STL basic_string 长度（含空字符）
为什么您可以在 std::basic_string 中插入 '\0' 字符并且 .length() 方法不受影响，但如果您调用 char_traits
例如
string str("abcdefgh");
cout << str.length(); // 8
str[4] = '\0';
cout << str.length(); // 8
cout << char_traits<char>::length(str.c_str()); // 4
Why is it that you can insert a '\0' char in a std::basic_string and the .length() method is unaffected but if you call char_traits<char>::length(str.c_str()) you get the length of the string up until the first '\0' character?
e.g.
string str("abcdefgh");
cout << str.length(); // 8
str[4] = '\0';
cout << str.length(); // 8
cout << char_traits<char>::length(str.c_str()); // 4
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好问题！
原因是 C 样式字符串被定义为以空字节结尾的字节序列。当您使用
.c_str()从 C++std::string中获取 C 样式字符串时，您将返回 C++ 字符串存储的序列其后有一个空字节。当您将其传递给strlen时，它将扫描字节，直到遇到空字节，然后报告在此之前找到的字符数。如果string包含空字节，则strlen将报告一个小于字符串整个长度的值，因为它会在到达字符串的真正末尾之前停止。一个重要的细节是::eq(s[i], charT()) 为真。对于
strlen和char_traits::length 不是同一个函数。但是，char_traits::length (§21.1.1) 的 C++ ISO 规范规定char_traits::length(s) 返回 最小i使得 char_traitschar_traits，eq函数仅通过执行==比较来返回两个字符是否相等，并通过以下方式构造字符编写char()会产生一个空字节，因此这相当于说“字符串中的第一个空字节在哪里？”这本质上就是strlen的工作原理，尽管这两个函数在技术上是不同的。然而，C++
std::string是“任意字符序列”的更通用概念。其实现的细节对外界是隐藏的，尽管它可能由开始和停止指针或指针和长度表示。由于这种表示形式不依赖于存储的字符，因此询问 std::string 的长度可以告诉您有多少个字符，而不管这些字符实际上是什么。希望这有帮助！
Great question!
The reason is that a C-style string is defined as a sequence of bytes that ends with a null byte. When you use
.c_str()to get a C-style string out of a C++std::string, then you're getting back the sequence the C++ string stores with a null byte after it. When you pass this intostrlen, it will scan across the bytes until it hits a null byte, then report how many characters it found before that. If thestringcontains a null byte, thenstrlenwill report a value that's smaller than the whole length of the string, since it will stop before hitting the real end of the string.An important detail is that
strlenandchar_traits<char>::lengthare NOT the same function. However, the C++ ISO spec forchar_traits<charT>::length(§21.1.1) says thatchar_traits<charT>::length(s)returns the smallestisuch thatchar_traits<charT>::eq(s[i], charT())is true. Forchar_traits<char>, theeqfunction just returns if the two characters are equal by doing a==comparison, and constructing a character by writingchar()produces a null byte, and so this is equal to saying "where is the first null byte in the string?" It's essentially howstrlenworks, though the two are technically different functions.A C++
std::string, however, it a more general notion of "an arbitrary sequence of characters." The particulars of its implementation are hidden from the outside world, though it's probably represented either by a start and stop pointer or by a pointer and a length. Because this representation does not depend on what characters are being stored, asking thestd::stringfor its length tells you how many characters are there, regardless of what those characters actually are.Hope this helps!