From: Seungbeom Kim on
On 2010-07-06 19:20, Brendan wrote:
> With vectors, you can take the address of the first element in order
> to pass the underlying data to API's that take char*'s like so:
> vector<char> v(BUF_SIZE);
> // for some function: void write_to_buf(char* buf, size_t buf_len)
> write_to_buf(&v[0], BUF_SIZE);
> With std::strings I've always been leery of doing the same thing since
> I know that some std::string implementations, gcc's in particular, are
> Copy On Write. To make this safe, a COW string str would have to make
> a copy when you do a str[index] if there is more than one reference to
> the underlying string. Does the standard require this to be safe?

I don't think you are allowed to pass the internal buffer of a std::
basic_string and have it modified, even without reference-counting.

const_reference operator[](size_type pos) const;
reference operator[](size_type pos);
Returns: if pos < size(), returns data()[pos]. Otherwise, if pos ==
size(), the const version returns charT(). Otherwise, the behavior
is undefined.

const charT* data() const;
Returns: if size() is nonzero, the member returns a pointer to the
initial element of an array whose first size() elements equal the
corresponding elements of the string controlled by *this. If size()
is zero, the member returns a non-null pointer that is copyable and
can have zero added to it.
Requires: The program shall not alter any of the values stored in the
character array. Nor shall the program treat the returned value as
a valid pointer value after any subsequent call to a non-const member
function of basic_string that designates the same object as this.

Therefore, it is clear that std::basic_string only gives you a read-
only view of the string value, and it doesn't even guarantee that
the array pointed to by data() is the same array as the "original"
string; it is allowed to return a copy.

In summary, std::basic_string claims exclusive access rights to its
internals. If you want a direct access to the internals, you should
use std::vector instead.

Seungbeom Kim

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