问题
Is std::string reference-counted when using gcc 4 with -std=c++0x or -std=c++11?
回答1:
Looking at libstdc++ documentation I find (see the link for more info):
A string looks like this:
[_Rep]
_M_length
[basic_string<char>] _M_capacity
_M_dataplus _M_refcount
_M_p ----------------> unnamed array of char_type
So, yes it is ref counted. Also, from the discussion here:
Yes, std::string will be made non-reference counting at some point, but as a non-reference-counted string is valid in C++98 as well, one option would be to switch to a non-ref-counted string for both -std=c++98 and -std=c++11 modes. I'm not saying that's what will happen, but it could be.
So, it seems there are plans to change it to be conforming (I don't know how the progress is going though).
Update
As emsr points out in the comments, there is currently a non-reference counted extension called vstring.h, and it seems the only reason it hasn't replaced std::string is because of ABI compatibility. There is an SO question about it here.
回答2:
C++11 added specific language forbidding std::string from being reference counted. So if it is, then it's a pretty significant failing in GCC's C++11 standard library.
回答3:
Adding some useful information that post-dates this question.
std::string will no longer be reference-counted with the release of GCC 5, to address this C++11 requirement.
From https://gcc.gnu.org/gcc-5/changes.html
A new implementation of std::string is enabled by default, using the small string optimization instead of copy-on-write reference counting.
来源：https://stackoverflow.com/questions/12520192/is-stdstring-refcounted-in-gcc-4-x-c11