修改std :: set中的元素 [英] Modifying elements in std::set

问题描述

我有以下代码-:

int main()
{
    set<string> s;
    s.insert( "asas" );
    s.insert( "abab" );

    for ( auto item : s )
    {
        cout << item << "\n";
        reverse( item.begin(), item.end() );
    }

    cout << "\n";

    for ( auto item : s )
    {
        cout << item << "\n";
    }
}

输出-:

abab
asas

abab
asas

reverse()函数完全不修改集合中的元素.
我怀疑集合中的元素根本无法修改.但是，如果是这种情况，为什么编译器本身不首先给出错误?

The elements of the set are not being modified at all by the reverse() function.
I suspect that the elements inside a set cannot be modified at all. But, if this is the case, why doesn't the compiler give an error in the first place itself ?

我在Windows 7上使用带有-std=c++14标志的TDM-GCC 4.9.2.

I am using TDM-GCC 4.9.2 with -std=c++14 flag on Windows 7.

推荐答案

std::set的元素为const.如果您想对元素进行突变，则需要进行插入和删除操作，以将容器置于所需的状态.

Elements of a std::set are const. If you want to mutate the elements you need to do insertions and removals to put the container in the state you want.

在您的代码示例中:

for (auto item : s)

被翻译成类似以下内容的

gets translated into something like:

for (auto iter = s.begin(); iter != s.end(); ++iter)
{
    auto item = *iter; // Copy!
    // loop body
}

循环感应变量item是set中元素的副本，而不是对set中实际元素的引用.结果，set不变.要更改集合，您需要调用set的成员函数，例如insert或erase.

The loop induction variable item is a copy of the element from the set, not a reference to the actual element in the set. As a result, the set is not changed. To change a set you need to call a member function of set, e.g. insert or erase.

将item更改为&item此处无济于事；如果执行此操作，则由于集合中的元素为const，将导致编译时错误，因此无法对它们应用reverse.

Changing item to &item won't help here; if you do this you'll get a compile-time error because the elements of the set are const, so you can't apply reverse to them.

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