我如何使用std :: shared_ptr实现多态？ [英] How do I implement polymorphism with std::shared_ptr?

问题描述

我已经看到关于此主题的其他一些问题，但仍未找到答案 - 我想我错过了一些东西：

我定义了两个简单的测试类：

I defined two simple test classes:

class TestBase
{

  public:

    TestBase ( ) { };
    ~ TestBase ( ) { };

  protected:

    inline virtual int getInt ( )
    {
        return 0;
    }

};

class TestDerived : public TestBase
{

  protected:

    inline int getInt ( ) override
    {
        return 1;
    }

};

我使用 std :: shared_ptr声明typedefs以简化它们的使用：

typedef std::shared_ptr<TestBase> spBase;
typedef std::shared_ptr<TestDerived> spDerived;

问题：我无法编译代码来使用这些 shared_ptr base 在所有这些情况下实际上都是 spDerived 的实例：

Problem: I cannot compile code to use these shared_ptr declarations polymorphically, even though base in all these cases is actually an instance of spDerived:

spBase base;
spDerived derived = static_cast < spDerived > ( base );

错误：没有匹配函数调用'std :: shared_ptr :: shared_ptr（spBase&）

error: no matching function for call to ‘std::shared_ptr::shared_ptr(spBase&)

spDerived derived = dynamic_cast < spDerived > ( base );

error：can not dynamic_cast'base'（type'spBase {aka class std :: shared_ptr}'（target不是指针或引用）

error: cannot dynamic_cast ‘base’ (of type ‘spBase {aka class std::shared_ptr}’) to type ‘spDerived {aka class std::shared_ptr}’ (target is not pointer or reference)

spDerived derived = static_pointer_cast < spDerived > ( base );

错误：从'std :: shared_ptr'转换为非标量键入
'spDerived {aka std :: shared_ptr}'请求
$ b

error: conversion from ‘std::shared_ptr >’ to non-scalar type ‘spDerived {aka std::shared_ptr}’ requested

spDerived derived = dynamic_pointer_cast < spDerived > ( base );

错误：从'std :: shared_ptr'转换为非标量键入
'spDerived {aka std :: shared_ptr}'请求

error: conversion from ‘std::shared_ptr >’ to non-scalar type ‘spDerived {aka std::shared_ptr}’ requested

在默认的GCC工具链的Ubuntu 14.04盒子上使用C ++ 11。编译器是gcc-4.9。
我做错了什么？不能使用多态的shared_pointer？

I'm using C++11 on an Ubuntu 14.04 box with the default GCC tool chain. Compiler is gcc-4.9. What am I doing wrong? Can't a shared_pointer be used polymorphically?

推荐答案

传入 std :: static_pointer_cast 和 std :: dynamic_pointer_cast 作为第一个类型模板参数是转换的指针类型本身的类型，而不是 smart 指针类型：

static_pointer_cast<T>(arg);
                .~~~^  
                v 
template <class T, class U> 
           .~~~~^  
           v 
shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r);


dynamic_pointer_cast<T>(arg);
                .~~~~^  
                v 
template <class T, class U> 
           .~~~~^  
           v 
shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r);

有了这个说法，你可以这样称呼它：

With that said, you could call it like below:

spBase base = std::make_shared<TestDerived>();
spDerived derived = std::dynamic_pointer_cast<spDerived::element_type>(base);
// or:
spDerived derived2 = std::dynamic_pointer_cast<TestDerived>(base);

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