}

    T* operator->() const { return impl_.get();}

    T* get() const { return impl_.get();}

    bool unique() const { return impl_.unique();}

    long use_count() const { return impl_.use_count();}

    void swap(safe_ptr& other) { impl_.swap(other.impl_); } 

    operator std::shared_ptr<T>() const { return impl_;}

    template<class U>
    bool owner_before(const safe_ptr<T>& ptr){ return impl_.owner_before(ptr.impl_); }

    template<class U>
    bool owner_before(const std::shared_ptr<U>& ptr){ return impl_.owner_before(ptr); }

    template<class D, class U> 
    D* get_deleter(safe_ptr<U> const& ptr) { return impl_.get_deleter(); }

private:    
    std::shared_ptr<T> impl_;
};

template<class T, class U>
bool operator==(const safe_ptr<T>& a, const safe_ptr<U>& b)
{
    return a.get() == b.get();
}

template<class T, class U>
bool operator!=(const safe_ptr<T>& a, const safe_ptr<U>& b)
{
    return a.get() != b.get();
}

template<class T, class U>
bool operator<(const safe_ptr<T>& a, const safe_ptr<U>& b)
{
    return a.get() < b.get();
}

template<class T, class U>
bool operator>(const safe_ptr<T>& a, const safe_ptr<U>& b)
{
    return a.get() > b.get();
}

template<class T, class U>
bool operator>=(const safe_ptr<T>& a, const safe_ptr<U>& b)
{
    return a.get() >= b.get();
}

template<class T, class U>
bool operator<=(const safe_ptr<T>& a, const safe_ptr<U>& b)
{
    return a.get() <= b.get();
}

template<class E, class T, class U>
std::basic_ostream<E, T>& operator<<(std::basic_ostream<E, T>& out, const safe_ptr<U>& p)
{
    return out << p.get();
}

template<class T> 
void swap(safe_ptr<T>& a, safe_ptr<T>& b)
{
    a.swap(b);
}

template<class T> 
T* get_pointer(safe_ptr<T> const& p)
{
    return p.get();
}

template <class T, class U>
safe_ptr<T> static_pointer_cast(const safe_ptr<U>& p)
{
    return safe_ptr<T>(std::static_pointer_cast<T>(std::shared_ptr<U>(p)));
}

template <class T, class U>
safe_ptr<T> const_pointer_cast(const safe_ptr<U>& p)
{
    return safe_ptr<T>(std::const_pointer_cast<T>(std::shared_ptr<U>(p)));
}

template <class T, class U>
safe_ptr<T> dynamic_pointer_cast(const safe_ptr<U>& p)
{
    auto temp = std::dynamic_pointer_cast<T>(std::shared_ptr<U>(p));
    if(!temp)
        throw std::bad_cast();
    return safe_ptr<T>(temp);
}

template<typename T>
safe_ptr<T> make_safe()
{
    return safe_ptr<T>();
}

template<typename T, typename P0>
safe_ptr<T> make_safe(P0&& p0)
{
    return safe_ptr<T>(std::make_shared<T>(std::forward<P0>(p0)));
}

template<typename T, typename P0, typename P1>
safe_ptr<T> make_safe(P0&& p0, P1&& p1)
{
    return safe_ptr<T>(std::make_shared<T>(std::forward<P0>(p0), std::forward<P1>(p1)));
}

template<typename T, typename P0, typename P1, typename P2>
safe_ptr<T> make_safe(P0&& p0, P1&& p1, P2&& p2)
{
    return safe_ptr<T>(std::make_shared<T>(std::forward<P0>(p0), std::forward<P1>(p1), std::forward<P2>(p2)));
}

template<typename T, typename P0, typename P1, typename P2, typename P3>
safe_ptr<T> make_safe(P0&& p0, P1&& p1, P2&& p2, P3&& p3)
{
    return safe_ptr<T>(std::make_shared<T>(std::forward<P0>(p0), std::forward<P1>(p1), std::forward<P2>(p2), std::forward<P3>(p3)));
}

template<typename T, typename P0, typename P1, typename P2, typename P3, typename P4>
safe_ptr<T> make_safe(P0&& p0, P1&& p1, P2&& p2, P3&& p3, P4&&)
{
    return safe_ptr<T>(std::make_shared<T>(std::forward<P0>(p0), std::forward<P1>(p1), std::forward<P2>(p2), std::forward<P3>(p3), std::forward<P3>(p4)));
}

Answer 1

你得到了 is_convertible 的参数。你想检查 Y* 到 T* 表示为:

std::is_convertible<Y*, T*>

关于您的编辑:是的，您需要一个好友声明

// Within the body of the class
template <typename> friend class safe_ptr; // the syntax is peculiar...

此外，您可能希望为 safe_ptr 提供默认构造函数，它自然会默认构造对象。我忘记调用我在回答您上一个问题时指向的对象的默认构造函数 :)