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

[Davor Cubranic]

On 13-02-07 07:24 PM, Davor Cubranic wrote:
> 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.

To be more precise, it was introduced after 0.10.0 and before 0.10.1.

Davor

>
> 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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>>
>>
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