const成员和operator = [英] const members and operator=

问题描述

我有一个 struct 和一些 const 变量

struct HashData
{
    const HashKey key;
    const void* data;
    HashData(const HashKey& key_, const void* data_) : key(key_), data(data_) {}
    /* How to write this ?
    HashData operator=(const HashData & data)
    {
        key = std::move(data.key);
        return *this;
    }
    */
};

和另一个使用它的类。

class HashTable
{
    std::vector< std::list<HashData> > hashTable; ///< Collisions resolution by chaining.
    public:
    void insertAtIndex(const std::size_t index, const HashData& data) {
        hashTable[index].insert(std::begin(hashTable[index]), data);
    }
};

class HashTable p>

class HashTable compiles but another class

class OpenAddressHashTable 
{
    std::vector<HashData> hashTable;
    public:    
    void insert(const HashData & data) throw() {
        if (data.key == NULLKEY)
            throw std::runtime_error("Do not use NullKey");

        size_t iteration = 0;
        do {
            const size_t index = (*hashFunc)(data.key, iteration);
            if (hashTable[index].key == NULLKEY) {
                // space is free
     // ** IMPORTANT **///// Line 131 is next line 
                hashTable[index] = data;
                return ;
            }
            iteration++;
        } while(iteration < hashTable.size());

        throw std::runtime_error("No space left");
    }
 };

我收到此错误：

g++ -W -Wall -pedantic -std=c++11 hash.cpp 
hash.cpp: In member function 'void OpenAddressHashTable::insert(const HashData&)':
hash.cpp:131:24: error: use of deleted function 'HashData& HashData::operator=(const HashData&)'
hash.cpp:26:8: note: 'HashData& HashData::operator=(const HashData&)' is implicitly deleted because the default definition would be ill-formed:
hash.cpp:26:8: error: non-static const member 'const HashKey HashData::key', can't use default assignment operator

std： ：list 是否需要将数据放入我的向量？
我需要在 hashTable ？

What is it that std::list does that i need to put the data in my vector ? Do i need to use pointers in my hashTable ?

推荐答案

p>您正在进行作业：

You are doing an assignment:

hashTable[index] = data;

如果你有 const 成员，因为您不能复制或移动到 const 。编译器错误很明显：

There is simply no way for that to work if you have const members because you cannot copy or move into const. The compiler error is pretty explicit:

[赋值运算符]被隐式删除，因为默认定义将是错误的

[the assignment operator] is implicitly deleted because the default definition would be ill-formed

您期望分配到键和 ？最简单的办法是删除 const 并在用户的界面上强制使用 - 以便他们不能从下面更改键。例如：

What would you expect the assignment to do to key and data? The simplest thing would be to drop the const and enforce it on your interface with the user - so that they cannot change the key out from under you. For instance:

using InternalHashData = std::pair<HashKey, void*>; 
using ExternalHashData = std::pair<const HashKey, void*>;

InternalHashData& something_to_be_returned = ..; // never expose this
return reinterpret_cast<ExternalHashData&>(something_to_be_returned); // this is OK

我可以想到的唯一方法是 const 将更改您的表格：

The only way I can think of to keep the const would be to change your table from:

std::vector<HashData> hashTable;

到

std::vector<std::unique_ptr<HashData>> hashTable;

但是，你在每个插入上做一个额外的分配只是为了保留 const -ness，这似乎不是一个很好的权衡，特别是对于一个容器，其唯一的目的是性能。

But then you're doing an extra allocation on each insert just to preserve const-ness, which doesn't seem like a good tradeoff to me at all, especially for a container whose sole purpose is performance.

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