Why is COW std :: string optimization still included in GCC 5.1?

Question

According to the GCC 5 version change page ( https://gcc.gnu.org/gcc-5/changes.html ):

The new std :: string implementation is enabled by default, using a little string optimization instead of counting copy-to-write links

I decided to check this out and wrote a simple program:

int main()
{
    std::string x{"blah"};
    std::string y = x;
    printf("0x%X\n", x.c_str());
    printf("0x%X\n", y.c_str());
    x[0] = 'c';
    printf("0x%X\n", x.c_str());
    printf("0x%X\n", y.c_str());
}

And the result:

0x162FC38
0x162FC38
0x162FC68
0x162FC38

Note that the pointer x.c_str () changes after x [0] = 'c'. This means that the internal buffer is copied when writing. It looks like COW is still working. Why?

I am using g ++ 5.1.0 on Ubuntu.

Answer 1

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