常量映射迭代器不会设置为 mymap.begin()

发布于 2024-11-04 16:38:08 字数 327 浏览 1 评论 0原文

map<string,Shopable*>::iterator it = mymap.begin();

迭代器看起来是常量，但 items.begin() 不返回常量迭代器。或者，这就是我的想法，因为鼠标悬停错误类似于：

"No conversion from 'std::Tree_const_iterator<...> to std::Tree_iterator<...> exists'".

为什么？

原文

map<string,Shopable*>::iterator it = mymap.begin();

The iterator appears to be constant, but items.begin() doesn't return a constant iterator. Or, that's what I think because the mouseover error is something like:

"No conversion from 'std::Tree_const_iterator<...> to std::Tree_iterator<...> exists'".

Why?

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牵强ㄟ 2024-11-11 16:38:08

使用 const_iterator as ：

map<string,Shopable*>::const_iterator it = mymap.begin();

从错误中可以清楚地看出，mymap.begin() 返回 const_iterator。这是因为 mymap 在您编写此代码的函数中是 const，如下所示：

void f(const std::map<int,int> & m)
{    //^^^^^ note this

      std::map<int,int>::const_iterator it = m.begin(); //m is const in f()
                       //^^^^^ note this
}

void g(std::map<int,int> & m)
{
      std::map<int,int>::iterator it = m.begin(); //m is non-const in g()
}

即 const 容器（无论其 < code>std::map、std::vector 等）返回 const_iterator，非常量容器返回 iterator。

每个容器都有重载函数 begin() 和 end()。因此，const 容器调用重载的 begin()，它返回 const_iterator，非常量容器调用另一个重载的 begin() code> 返回迭代器。对于 end() 重载函数也是如此。

Use const_iterator as :

map<string,Shopable*>::const_iterator it = mymap.begin();

From the error, its clear that mymap.begin() returns const_iterator. That is because mymap is const in the function where you've written this, something like following:

void f(const std::map<int,int> & m)
{    //^^^^^ note this

      std::map<int,int>::const_iterator it = m.begin(); //m is const in f()
                       //^^^^^ note this
}

void g(std::map<int,int> & m)
{
      std::map<int,int>::iterator it = m.begin(); //m is non-const in g()
}

That is, const container (whether its std::map, std::vector etc) returns const_iterator and non-const container returns iterator.

Every container has overloaded functions of begin() and end(). So const container invokes the overloaded begin() which returns const_iterator and non-const container invokes the other overloaded begin() which returns iterator. And same for end() overloaded functions.