C ++ 11内部std ::字符串表示(libstdc ++) [英] C++11 internal std::string representation (libstdc++)
问题描述
如何在c ++ 11(libstdc ++)中内部表示std :: string?
How std::string is internally represented in c++11 (libstdc++)?
在实现过程中,我发现:
While digging inside the implementation, I found:
/* A string looks like this:
*
* [_Rep]
* _M_length
* [basic_string<char_type>] _M_capacity
* _M_dataplus _M_refcount
* _M_p ----------------> unnamed array of char_type
*
* Where the _M_p points to the first character in the string, and
* you cast it to a pointer-to-_Rep and subtract 1 to get a
* pointer to the header.
*
* This approach has the enormous advantage that a string object
* requires only one allocation. All the ugliness is confined
* within a single %pair of inline functions, which each compile to
* a single @a add instruction: _Rep::_M_data(), and
* string::_M_rep(); and the allocation function which gets a
* block of raw bytes and with room enough and constructs a _Rep
* object at the front.
*
* The reason you want _M_data pointing to the character %array and
* not the _Rep is so that the debugger can see the string
* contents. (Probably we should add a non-inline member to get
* the _Rep for the debugger to use, so users can check the actual
* string length.)
*
* Note that the _Rep object is a POD so that you can have a
* static <em>empty string</em> _Rep object already @a constructed before
* static constructors have run. The reference-count encoding is
* chosen so that a 0 indicates one reference, so you never try to
* destroy the empty-string _Rep object.
*/
// _Rep: string representation
// Invariants:
// 1. String really contains _M_length + 1 characters: due to 21.3.4
// must be kept null-terminated.
// 2. _M_capacity >= _M_length
// Allocated memory is always (_M_capacity + 1) * sizeof(_CharT).
// 3. _M_refcount has three states:
// -1: leaked, one reference, no ref-copies allowed, non-const.
// 0: one reference, non-const.
// n>0: n + 1 references, operations require a lock, const.
// 4. All fields==0 is an empty string, given the extra storage
// beyond-the-end for a null terminator; thus, the shared
// empty string representation needs no constructor.
struct _Rep_base
{
size_type _M_length;
size_type _M_capacity;
_Atomic_word _M_refcount;
};
我不太明白这些意见:
- 是std :: string ref么?怎么样?我的意思是_M_refcount不是指针,所以如果一个字符串修改它,其他人看不到它。
- 缓冲区紧接在标题后面?如果是这样,我真的不明白为什么。
推荐答案
引用计数的字符串遵循c ++ 11标准,但请注意,您的程序可能会将其用作ABI兼容性实现的一部分。
GCC did move away from the refcounted string to follow the c++11 standard, but note that it is possible that your program will use it as part of the ABI compatibility implementation.
std :: string
没有 _Rep_Base
成员,但指向 _Rep
与 _Rep
的指针 c>继承 _Rep_Base
std::string
doesn't have a _Rep_Base
member but a pointer to _Rep
with _Rep
inheriting from _Rep_Base
这里解释的是:
* Where the _M_p points to the first character in the string, and
* you cast it to a pointer-to-_Rep and subtract 1 to get a
* pointer to the header.
缓冲区位于标题...之后
是的,但是在_Rep对象的头之后,你的字符串只有一个指向它的指针。
The buffer lies after the header...
Yes, but after the header of the _Rep object, and your string only has a pointer to it.
这篇关于C ++ 11内部std ::字符串表示(libstdc ++)的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!