Re: [boost] Re: static sized strings
Rob Stewart wrote:
From: "Reece Dunn" <msclrhd@hotmail.com>
What I am interested in is a comparison between
* I think that data() would be better named "buffer." The
This would go with the renaming you suggest for functions that are the same in the base and implementation classes (hiding issue).
* Should fixed_string support zero-length strings?
You can have zero-length strings in my implementation (this is what fized_string() defaults to). What you *can't* have is a zero-*capacity* string, which is the same as CharT[ 0 ].
* I'm not sure that there's enough value in const_iter_offset()/iter_offset() versus begin()/end() (const and non-const) in the derived class. You eliminate four trivial functions, but you may eliminate opportunities for optimizations. (I'm thinking there may be derived types that have a better way of computing end and that returning the result of adding zero to a pointer is slower than simply returning the pointer.)
This is true. I will probably expand the interface to the four begin/end functions.
* fixed_string doesn't have a const overload of at().
I shall fix this.
Calling fixed_string's at() "at" seems wrong. You should take the tack of requiring a derivate of detail::basic_string_impl to provide uniquely named, implementation functions that basic_string_impl uses to provide the public interface. You could even suggest that such functions be made private with the basic_string_impl specialization made a friend. Your current approach will generate many "hiding" warnings, too.
I will rename the conflicting names.
* fixed_string's at() shouldn't test the length and throw an exception since detail::basic_string_impl::at() does it and detail::basic_string_impl::operator[] calls it.
fixed_string tests against capacity as a measure against [] usage that would cause buffer overflow, e.g. fixed_srting< 5 > str; str[ 10 ] = 'a'; // ok: exception vs char str2[ 5 ]; str[ 10 ] = 'a'; // foobar
* fixed_string::swap() is missing. * detail::basic_string_impl::get_allocator() expects string_type::get_allocator() but that isn't listed in the requirements levied on the derived type and isn't defined in fixed_string.
These are currently missing becase (1) swap requires a deep copy and haven't implemented that yet; (2) allocators are not used by fixed_string (no need to allocate any data) so it is not implemented (I am considering if/how it should be implemented). NOTE: it should compile despite these missing from fixed_string unless you attempt to call those functions.
* I wouldn't expect fixed_string's StringPolicy to govern how to determine the length of a string passed to a mf as a CharT *.
It does not. string_type::length() returns the value of the string held in the class (return a variable value, defer to container, use traits_type, etc.). traits_type::length( const CharT * ) calculates the length of a string passed to it, wher traits_type is a char_triats compatible interface.
* If I kept looking, I'm sure I'd find many more things, but you are more interested in philosophical differences, so I'll stop here. Look at the end of the message for my summary.
I am actually interested in both. The feedback is appreciated and I will use it to improve the code.
I think John's approach is better, only because it does less type indirection. It relies on the more ordinary pvf approach to the core functionality. Otherwise, the two libraries provide the same functionality (or can when complete).
To simplify my design, I could implement basic_string_impl using the policy approach that flex_string uses. The problem with this is that it requires a usage like: flex_string< fixed_string< 10, char > > str; so in order to get the fixed_string< n > syntax, I would need two classes instead of just the one: this was something I was trying to move away from, but the approach I use has several problems: [1] the code is less optimal (7.5k vs 5k on MS VC 7.1) in my approach, due to the indirection used; the flex_string policy approach would allow compilers to aggressively inline. [2] the implementation functions are exposed in the interface; a policy approach would allow the implementation to be hidden because it can be derived from it like flex_string. [3] in my current approach, I can't use typedefs from the StringPolicy in basic_string_impl (they are not defined yet); seperate classes will solve this. There may be others to this list. So I am going to change to the flex_string approach and see what happens :). Regards, Reece _________________________________________________________________ Express yourself with cool new emoticons http://www.msn.co.uk/specials/myemo
From: "Reece Dunn" <msclrhd@hotmail.com>
Rob Stewart wrote:
* I think that data() would be better named "buffer." The
This would go with the renaming you suggest for functions that are the same in the base and implementation classes (hiding issue).
data() is a const mf and returns CharT const *, but buffer() is a non-const mf and can return CharT *.
* Should fixed_string support zero-length strings?
You can have zero-length strings in my implementation (this is what fized_string() defaults to). What you *can't* have is a zero-*capacity* string, which is the same as CharT[ 0 ].
I did mean zero-capacity. I didn't look at your implementation at this point, but I did notice that John's class declares a size N array, so for N == 0, his would fail to compile. Is the same true of yours?
* fixed_string's at() shouldn't test the length and throw an exception since detail::basic_string_impl::at() does it and detail::basic_string_impl::operator[] calls it.
fixed_string tests against capacity as a measure against [] usage that would cause buffer overflow, e.g. fixed_srting< 5 > str; str[ 10 ] = 'a'; // ok: exception vs char str2[ 5 ]; str[ 10 ] = 'a'; // foobar
Fine, but there should only need to be one test per call to at(). If size() can never exceed capacity, by enforcing invariants elsewhere, then at() only needs to check that the index doesn't exceed size().
* fixed_string::swap() is missing. * detail::basic_string_impl::get_allocator() expects string_type::get_allocator() but that isn't listed in the requirements levied on the derived type and isn't defined in fixed_string.
These are currently missing becase (1) swap requires a deep copy and haven't implemented that yet; (2) allocators are not used by fixed_string (no need to allocate any data) so it is not implemented (I am considering if/how it should be implemented). NOTE: it should compile despite these missing from fixed_string unless you attempt to call those functions.
If you're not going to implement get_allocator(), then it should be in basic_string_impl and shouldn't be among the requirements levied upon derivates.
* I wouldn't expect fixed_string's StringPolicy to govern how to determine the length of a string passed to a mf as a CharT *.
It does not. string_type::length() returns the value of the string held in the class (return a variable value, defer to container, use traits_type, etc.). traits_type::length( const CharT * ) calculates the length of a string passed to it, wher traits_type is a char_triats compatible interface.
From your fixed_string.hpp:
inline CharT * append( const CharT * s, size_type l = npos ) { if( l == npos ) l = StringPolicy::length( s ) + 1; l = (( l + len ) > n ) ? ( n - len ) : l; CharT * ret = StringPolicy::copy( str + len - 1, s, l ); len += l - 1; str[ len ] = CharT(); return( ret ); } Notice the expression, "StringPolicy::length( s )?" That's using fixed_string's StringPolicy to compute the length of a CharT const * argument.
* If I kept looking, I'm sure I'd find many more things, but you are more interested in philosophical differences, so I'll stop here. Look at the end of the message for my summary.
I am actually interested in both. The feedback is appreciated and I will use it to improve the code.
I'm starting to wonder if it would be good to reach some consensus as to which approach is better, by some definition of "better," so that the two of you can collaborate or one can stop work. I'm not certain we've reached that point, but it may be wise to write a common set of tests that you both can run so that performance can be compared (memory and speed) and binary size can be assessed. Given the adherence to std::basic_string's I/F, both classes should work with the same tests, and usability comes down to non-interface issues. It would be useful to see the sorts of error messages generated by the two approaches, too. For example, if a derived class fails to provide the right implementation function, how sensible is the error message from the compiler? If one generates much more verbose and harder to decipher error messages, and nothing else favors that one, then I would vote against that version.
I think John's approach is better, only because it does less type indirection. It relies on the more ordinary pvf approach to the core functionality. Otherwise, the two libraries provide the same functionality (or can when complete).
To simplify my design, I could implement basic_string_impl using the policy approach that flex_string uses. The problem with this is that it requires a usage like:
flex_string< fixed_string< 10, char > > str;
so in order to get the fixed_string< n > syntax, I would need two classes instead of just the one: this was something I was trying to move away from,
Sorry, I don't know anything about flex_string, but what you've shown would only serve to make your string class more painful than John's (or your) current approach. -- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;
participants (2)
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Reece Dunn -
Rob Stewart