在C ++中存储任意对象的列表 [英] Storing a list of arbitrary objects in C++

问题描述

在Java中，您可以有一个对象列表。您可以添加多个类型的对象，然后检索它们，检查它们的类型，并对该类型执行相应的操作。

例如：（如果代码不完全正确，从内存中删除）

In Java, you can have a List of Objects. You can add objects of multiple types, then retrieve them, check their type, and perform the appropriate action for that type.
For example: (apologies if the code isn't exactly correct, I'm going from memory)

List<Object> list = new LinkedList<Object>();

list.add("Hello World!");
list.add(7);
list.add(true);

for (object o : list)
{
    if (o instanceof int)
        ; // Do stuff if it's an int
    else if (o instanceof String)
        ; // Do stuff if it's a string
    else if (o instanceof boolean)
        ; // Do stuff if it's a boolean
}

复制此行为的最佳方式

推荐答案

使用Boost.Variant和访问者的示例：

Your example using Boost.Variant and a visitor:

#include <string>
#include <list>
#include <boost/variant.hpp>
#include <boost/foreach.hpp>

using namespace std;
using namespace boost;

typedef variant<string, int, bool> object;

struct vis : public static_visitor<>
{
    void operator() (string s) const { /* do string stuff */ }
    void operator() (int i) const { /* do int stuff */ }
    void operator() (bool b) const { /* do bool stuff */ }		
};

int main() 
{
    list<object> List;

    List.push_back("Hello World!");
    List.push_back(7);
    List.push_back(true);

    BOOST_FOREACH (object& o, List) {
    	apply_visitor(vis(), o);
    }

    return 0;
}

使用这种技术的一个好处是，另一个类型到变体，你忘记修改一个访问者以包括该类型，它将无法编译。您已支持所有可能的情况。而如果你使用switch或cascading if语句，很容易忘记在任何地方进行更改并引入一个错误。

One good thing about using this technique is that if, later on, you add another type to the variant and you forget to modify a visitor to include that type, it will not compile. You have to support every possible case. Whereas, if you use a switch or cascading if statements, it's easy to forget to make the change everywhere and introduce a bug.

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