c++ - 字符串超出范围c++

Question

当我尝试检查 str[index] 是否等于时出现异常 String out of range

std::string Test::getTheText(std::string str) {
    int index = 7;
    string text;
    cout << str[index] << endl; // Work!!

    while (str[index] != '\"') // Exception,why?? also try while(str[index]!=34)
        text += str[index++];
    return text;
}

我的字符串是:文本 - bla bla

Answer 1

Why the below code works?

std::string str = "bla bla";
int index = 7;
cout << str[index] << endl; // Work!!

如果index等于字符串的长度(在您的例子中是 它是, 7 == strlen("bla bla") )，访问运算符返回对默认值 charT 的引用在 std::basic_string<charT>实例化，对于 char它是 \0 ).

C++ string::operator[] reference

If pos is not greater than the string length, the function never throws exceptions (no-throw guarantee).

但是，稍后您尝试访问另一个元素:

str[index++] // in second iteration, the first is ok though

只有这样你才会陷入:

[...], it causes undefined behavior.

C++ 标准引用:

§ 21.4.5 basic_string element access [string.access]

const_reference operator[](size_type pos) const;
reference operator[](size_type pos);

1. Requires: pos <= size()

2. Returns: *(begin() + pos) if pos < size(). Otherwise, returns a reference to an object of type charT with value charT(), where modifying the object leads to undefined behavior.

3. Throws: Nothing.

也就是只要pos <= size()满足条件，该方法永远不会抛出异常，否则行为是未定义，抛出异常就是其中一个例子。