c++ - `substr` 是否使用相同的底层内存？

Question

我听说 std::string 使用底层引用计数器来避免复制 std::string 数据。

substr 方法是使用它还是创建原始 std::string 的拷贝？

由于它是非常特定于实现的，所以让我们首先关注 GNU 的实现。

Answer 1

来自 cplusplus.com，string::substr()引用文献(强调):

Generate substring

Returns a newly constructed string object with its value initialized to a copy of a substring of this object.

The substring is the portion of the object that starts at character position pos and spans len characters (or until the end of the string, whichever comes first).

一窥 GNU 的实现表明它确实构造了一个新字符串，使用 substring constructor :

basic_string (const basic_string& str, size_type pos, size_type len = npos,
          const allocator_type& alloc = allocator_type());
// or string (const string& str, size_t pos, size_t len = npos);

substring constructor

Copies the portion of str that begins at the character position pos and spans len characters (or until the end of str, if either str is too short or if len is basic_string::npos).

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进一步测试 GNU 的实现，显然复制构造函数确实使用了引用计数，而子字符串构造函数没有。