支持“对于每个”在自定义const本地C ++容器类 [英] Supporting "for each" on custom const native C++ container class

问题描述

我想实现一个简单的本地C ++固定容量数组模板类，为了方便起见，支持基于范围的for each语法，并且开销最小。

<

使用此实现：

  template<类T，size_t Capacity> 
 class List 
 {
 public：
 List（）{mSize = 0; } 
 
 const T * begin（）const {return mItems; } 
 const T * end（）const {return mItems + mSize; } 
 
 T * begin（）{return mItems; } 
 T * end（）{return mItems + mSize; } 
 
 private：
 size_t mSize; 
 T mItems [Capacity]; 
}; 
  

和此用法：

  const List< int，5> myInts; 
（const int myInt in myInts）
 {
 continue; 
} 
  

我收到此错误：

$ b b

 错误C2440：'initializing'：无法从'const int *'转换为'int *'
转换失去限定符
  

此用法不会引起抱怨：

 列表< int，5> myInts; 
（const int myInt in myInts）
 {
 continue; 
} 
  

这个（不需要的）实现不会抱怨：

  template<类T，size_t Capacity> 
 class List 
 {
 public：
 List（）{mSize = 0; } 
 
 T * begin（）const {return const_cast< List *>（this） - > mItems; } 
 T * end（）const {return const_cast< List *>（this） - > mItems + mSize; } 
 
 private：
 size_t mSize; 
 T mItems [Capacity]; 
}; 
  

发生什么事情，我不明白？它是什么关于std :: vector<>处理这个正确？谢谢！

解决方案

你的使用情况似乎有点奇怪，因为你没有C ++中的每个结构，它下来。在C ++ 11中引入了用于和基于范围的用于。我只能猜测你的真实用例是什么，但最可能的编译器抱怨由于const正确性错误。我不能真正地确定你的错误，没有一个真正的代码，你试图运行。无论如何，下面是一个演示两种用法的工作示例。希望这是有帮助的，但如果你有任何问题 - 随时跟进，我会尝试解释。

  #include < cstdlib> 
 #include< iostream> 
 
 template< typename T，std :: size_t Capacity> 
 class List {
 public：
 List（）：mSize（0）{} 
 
 const T * begin（）const {return mItems; } 
 const T * end（）const {return mItems + mSize; } 
 
 T * begin（）{return mItems; } 
 T * end（）{return mItems + mSize; } 
 
 void add（int v）
 {
 // TODO：在这里检查是否超出范围... 
 mItems [mSize ++] = v; 
} 
 
 private：
 size_t mSize; 
 T mItems [Capacity]; 
}; 
 
 int main（）
 {
 / * const * / List< int，10>数组; 
 
 array.add（1）; 
 array.add（11）; 
 array.add（15）; 
 array.add（3）; 
 
 // C ++ 11风格（基于范围）
 for（int p：array）{
 std :: cout< p < '\\\
'; 
} 
 
 // Pre C ++ 11 style 
 for（const int * from = array.begin（），* to = array.end（）; from！= to; ++ from）
 {
 int p = * from; 
 std :: cout<< p < '\\\
'; 
} 
} 
  

I'd like to implement a simple native C++ fixed-capacity array template class, supporting the range-based "for each" syntax for convenience, with minimal overhead.

I'm having trouble with supporting it on const instances.

With this implementation:

template< class T, size_t Capacity >
class List
{
public:
    List() { mSize = 0; }

    const T* begin() const { return mItems; }
    const T* end() const { return mItems + mSize; }

    T* begin() { return mItems; }
    T* end() { return mItems + mSize; }

private:
    size_t mSize;
    T mItems[ Capacity ];
};

and this usage:

const List< int, 5 > myInts;
for each( const int myInt in myInts )
{
    continue;
}

I get this error:

error C2440: 'initializing' : cannot convert from 'const int *' to 'int *'
    Conversion loses qualifiers

This usage doesn't complain:

List< int, 5 > myInts;
for each( const int myInt in myInts )
{
    continue;
}

And this (undesirable) implementation doesn't complain:

template< class T, size_t Capacity >
class List
{
public:
    List() { mSize = 0; }

    T* begin() const { return const_cast< List* >( this )->mItems; }
    T* end() const { return const_cast< List* >( this )->mItems + mSize; }

private:
    size_t mSize;
    T mItems[ Capacity ];
};

What is going on under the hood that I'm not understanding? What is it about std::vector<> that handles this properly? Thanks!

解决方案

Your use-case seems a bit strange for me as there is no for each constructs in C++ as you have written it down. There is a regular for and range-based for introduced in C++11. I can only guess what your real use case is, but most likely compiler complains due to const-correctness mistake. I cannot really pin down your mistake exactly without a real code that you are trying to run. At any rate, below is a working example demonstrating both usages. Hope it is helpful, but if you have any questions — feel free to follow up, I will try to explain.

#include <cstdlib>
#include <iostream>

template <typename T, std::size_t Capacity>
class List {
  public:
    List() : mSize(0) {}

    const T *begin() const { return mItems; }
    const T *end() const { return mItems + mSize; }

    T *begin() { return mItems; }
    T *end() { return mItems + mSize; }

    void add(int v)
    {
        // TODO: Check for out of range here...
        mItems[mSize++] = v;
    }

  private:
    size_t mSize;
    T      mItems[Capacity];
};

int main()
{
    /* const */ List<int, 10> array;

    array.add(1);
    array.add(11);
    array.add(15);
    array.add(3);

    // C++11 style (range-based for)
    for (int p : array) {
        std::cout << p << '\n';
    }

    // Pre C++11 style
    for (const int *from = array.begin(), *to = array.end(); from != to; ++from)
    {
        int p = *from;
        std::cout << p << '\n';
    }
}

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