STL basic_string长度为空字符 [英] STL basic_string length with null characters

问题描述

为什么可以在std :: basic_string中插入一个'\0'char，而且.length（）方法不受影响，但是如果调用 char_traits< char> :: length str.c_str（））你得到字符串的长度，直到第一个'\0'字符为止。

>

  string str（abcdefgh）; 
 cout<< str.length（）; // 8 
 str [4] ='\0'; 
 cout<< str.length（）; // 8 
 cout<< char_traits< char> :: length（str.c_str（））; // 4 
  

解决方案

b
$ b

原因是C风格字符串被定义为以空字节结尾的字节序列。当使用 .c_str（）从C ++ std :: string 中获取C风格的字符串时，你得到的序列C ++字符串存储与空字节之后。当你将它传递给 strlen 时，它将扫描字节，直到它命中一个空字节，然后报告它之前找到了多少字符。如果 string 包含一个空字节，则 strlen 将报告小于字符串整个长度的值，因为它会在到达字符串的真实结尾之前停止。

一个重要的细节是 strlen 和 char_traits< char> :: length 不是相同的函数。但是， char_traits< charT> :: length （&Sect; 21.1.1）的C ++ ISO规范表示 char_traits< charT> :: length s）返回最小 i ，以使 char_traits< charT> :: eq [i]，charT（））为true。对于 char_traits< char> ， eq 函数只是返回如果两个字符相等， c> == 比较，并通过写 char（）构造一个字符产生一个空字节，因此这等于说where is字符串中的第一个空字节？它实际上是如何 strlen 工作，虽然这两个是技术上不同的功能。

A C ++ std :: string ，但是，它是一个更一般的概念一个任意的字符序列。其实现的细节从外部世界隐藏，尽管它可能由开始和停止指针或由指针和长度表示。因为这个表示不依赖于什么字符被存储，要求 std :: string 它的长度告诉你有多少字符，无论这些字符实际上是什么。

希望这有助于！

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

解决方案

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 into strlen, it will scan across the bytes until it hits a null byte, then report how many characters it found before that. If the string contains a null byte, then strlen will 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 strlen and char_traits<char>::length are NOT the same function. However, the C++ ISO spec for char_traits<charT>::length (§21.1.1) says that char_traits<charT>::length(s) returns the smallest i such that char_traits<charT>::eq(s[i], charT()) is true. For char_traits<char>, the eq function just returns if the two characters are equal by doing a == comparison, and constructing a character by writing char() produces a null byte, and so this is equal to saying "where is the first null byte in the string?" It's essentially how strlen works, 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 the std::string for its length tells you how many characters are there, regardless of what those characters actually are.

Hope this helps!

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