问题描述
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() { }
};
这篇关于自己的容器类的 C++ 迭代器和 const_iterator 问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！