c++ - 访问私有(private)类变量的段错误

Question

#define TABLE_SIZE 100489 // must be a power of 2
typedef map<string,int> MAP_TYPE;
typedef pair<string, int> PAIR_TYPE;

class HashTable
{
public:                                     //public functions
    HashTable();
    ~HashTable();
    int find(string);
    bool insert(string, int);
private:
    int hash( const char* );
    vector< MAP_TYPE > v;
};

//HashTable constructor
HashTable::HashTable()
{
    vector< MAP_TYPE > v;               //initialize vector of maps
    v.reserve(TABLE_SIZE);                  //reserve table size
    MAP_TYPE m;
    for (int i = 0; i < TABLE_SIZE; ++i)    //fill vector with empty maps
    v.push_back(m);
    cout << v.size();
}

int HashTable::find(string key)
{
    cout << "in find" << '\n';
    //String to const char* for hash function
    const char *c = key.c_str();
    //find bucket where key is located
    int hashValue = HashTable::hash(c);
    cout << hashValue << '\n';
    string s = key;
    cout << v.size(); //Prints 0 but should be TABLE_SIZE
    //look for key in map in bucket
    MAP_TYPE::const_iterator iter = v[hashValue].find(s);
    if ( iter != v[hashValue].end()) //check if find exists
        return iter->second; //return value of key
    else
        return -1; //arbitrary value signifying failure to find key
}

int main()
{
    HashTable my_hash;
    string s = "hi";
    int z = my_hash.find(s);
    cout << z; //should return -1
    return 0;
}

我正在测试哈希表的查找函数，但它返回了段错误。即使我在 find 函数中构造了正确大小的 vector v，但现在大小为 0？我不认为它正在访问同一个变量。哈希函数很好。怎么了？

Answer 1

在 C++ 中，类变量最好在初始化列表中初始化:

HashTable::HashTable() : v(TABLE_SIZE, MAP_TYPE()) // **here**
{}

std::vector 有一个带有大小和默认值的构造函数，因此您可以直接调用它。事实上，由于要使用的默认值实际上是使用默认构造函数创建的映射，您实际上可以只编写以下内容，因为在这种情况下可以省略第二个参数:

HashTable::HashTable() : v(TABLE_SIZE)
{}