std::set 迭代器自动 const

std::set iterator automatically const(std::set 迭代器自动 const)

问题描述

可能重复:
C++ STL 集更新很繁琐: 我不能原地改变元素

为了简单起见，我已经提取了问题并更改了名称.

I have extracted the problem and changed names and so for simplicities sake.

基本上我实例化一个类并将其存储在 std::set 中，稍后我想引用该类，以便我可以检查它的值并修改它们...

Basically I instantiate a class and I stock it in a std::set, later on I'd like to have a reference to the class so that I can check its values and modify them...

简化代码:

        MyClass tmpClass;
        std::set<MyClass > setMyClass;
        setMyClass.insert(tmpClass);
        std::set<MyClass >::iterator iter;
        iter=setMyClass.begin();
        MyClass &tmpClass2=*iter;

和错误:

error C2440: 'initializing' : cannot convert from 'const MyClass' to 'MyClass &'

(我也删除了部分错误消息MVB::Run::"以清除它.)

(I removed parts of the error message, "MVB::Run::" to clear it up too.)

如果我在最后一行代码中添加一个前面的const"，那么一切正常，但我无法更改值...

if I add a preceding 'const' to the last line of code then everything works well but then I can't change the value...

这是正常行为吗?比如说，我必须删除数据、更改值并重新放回去?

Is this normal behaviour and I have to, say, remove the data, change values and put it back again?

我感觉这与集合的排序有关，但我不会触及用于排序的变量.

I have the feeling that this has something to do with the sorting of the set but I won't touch the variables that are used for the sorting.

推荐答案

我确定你 不会触及用于排序的变量，你可以通过使用const_cast 像这样:

I you are certain you won't touch the variables that are used for the sorting the you could work around this by using a const_cast like this:

    MyClass tmpClass;
    std::set<MyClass > setMyClass;
    setMyClass.insert(tmpClass);
    std::set<MyClass >::iterator iter;
    iter=setMyClass.begin();
    const MyClass &tmpClass2=*iter;

    MyClass &tmpClass3 = const_cast<MyClass&>(tmpClass2);

或者，您可以将要更改的类的成员声明为可变的.

Alternatively, you could declare the members of the class that you intend to change as mutable.

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