在unordered_map中使用抽象类作为值

Question

在unordered_map中使用抽象类作为值

4

我在使用抽象类作为值时无法正确编译抽象类。理想情况下，我希望像下面这样做：

unordered_map<string, Process_Base> func_map;

Process_Base的外观如下：

//Contained in Process_Base.hpp
class Process_Base{

public:
    virtual ~Process_Base(){};
    virtual void process() const = 0;

};

子类的代码将会类似于：

#include "Process_Base.hpp"

class Process_Message : public Process_Base {

public:
    ~Process_Message(){};
    virtual void process();

};

#include <stdio.h>
#include <string.h>
#include "Process_Base.hpp"

class Process_Message{

public: 
    void process(){
        printf("%s", "Hello");
    }
};

这个想法是我可以将子类添加到地图上，并有一个简单的函数，该函数将查看键值并调用子类的处理函数。

当我在CentOS 5.8上编译时，使用的是：

g++44 -Wall -c -std=c++0x -I/usr/include -g Source.cpp

我收到了以下一系列错误：

In file included from /usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/bits/stl_algobase.h:66,
             from /usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/bits/char_traits.h:41,
             from /usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/string:42,
             from Source.cpp:2:
/usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/bits/stl_pair.h: In instantiation of ‘std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base>’:
/usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/bits/stl_function.h:482:   instantiated from ‘std::_Select1st<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base> >’
/usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/tr1_impl/hashtable_policy.h:790:   instantiated from ‘std::__detail::_Hash_code_base<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base>, std::_Select1st<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base> >, std::equal_to<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::hash<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, false>’
/usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/tr1_impl/hashtable:137:   instantiated from ‘std::_Hashtable<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base>, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base> >, std::_Select1st<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base> >, std::equal_to<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::hash<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>’
/usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/tr1_impl/unordered_map:48:   instantiated from ‘std::__unordered_map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base, std::hash<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base> >, false>’
/usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/tr1_impl/unordered_map:190:   instantiated from ‘std::unordered_map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base, std::hash<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base> > >’
Source.cpp:14:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux6E/4.4.6/../../../../include/c++/4.4.6/bits/stl_pair.h:73: error: cannot declare field ‘std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, Process_Base>::second’ to be of abstract type ‘Process_Base’
Process_Base.hpp:1: note:   because the following virtual functions are pure within ‘Process_Base’:
Process_Base.hpp:5: note:       virtual void Process_Base::process() const
make: *** [Source.o] Error 1

有人可以帮我理解这些编译错误吗？是否尝试制作函数指针的映射比子类更好？如果是，那么如何创建指向像“处理消息”这样的类中函数的指针映射呢？

- JME

将其更改为unordered_map<string，Process_Base*>。您只能拥有指向抽象类的指针。 - alecbz

当我这样做时，我会得到一个不同的错误Source.cpp:14: error: expected constructor, destructor, or type conversion before ‘<’ token. - JME

如果你想在帖子中获得关于这个的帮助，你需要在大约第14行附上Source.cpp文件（或者指出如果它已经存在的话）。 - alecbz

您可以忽略我之前的评论，那是我的拼写错误。 - JME

1个回答

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- alecbz · Accepted Answer

抽象类不能直接实例化，但是你可以拥有指向它们的指针，其值最终被实例化为非抽象子类。

class Abstract {
 public:
  virtual void Foo() = 0;
};

Abstract x;   // error
Abstract* p;  // fine

class Derived : Abstract {
 public:
  virtual void Foo() {
    printf("Hello!\n");
  }
};

p = new Derived();

// or:
Derived d;
p = &d;

由于这个原因，您也不能拥有抽象类的容器，只能拥有指向抽象类的指针。

unordered_map<string, Process_Base*> func_map;

根据您是如何填充（或尝试填充）原始的func_map而定，使用unique_ptr代替普通指针可能会更有益。

unoredered_map<string, unique_ptr<Process_Base>> func_map;

这样，你就可以做一些类似于 func_map["foo"] = new Derived() 的事情，而不必担心显式调用 delete func_map["foo"]。