will taking the address of the first element of a string break COW strings?
in [C++]
|
Prev: will taking the address of the first element of a string break COW strings?
Next: will taking the address of the first element of a string break COW strings?
From: Daniel Krügler on 6 Jul 2010 22:28 On 7 Jul., 04:20, Brendan <catph...(a)catphive.net> wrote: > 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? Yes. The C++03 standard does handle this problem in two ways: 1) It defines restrictions in regard to invalidation of pointers, references, and iterators as follows ([lib.basic.string]/5): "References, pointers, and iterators referring to the elements of a basic_string sequence may be invalidated by the following uses of that basic_string object: [..] � Calling data() and c_str() member functions. [..]" This means that a user holding a *reference/iterator/pointer* to a string character, is responsible to honor invalidations of this particular string object (I left only the interesting ones above, because they exist purely to support reference-counted implementations). 2) The standard adds a note in p. 6 that gives a hint for implementations, that basic use-cases should be possible: "[Note: These rules are formulated to allow, but not require, a reference counted implementation. A reference counted implementation must have the same semantics as a non-reference counted implementation. [Example: string s1("abc"); string::iterator i = s1.begin(); string s2 = s1; *i = �a�; // Must modify only s1 �end example] �end note]" I assume that your example was supposed to describe a similar scenario as this example. HTH & Greetings from Bremen, Daniel Kr�gler -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Mathias Gaunard on 6 Jul 2010 22:27
On Jul 7, 3:20 am, Brendan <catph...(a)catphive.net> 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. More importantly, strings are not even guaranteed to be stored contiguously... > 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. If your string is not a copy of any other string, then it should work. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |