c++智能指针转化:static_pointer_cast、dynamic_pointer_cast、const_pointer_cast、reinterpret_pointer_cast

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转自：https://blog.csdn.net/sunlin972913894/article/details/108427587

C++基类和派生类的智能指针转换：static_pointer_cast、dynamic_pointer_cast、const_pointer_cast、reinterpret_pointer_cast
当我们用“裸”指针进行类层次上的上下行转换时，可以使用dynamic_cast。当然我们也可以使用static_cast，只是dynamic_cast在进行下行转换的时候（即基类到派生类）具有类型检查功能，而static_cast没有。因此存在安全问题。

当我们使用智能指针时，如果需要进行类层次上的上下行转换时，可以使用std::static_pointer_cast()、std::dynamic_pointer_cast、std::const_pointer_cast()和std::reinterpret_pointer_cast()。它们的功能和std::static_cast()、std::dynamic_cast、std::const_cast()和std::reinterpret_cast()类似，只不过转换的是智能指针std::shared_ptr，返回的也是std::shared_ptr类型。

1、std::static_pointer_cast()：当指针是智能指针时候，向上转换，用static_cast 则转换不了，此时需要使用static_pointer_cast。

2、std::dynamic_pointer_cast()：当指针是智能指针时候，向下转换，用dynamic_cast 则转换不了，此时需要使用dynamic_pointer_cast（此处注意：base基类需要至少有一个virtual成员函数（即多态类型）才能允许动态强制转换，否则编译报错）。

3、std::const_pointer_cast()：功能与std::const_cast()类似

4、std::reinterpret_pointer_cast()：功能与std::reinterpret_cast()类似

Defined in header <memory>

template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;

(1) (since C++11)
template< class T, class U >
std::shared_ptr<T> static_pointer_cast( std::shared_ptr<U>&& r ) noexcept;

(2) (since C++20)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;

(3) (since C++11)
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( std::shared_ptr<U>&& r ) noexcept;

(4) (since C++20)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept;

(5) (since C++11)
template< class T, class U >
std::shared_ptr<T> const_pointer_cast( std::shared_ptr<U>&& r ) noexcept;

(6) (since C++20)
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept;

(7) (since C++17)
template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( std::shared_ptr<U>&& r ) noexcept;

(8) (since C++20)
基类和派生类的智能指针转换要使用std::dynamic_pointer_cast和std::static_pointer_cast。由于std::dynamic_pointer_cast和dynamic_cast原理一样，std::static_pointer_cast和static_cast原理一样

Creates a new instance of std::shared_ptr whose stored pointer is obtained from r's stored pointer using a cast expression.

If r is empty, so is the new shared_ptr (but its stored pointer is not necessarily null). Otherwise, the new shared_ptr will share ownership with the initial value of r, except that it is empty if the dynamic_cast performed by dynamic_pointer_cast returns a null pointer.

Let Y be typename std::shared_ptr<T>::element_type, then the resulting std::shared_ptr's stored pointer will be obtained by evaluating, respectively:

1-2) static_cast<Y*>(r.get()).

3-4) dynamic_cast<Y*>(r.get()) (If the result of the dynamic_cast is a null pointer value, the returned shared_ptr will be empty.)

5-6) const_cast<Y*>(r.get()).

7-8) reinterpret_cast<Y*>(r.get())

The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed:

1-2) The behavior is undefined unless static_cast<T*>((U*)nullptr) is well formed.

3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed.

5-6) The behavior is undefined unless const_cast<T*>((U*)nullptr) is well formed.

7-8) The behavior is undefined unless reinterpret_cast<T*>((U*)nullptr) is well formed.

After calling the rvalue overloads (2,4,6,8), r is empty and r.get() == nullptr, except that r is not modified for dynamic_pointer_cast (4) if the dynamic_cast fails.

(since C++20)
Parameters
r - The pointer to convert
Notes
The expressions std::shared_ptr<T>(static_cast<T*>(r.get())), std::shared_ptr<T>(dynamic_cast<T*>(r.get())) and std::shared_ptr<T>(const_cast<T*>(r.get())) might seem to have the same effect, but they all will likely result in undefined behavior, attempting to delete the same object twice!

Possible implementation
1、std::static_pointer_cast():

template< class T, class U >
std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept
{
auto p = static_cast<typename std::shared_ptr<T>::element_type*>(r.get());
return std::shared_ptr<T>(r, p);
}
2、std::dynamic_pointer_cast()
template< class T, class U >
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept
{
if (auto p = dynamic_cast<typename std::shared_ptr<T>::element_type*>(r.get())) {
return std::shared_ptr<T>(r, p);
} else {
return std::shared_ptr<T>();
}
}
3、std::const_pointer_cast()

template< class T, class U >
std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept
{
auto p = const_cast<typename std::shared_ptr<T>::element_type*>(r.get());
return std::shared_ptr<T>(r, p);
}
4、std::reinterpret_pointer_cast()

template< class T, class U >
std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept
{
auto p = reinterpret_cast<typename std::shared_ptr<T>::element_type*>(r.get());
return std::shared_ptr<T>(r, p);
}
使用示例：

#include <iostream>
#include <memory>

struct Base
{
int a;
virtual void f() const { std::cout << "I am base!\n";}
virtual ~Base(){}
};

struct Derived : Base
{
void f() const override
{ std::cout << "I am derived!\n"; }
~Derived(){}
};

int main(){
auto basePtr = std::make_shared<Base>();
std::cout << "Base pointer says: ";
basePtr->f();

auto derivedPtr = std::make_shared<Derived>();
std::cout << "Derived pointer says: ";
derivedPtr->f();

// static_pointer_cast to go up class hierarchy
basePtr = std::static_pointer_cast<Base>(derivedPtr);
std::cout << "Base pointer to derived says: ";
basePtr->f();

// dynamic_pointer_cast to go down/across class hierarchy
auto downcastedPtr = std::dynamic_pointer_cast<Derived>(basePtr);
if(downcastedPtr)
{
std::cout << "Downcasted pointer says: ";
downcastedPtr->f();
}

// All pointers to derived share ownership
std::cout << "Pointers to underlying derived: "
<< derivedPtr.use_count()
<< "\n";
}
Output:

Base pointer says: I am base!
Derived pointer says: I am derived!
Base pointer to derived says: I am derived!
Downcasted pointer says: I am derived!
Pointers to underlying derived: 3
示例2
#include <iostream> // std::cout std::endl
#include <memory> // std::shared_ptr std::dynamic_pointer_cast std::static_pointer_cast

class base
{
public:
virtual ~base(void) = default;
};

class derived : public base
{
};

class test : public base
{
};

int main(void)
{
std::cout << std::boolalpha;

// 两个不同的派生类对象
auto derivedobj = std::make_shared<derived>();
auto testobj = std::make_shared<test>();

// 隐式转换 derived->base
std::shared_ptr<base> pointer1 = derivedobj;

// static_pointer_cast derived->base
auto pointer2 = std::static_pointer_cast<base>(derivedobj);

// dynamic_pointer_cast base->derived
auto pointer3 = std::dynamic_pointer_cast<derived>(pointer1);
std::cout << (pointer3 == nullptr) << std::endl;

// dynamic_pointer_cast base->derived
auto pointer4 = std::dynamic_pointer_cast<test>(pointer1);
std::cout << (pointer4 == nullptr) << std::endl;

return 0;
}
输出结果：

false
true
std::reinterpret_pointer_cast()和std::const_pointer_cast()示例：

#include <memory>
#include <cassert>
#include <cstdint>

int main()
{
std::shared_ptr<int> foo;
std::shared_ptr<const int> bar;

foo = std::make_shared<int>(10);

bar = std::const_pointer_cast<const int>(foo);

std::cout << "*bar: " << *bar << std::endl;
*foo = 20;
std::cout << "*bar: " << *bar << std::endl;

std::shared_ptr<std::int8_t[]> p(new std::int8_t[4]{1, 1, 1, 1});
std::shared_ptr<std::int32_t[]> q = std::reinterpret_pointer_cast<std::int32_t[]>(p);

std::int32_t r = q[0];

std::int32_t x = (1 << 8) | (1 << 16) | (1 << 24) | 1;
assert(r == x);

return 0;
}
输出：

*bar: 10
*bar: 20
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