自己的容器类的 C++ 迭代器和 const_iterator 问题

2022-01-07 00:00:00 containers c++ stl const-iterator

我正在编写一个自己的容器类，但遇到了一个我无法理解的问题.这是显示问题的基本示例.

I'm writing an own container class and have run into a problem I can't get my head around. Here's the bare-bone sample that shows the problem.

它由一个容器类和两个测试类组成:一个使用 std:vector 的测试类可以很好地编译，第二个测试类尝试以完全相同的方式使用我自己的容器类，但编译失败.

It consists of a container class and two test classes: one test class using a std:vector which compiles nicely and the second test class which tries to use my own container class in exact the same way but fails miserably to compile.

#include <vector>
#include <algorithm>
#include <iterator>

using namespace std;

template <typename T>
class MyContainer
{
public:
  
  class iterator
  {
  public:
    typedef iterator self_type;
    inline iterator() { }
  };

  class const_iterator
  {
  public:
    typedef const_iterator self_type;
    inline const_iterator() { }
  };
  
  iterator begin() {
    return iterator();
  }

  const_iterator begin() const {
    return const_iterator();
  }
};

// This one compiles ok, using std::vector
class TestClassVector
{
public:
  void test() {
    vector<int>::const_iterator I=myc.begin();
  }

private:
  vector<int> myc;
};

// this one fails to compile. Why?
class TestClassMyContainer
{
public:
  void test(){
    MyContainer<int>::const_iterator I=myc.begin();
  }

private:
  MyContainer<int> myc;
};


int main(int argc, char ** argv)
{
  return 0;
}

gcc 告诉我:

test2.C:在成员函数‘void TestClassMyContainer::test()’中:

test2.C: In member function ‘void TestClassMyContainer::test()’:

test2.C:51: 错误:请求从MyContainer::iterator"转换为非标量类型MyContainer::const_iterator"

test2.C:51: error: conversion from ‘MyContainer::iterator’ to non-scalar type ‘MyContainer::const_iterator’ requested

我不确定编译器在哪里以及为什么要将迭代器转换为我自己的类的 const_iterator 而不是 STL 向量类.我做错了什么?

I'm not sure where and why the compiler wants to convert an iterator to a const_iterator for my own class but not for the STL vector class. What am I doing wrong?

推荐答案

当你调用 begin() 时，编译器默认会创建一个对非常量 begin() 的调用代码>.由于 myc 不是 const，它无法知道您的意思是使用 const begin() 而不是非常量 begin().

When you call begin() the compiler by default creates a call to the non-const begin(). Since myc isn't const, it has no way of knowing you mean to use the const begin() rather than the non-const begin().

STL 迭代器包含一个强制转换运算符，它允许将 iterator 静默转换为 const_iterator.如果你想让它工作，你需要像这样添加一个:

The STL iterator contains a cast operator which allows an iterator to be silently converted to a const_iterator. If you want this to work you need to add one as well like so:

class iterator
{
public:
    typedef iterator self_type;
    inline iterator() { }

    operator const_iterator() { return const_iterator(); }
};

或允许 const_iterator 像这样从 iterator 构造:

or allow const_iterator to be constructed from an iterator like so:

class const_iterator
{
public:
    typedef const_iterator self_type;

    const_iterator(iterator& ) {}
    inline const_iterator() { }
};

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