Difference between revisions of "cpp/memory/shared ptr/pointer cast"

Latest revision as of 06:58, 7 October 2023

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)

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)

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}; }
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>{}; }
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}; }
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}; }

[edit] Example

Run this code

#include <iostream>
#include <memory>
 
class Base
{
public:
    int a;
    virtual void f() const { std::cout << "I am base!\n"; }
    virtual ~Base() {}
};
 
class Derived : public Base
{
public:
    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

[edit] See also

(constructor)

constructs new shared_ptr
(public member function) [edit]

@@ Line 1: / Line 1: @@
-{{cpp/title | static_pointer_cast | dynamic_pointer_cast | const_pointer_cast}}
+{{cpp/title|static_pointer_cast|dynamic_pointer_cast|const_pointer_cast|reinterpret_pointer_cast}}
 {{cpp/memory/shared_ptr/navbar}}
 {{dcl begin}}
-{{dcl | num=1 | since=c++11 | 1=
+{{dcl header|memory}}
-template< class T, class U >
+{{dcla|num=1|since=c++11|1=
-shared_ptr<T> static_pointer_cast( const shared_ptr<U>& r );
+template< class T, class U >
+std::shared_ptr<T> static_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
 }}
-{{dcl | num=2 | since=c++11 | 1=
+{{dcl|num=2|since=c++20|1=
 template< class T, class U >
-shared_ptr<T> dynamic_pointer_cast( const shared_ptr<U>& r );
+std::shared_ptr<T> static_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
 }}
-{{dcl | num=3 | since=c++11 | 1=
+{{dcla|num=3|since=c++11|1=
 template< class T, class U >
-shared_ptr<T> const_pointer_cast( const shared_ptr<U>& r );
+std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
+}}
+{{dcl|num=4|since=c++20|1=
+template< class T, class U >
+std::shared_ptr<T> dynamic_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
+}}
+{{dcla|num=5|since=c++11|1=
+template< class T, class U >
+std::shared_ptr<T> const_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
+}}
+{{dcl|num=6|since=c++20|1=
+template< class T, class U >
+std::shared_ptr<T> const_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
+}}
+{{dcla|num=7|since=c++17|1=
+template< class T, class U >
+std::shared_ptr<T> reinterpret_pointer_cast( const std::shared_ptr<U>& r ) noexcept;
+}}
+{{dcl|num=8|since=c++20|1=
+template< class T, class U >
+std::shared_ptr<T> reinterpret_pointer_cast( std::shared_ptr<U>&& r ) noexcept;
 }}
 {{dcl end}}
-Creates a new instance of {{lc|std::shared_ptr}} whose managed object type is obtained from the {{tt|r}}'s managed object type using a cast expression. Both smart pointers will share the ownership of the managed object.
+Creates a new instance of {{lc|std::shared_ptr}} whose stored pointer is obtained from {{c|r}}'s stored pointer using a cast expression.
-The resulting {{lc|std::shared_ptr}}'s managed object will be obtained by calling (in respective order):
+If {{c|r}} is empty, so is the new {{tt|shared_ptr}} (but its stored pointer is not necessarily null). Otherwise, the new {{tt|shared_ptr}} will share ownership with the initial value of {{c|r}}, except that it is empty if the {{tt|dynamic_cast}} performed by {{tt|dynamic_pointer_cast}} returns a null pointer.
-@1@ {{tt|static_cast<T*>(r.get())}}.
+Let {{tt|Y}} be {{c|typename std::shared_ptr<T>::element_type}}, then the resulting {{lc|std::shared_ptr}}'s stored pointer will be obtained by evaluating, respectively:
-@2@ {{tt|dynamic_cast<T*>(r.get())}} (If the result of the {{tt|dynamic_cast}} is 0, the returned shared_ptr will be empty).
+@1,2@ {{c|static_cast<Y*>(r.get())}}
+@3,4@ {{c|dynamic_cast<Y*>(r.get())}}. If the result of the {{tt|dynamic_cast}} is a null pointer value, the returned {{tt|shared_ptr}} will be empty.
+@5,6@ {{c|const_cast<Y*>(r.get())}}
+@7,8@ {{c|reinterpret_cast<Y*>(r.get())}}
-@3@ {{tt|const_cast<T*>(r.get())}}.
+The behavior of these functions is undefined unless the corresponding cast from {{tt|U*}} to {{tt|T*}} is well formed:
-In any case, if the parameter {{tt|r}} is an empty {{lc|std::shared_ptr}} the result will be a new empty {{lc|std::shared_ptr}}.
+@1,2@ The behavior is undefined unless {{c|static_cast<T*>((U*)nullptr)}} is well formed.
+@3,4@ The behavior is undefined unless {{c|dynamic_cast<T*>((U*)nullptr)}} is well formed.
+@5,6@ The behavior is undefined unless {{c|const_cast<T*>((U*)nullptr)}} is well formed.
+@7,8@ The behavior is undefined unless {{c|reinterpret_cast<T*>((U*)nullptr)}} is well formed.
+{{rrev|since=c++20|
+After calling the rvalue overloads {{v|2,4,6,8}}, {{c|r}} is empty and {{c|r.get() {{==}} nullptr}}, except that {{c|r}} is not modified for {{tt|dynamic_pointer_cast}} {{v|4}} if the {{c|dynamic_cast}} fails.
+}}
 ===Parameters===
 {{par begin}}
-{{par | r | The pointer to convert}}
+{{par|r|the pointer to convert}}
 {{par end}}
-===Exceptions===
+===Notes===
-{{noexcept}}
+The expressions {{c|std::shared_ptr<T>(static_cast<T*>(r.get()))}}, {{c|std::shared_ptr<T>(dynamic_cast<T*>(r.get()))}} and {{c|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===
+{{eq impl
+|title1=static_pointer_cast|ver1=1|1=
+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};
+}
+|title2=dynamic_pointer_cast|ver2=3|2=
+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>{};
+}
+|title3=const_pointer_cast|ver3=5|3=
+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};
+}
+|title4=reinterpret_pointer_cast|ver4=7|4=
+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};
+}
+}}
+===Example===
+{{example
+|code=
+#include <iostream>
+#include <memory>
+class Base
+{
+public:
+    int a;
+    virtual void f() const { std::cout << "I am base!\n"; }
+    virtual ~Base() {}
+};
+class Derived : public Base
+{
+public:
+    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
+}}
+===See also===
+{{dsc begin}}
+{{dsc inc|cpp/memory/shared_ptr/dsc constructor}}
+{{dsc end}}
-[[de:cpp/memory/shared ptr/pointer cast]]
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
-[[es:cpp/memory/shared ptr/pointer cast]]
-[[fr:cpp/memory/shared ptr/pointer cast]]
-[[it:cpp/memory/shared ptr/pointer cast]]
-[[ja:cpp/memory/shared ptr/pointer cast]]
-[[pt:cpp/memory/shared ptr/pointer cast]]
-[[ru:cpp/memory/shared ptr/pointer cast]]
-[[zh:cpp/memory/shared ptr/pointer cast]]

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Difference between revisions of "cpp/memory/shared ptr/pointer cast"

Latest revision as of 06:58, 7 October 2023

Contents

[edit] Parameters

[edit] Notes

[edit] Possible implementation

[edit] Example

[edit] See also

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