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。
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 |
||
| template< class T, class U > |
(1) | (since C++11) |
| template< class T, class U > |
(2) | (since C++20) |
| template< class T, class U > |
(3) | (since C++11) |
| template< class T, class U > |
(4) | (since C++20) |
| template< class T, class U > |
(5) | (since C++11) |
| template< class T, class U > |
(6) | (since C++20) |
| template< class T, class U > |
(7) | (since C++17) |
| template< class T, class U > |
(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), |
(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
Press <RETURN> to close this window...