c++ - 为什么有时cout输出字符串失败，而printf没有问题？

Question

问题：在哈希表中打印项目时，非顶级项目无法正确打印。我从文本文件中逐行添加项目。首先，确定行数，然后构造散列表，其大小在文件的第一个循环中找到。

哈希构造函数

hash::hash(unsigned int tSize)
{
    this->tableSize = tSize;
    hashTable.resize(tableSize);
    for (int i = 0; i < tableSize; i++)
    {
        hashTable[i] = new item;
        hashTable[i]->firstLetters = "__";
        hashTable[i]->str = "____";
        hashTable[i]->next = NULL;
    }
}

添加项目、设置值并打印它们

void hash::add(std::string key)
{
    int index = hafn(key);
    if (hashTable[index]->str == "____")
    {
        hashTable[index]->str = key;
        setFirstLetters(index,key[0],key[1]);
    }
    else
    {
        item *iter = hashTable[index];
        item *n_ptr = new item;
        n_ptr->str = key;
        setFirstLetters(n_ptr, key[0], key[1]);
        n_ptr->next = NULL;
        while (iter->next != NULL)
        {
            iter = iter->next;
        }
        iter->next = n_ptr;
    }
}

void hash::setFirstLetters(item *n_ptr, char a, char b)
{
    n_ptr->firstLetters[0] = a;
    n_ptr->firstLetters[1] = b;
}

void hash::setFirstLetters(int index, char a, char b)
{
    hashTable[index]->firstLetters[0] = a;
    hashTable[index]->firstLetters[1] = b;
}


void hash::print()
{
    int num;
    for (int i = 0; i < tableSize; i++)
    {
        num = numInIndex(i);
        printer(num, i);
        if (num > 1)
        {
            int c = 0;
            for (int j = num - 1; j > 0; j--)
            {
                printer(c, num, i);
                c++;
            }
        }
    }
}

void hash::printer(int num, int i)
{
    item *iter = hashTable[i];
    cout << "-------------------------------" << endl;
    cout << "index = " << i << endl;
    cout << iter->str << endl;
    cout << iter->firstLetters << endl;
    cout << "# of items = " << num << endl;
    cout << "-------------------------------" << endl;
    cout << endl;
}

void hash::printer(int numIn, int num, int i)
{
    item *iter = hashTable[i];
    for (int j = 0; j < numIn; j++)
    {
        iter = iter->next;
    }
    cout << "-------------------------------" << endl;
    cout << "index = " << i << endl;
    cout << iter->str << endl;
    cout << std::flush;
    cout << iter->firstLetters << endl; //this does not work, though the pointer is pointing to the correct values
    //printf("%s\n", iter->firstLetters.c_str()); //this works, even without flushing
    cout << "# of items = " << num << endl;
    cout << "-------------------------------" << endl;
    cout << endl;
}


    struct item
    {
        std::string str;
        std::string firstLetters;
        item *next;
    }; 

问题是firstLetters打印不正确。 firstLetters设置正确。但是，在第三级和更高级别的项目中（即，使用相同的哈希索引），firstLetters根本不打印。

为了更清楚，这里是一个输出示例：

-------------------------------
index = 15
will
wi
# of items = 3
-------------------------------

-------------------------------
index = 15
will
wi
# of items = 3
-------------------------------

-------------------------------
index = 15
good

# of items = 3
-------------------------------

请注意，在“添加项目、设置值和打印它们”标题下，在方法hash::add(std::string key)中，我使用setFirstLetters()访问内部元素std::string firstLetters而无需先初始化它们。本质上，这会导致这些值的任何更改丢失。在打印值时访问iter->firstLetters时，无法访问实际数据。为了避免这种未定义的行为，我在尝试更改它们之前 更改了 的定义hash::add(std::string key)以设置 的值。firstLetters

的新定义hash::add()

void hash::add(std::string key)
{
    int index = hafn(key);
    if (hashTable[index]->str == "____")
    {
        hashTable[index]->str = key;
        setFirstLetters(index,key[0],key[1]);
    }
    else
    {
        item *iter = hashTable[index];
        item *n_ptr = new item;
        n_ptr->firstLetters = "__";        //here
        n_ptr->str = key;
        setFirstLetters(n_ptr, key[0], key[1]);
        n_ptr->next = NULL;
        while (iter->next != NULL)
        {
            iter = iter->next;
        }
        iter->next = n_ptr;
    }
}

添加此行可修复输出。
这是新的输出：

-------------------------------
index = 15
will
wi
# of items = 3
-------------------------------

-------------------------------
index = 15
will
wi
# of items = 3
-------------------------------

-------------------------------
index = 15
good
go
# of items = 3
-------------------------------

这种类型的行为算作“未定义的行为”。这里有几个移动部件，这种行为的唯一原因是由于缺乏初始化。如果std::cout失败但printf()没有失败，请确保在尝试访问任何成员/变量之前对其进行初始化。在使用的特定情况下std::string，[]操作符只有在正常初始化后，使用=操作符或其他成员函数才能正常工作std::string。

标签： c++hashprintfcoutstring-hashing