std::string 可以重载“substr”吗？对于 rvalue *this 并窃取资源？

发布于 2024-10-17 21:35:37 字数 803 浏览 2 评论 0原文

我突然想到，当 std::string 的 substr 操作可以从 *this 窃取分配的内存时，它对于右值来说会更有效。

N3225 的标准库包含以下 std::string 成员函数声明

basic_string substr(size_type pos = 0, size_type n = npos) const;

可以为右值重载实现优化的 substr 实现，并提供两个版本，其中之一是哪个可以重用右值字符串的缓冲区？

basic_string substr(size_type pos = 0) &&;
basic_string substr(size_type pos, size_type n) const;

我想右值版本可以按如下方式实现，重用 *this 的内存，并将 *this 设置为移出状态。

basic_string substr(size_type pos = 0) && {
  basic_string __r;
  __r.__internal_share_buf(pos, __start + pos, __size - pos);
  __start = 0; // or whatever the 'empty' state is
  return __r;
}

这是否可以在常见字符串实现上以有效的方式工作，或者是否需要太多的内务处理？

原文

It just occurred to me I noticed that std::string's substr operation could be much more efficient for rvalues when it could steal the allocated memory from *this.

The Standard library of N3225 contains the following member function declaration of std::string

basic_string substr(size_type pos = 0, size_type n = npos) const;

Can an implementation that could implement an optimized substr for rvalues overload that and provide two versions, one of which could reuse the buffer for rvalue strings?

basic_string substr(size_type pos = 0) &&;
basic_string substr(size_type pos, size_type n) const;

I imagine the rvalue version could be implemented as follows, reusing the memory of *this an setting *this to a moved-from state.

basic_string substr(size_type pos = 0) && {
  basic_string __r;
  __r.__internal_share_buf(pos, __start + pos, __size - pos);
  __start = 0; // or whatever the 'empty' state is
  return __r;
}

Does this work in an efficient fashion on common string implementations or would this take too much housekeeping?

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最丧也最甜 2024-10-24 21:35:37

首先，实现不能添加窃取源的重载，因为这是可以检测到的：

std::string s="some random string";
std::string s2=std::move(s).substr(5,5);
assert(s=="some random string"); 
assert(s2=="rando");

如果实现从 s 窃取数据，第一个断言将会失败，并且 C++0x 措辞本质上禁止复制写时。

其次，这不一定是一种优化：您必须在 std::string 中添加额外的内务处理来处理它是较大字符串的子字符串的情况，这意味着保留当不再有任何字符串引用大字符串时，只有它的一些子字符串时，就会出现大块。

Firstly, an implementation cannot add an overload that steals the source, since that would be detectable:

std::string s="some random string";
std::string s2=std::move(s).substr(5,5);
assert(s=="some random string"); 
assert(s2=="rando");

The first assert would fail if the implementation stole the data from s, and the C++0x wording essentially outlaws copy on write.

Secondly, this wouldn't necessarily be an optimization anyway: you'd have to add additional housekeeping in std::string to handle the case that it's a substring of a larger string, and it would mean keeping large blocks around when there was no longer any strings referencing the large string, just some substring of it.

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