[Rcpp-devel] "expecting a string" and not finding it in a CharacterVector

[Davor Cubranic]

The version assigning an element of a const CharacterVector to a const 
std::string (named "bar" below) compiles and works fine in Rcpp 0.9.14, 
so the problem was introduced after that.

On the other hand, "as<std::string>" (function "baz") fails as in 
0.10.2, probably for the same reason Kevin identified in his last email 
(a check with ::Rf_isString).

Davor


On 13-02-07 10:55 AM, Davor Cubranic wrote:
> I tracked this down to const-ness:
>
>
>     bar <- cxxfunction(signature(xs_="character"), plugin='Rcpp', body='
>     const CharacterVector xs(xs_);
>     const std::string x(xs(0));
>     Rcout << x << std::endl;')
>
>
> fails with the compile error mentioned in my last message, while
>
>     foo <- cxxfunction(signature(xs_="character"), plugin='Rcpp', body='
>     CharacterVector xs(xs_);
>     std::string x(xs(0));
>     Rcout << x << std::endl;')
>
>     foo(month.abb)
>
>
> works fine. I'll be happy to add a unit test to the package if the devs
> want it.
>
> And to go back to my initial qustion: I'm still not sure why
> "as<std::string>" doesn't work:
>
>     baz <- cxxfunction(signature(xs_="character"), plugin='Rcpp', body='
>     CharacterVector xs(xs_);
>     std::string x = as<std::string>(xs(0));
>     Rcout << x << std::endl;')
>
>     baz(month.abb)
>
>
> will throw "expecting a string".
>
> Davor
>
>
> On 2013-02-07, at 10:27 AM, Davor Cubranic wrote:
>
>> On 2013-02-07, at 10:15 AM, Kevin Ushey wrote:
>>
>>> One trick to get around this is to 'as' the entire CharacterVector
>>> into a std::vector< std::string >, and then index based off of that.
>>>
>>> My guess is though, elements of CharacterVectors are 'const char*' s,
>>> so to convert them to strings you might want to just use std::string
>>> constructor, eg. std::string( xs(0) ).
>>
>> That used to work with 0.10.0, but when I upgraded this morning to
>> 0.10.2, gives me the following compile error:
>>
>>     rcppdb.cpp:131: error: no matching function for call to
>>     'std::basic_string<char, std::char_traits<char>,
>>     std::allocator<char> >::basic_string(SEXPREC* const&)'
>>     /usr/include/c++/4.2.1/bits/basic_string.tcc:233: note: candidates
>>     are: std::basic_string<_CharT, _Traits,
>>     _Alloc>::basic_string(typename
>>     _Alloc::rebind<_CharT>::other::size_type, _CharT, const _Alloc&)
>>     [with _CharT = char, _Traits = std::char_traits<char>, _Alloc =
>>     std::allocator<char>]
>>     /usr/include/c++/4.2.1/bits/basic_string.tcc:226: note:
>>           std::basic_string<_CharT, _Traits,
>>     _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT =
>>     char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]
>>     /usr/include/c++/4.2.1/bits/basic_string.tcc:219: note:
>>           std::basic_string<_CharT, _Traits,
>>     _Alloc>::basic_string(const _CharT*, typename
>>     _Alloc::rebind<_CharT>::other::size_type, const _Alloc&) [with
>>     _CharT = char, _Traits = std::char_traits<char>, _Alloc =
>>     std::allocator<char>]
>>     /usr/include/c++/4.2.1/bits/basic_string.tcc:208: note:
>>           std::basic_string<_CharT, _Traits,
>>     _Alloc>::basic_string(const std::basic_string<_CharT, _Traits,
>>     _Alloc>&, typename _Alloc::rebind<_CharT>::other::size_type,
>>     typename _Alloc::rebind<_CharT>::other::size_type, const _Alloc&)
>>     [with _CharT = char, _Traits = std::char_traits<char>, _Alloc =
>>     std::allocator<char>]
>>     /usr/include/c++/4.2.1/bits/basic_string.tcc:197: note:
>>           std::basic_string<_CharT, _Traits,
>>     _Alloc>::basic_string(const std::basic_string<_CharT, _Traits,
>>     _Alloc>&, typename _Alloc::rebind<_CharT>::other::size_type,
>>     typename _Alloc::rebind<_CharT>::other::size_type) [with _CharT =
>>     char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]
>>     /usr/include/c++/4.2.1/bits/basic_string.tcc:183: note:
>>           std::basic_string<_CharT, _Traits,
>>     _Alloc>::basic_string(const std::basic_string<_CharT, _Traits,
>>     _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>,
>>     _Alloc = std::allocator<char>]
>>     /usr/include/c++/4.2.1/bits/basic_string.tcc:191: note:
>>           std::basic_string<_CharT, _Traits,
>>     _Alloc>::basic_string(const _Alloc&) [with _CharT = char, _Traits
>>     = std::char_traits<char>, _Alloc = std::allocator<char>]
>>     /usr/include/c++/4.2.1/bits/basic_string.h:2065: note:
>>         std::basic_string<_CharT, _Traits, _Alloc>::basic_string()
>>     [with _CharT = char, _Traits = std::char_traits<char>, _Alloc =
>>     std::allocator<char>]
>>     make: *** [rcppdb.o] Error 1
>>
>> Perhaps this is caused by the same bug, introduced somewhere post-0.10.0.
>>
>> Davor
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