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建设网站如何弄好几张网站背景,沈阳建设工程信息网中项网,魔艺极速建站,免费发布租房信息网站当我们用“裸”指针进行类层次上的上下行转换时#xff0c;可以使用dynamic_cast。当然我们也可以使用static_cast#xff0c;只是dynamic_cast在进行下行转换的时候#xff08;即基类到派生类#xff09;具有类型检查功能#xff0c;而static_cast没有。因此存在安全问题…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 memorytemplate class T, class U std::shared_ptrT static_pointer_cast( const std::shared_ptrU r ) noexcept;(1) (since C11)template class T, class U std::shared_ptrT static_pointer_cast( std::shared_ptrU r ) noexcept;(2) (since C20)template class T, class U std::shared_ptrT dynamic_pointer_cast( const std::shared_ptrU r ) noexcept;(3) (since C11)template class T, class U std::shared_ptrT dynamic_pointer_cast( std::shared_ptrU r ) noexcept;(4) (since C20)template class T, class U std::shared_ptrT const_pointer_cast( const std::shared_ptrU r ) noexcept;(5) (since C11)template class T, class U std::shared_ptrT const_pointer_cast( std::shared_ptrU r ) noexcept;(6) (since C20)template class T, class U std::shared_ptrT reinterpret_pointer_cast( const std::shared_ptrU r ) noexcept;(7) (since C17)template class T, class U std::shared_ptrT reinterpret_pointer_cast( std::shared_ptrU r ) noexcept;(8) (since C20)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 rs 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_ptrT::element_type, then the resulting std::shared_ptrs stored pointer will be obtained by evaluating, respectively:1-2) static_castY*(r.get()).3-4) dynamic_castY*(r.get()) (If the result of the dynamic_cast is a null pointer value, the returned shared_ptr will be empty.)5-6) const_castY*(r.get()).7-8) reinterpret_castY*(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_castT*((U*)nullptr) is well formed.3-4) The behavior is undefined unless dynamic_castT*((U*)nullptr) is well formed.5-6) The behavior is undefined unless const_castT*((U*)nullptr) is well formed.7-8) The behavior is undefined unless reinterpret_castT*((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 C20)Parametersr - The pointer to convertNotesThe expressions std::shared_ptrT(static_castT*(r.get())), std::shared_ptrT(dynamic_castT*(r.get())) and std::shared_ptrT(const_castT*(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 implementation1、std::static_pointer_cast():template class T, class U std::shared_ptrT static_pointer_cast( const std::shared_ptrU r ) noexcept { auto p static_casttypename std::shared_ptrT::element_type*(r.get()); return std::shared_ptrT(r, p); }2、std::dynamic_pointer_cast()template class T, class U std::shared_ptrT dynamic_pointer_cast( const std::shared_ptrU r ) noexcept { if (auto p dynamic_casttypename std::shared_ptrT::element_type*(r.get())) { return std::shared_ptrT(r, p); } else { return std::shared_ptrT(); } }3、std::const_pointer_cast()template class T, class U std::shared_ptrT const_pointer_cast( const std::shared_ptrU r ) noexcept { auto p const_casttypename std::shared_ptrT::element_type*(r.get()); return std::shared_ptrT(r, p); }4、std::reinterpret_pointer_cast()template class T, class U std::shared_ptrT reinterpret_pointer_cast( const std::shared_ptrU r ) noexcept { auto p reinterpret_casttypename std::shared_ptrT::element_type*(r.get()); return std::shared_ptrT(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_sharedBase(); std::cout Base pointer says: ; basePtr-f(); auto derivedPtr std::make_sharedDerived(); std::cout Derived pointer says: ; derivedPtr-f(); // static_pointer_cast to go up class hierarchy basePtr std::static_pointer_castBase(derivedPtr); std::cout Base pointer to derived says: ; basePtr-f(); // dynamic_pointer_cast to go down/across class hierarchy auto downcastedPtr std::dynamic_pointer_castDerived(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_sharedderived(); auto testobj std::make_sharedtest(); // 隐式转换 derived-base std::shared_ptrbase pointer1 derivedobj; // static_pointer_cast derived-base auto pointer2 std::static_pointer_castbase(derivedobj); // dynamic_pointer_cast base-derived auto pointer3 std::dynamic_pointer_castderived(pointer1); std::cout (pointer3 nullptr) std::endl; // dynamic_pointer_cast base-derived auto pointer4 std::dynamic_pointer_casttest(pointer1); std::cout (pointer4 nullptr) std::endl; return 0; }输出结果falsetruestd::reinterpret_pointer_cast()和std::const_pointer_cast()示例代码#include memory #include cassert #include cstdint int main() { std::shared_ptrint foo; std::shared_ptrconst int bar; foo std::make_sharedint(10); bar std::const_pointer_castconst int(foo); std::cout *bar: *bar std::endl; *foo 20; std::cout *bar: *bar std::endl; std::shared_ptrstd::int8_t[] p(new std::int8_t[4]{1, 1, 1, 1}); std::shared_ptrstd::int32_t[] q std::reinterpret_pointer_caststd::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