C++11 compile failure

[richmattes]

When building hokuyoaist with a C++11 compiler, there are a lot of errors of the following variety:

/home/rich/fedora/hokuyoaist/hokuyoaist-b87a88aa66853d3c9d901d4e6be729c5fe69aae0/src/hokuyo_errors.cpp: In member function ‘virtual const char* hokuyoaist::BaseError::what() const’:
/home/rich/fedora/hokuyoaist/hokuyoaist-b87a88aa66853d3c9d901d4e6be729c5fe69aae0/src/hokuyo_errors.cpp:386:8: error: no match for ‘operator<<’ (operand types are ‘const stringstream {aka const std::__cxx11::basic_stringstream<char>}’ and ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’)
     ss << std::string(error_type_, sizeof(error_type_)) << " (" << desc_code_ << "): " <<
     ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/6.1.1/istream:39:0,
                 from /usr/include/c++/6.1.1/sstream:38,
                 from /home/rich/fedora/hokuyoaist/hokuyoaist-b87a88aa66853d3c9d901d4e6be729c5fe69aae0/include/hokuyoaist/hokuyo_errors.h:32,
                 from /home/rich/fedora/hokuyoaist/hokuyoaist-b87a88aa66853d3c9d901d4e6be729c5fe69aae0/src/hokuyo_errors.cpp:29:
/usr/include/c++/6.1.1/ostream:108:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(__ostream_type& (*__pf)(__ostream_type&))
       ^~~~~~~~
/usr/include/c++/6.1.1/ostream:108:7: note:   no known conversion for argument 1 from ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&) {aka std::basic_ostream<char>& (*)(std::basic_ostream<char>&)}’
/usr/include/c++/6.1.1/ostream:117:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
       operator<<(__ios_type& (*__pf)(__ios_type&))
       ^~~~~~~~
/usr/include/c++/6.1.1/ostream:117:7: note:   no known conversion for argument 1 from ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}’
/usr/include/c++/6.1.1/ostream:127:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(ios_base& (*__pf) (ios_base&))
       ^~~~~~~~
/usr/include/c++/6.1.1/ostream:127:7: note:   no known conversion for argument 1 from ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/6.1.1/ostream:166:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(long __n)
       ^~~~~~~~
/usr/include/c++/6.1.1/ostream:166:7: note:   no known conversion for argument 1 from ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘long int’
/usr/include/c++/6.1.1/ostream:170:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(unsigned long __n)
       ^~~~~~~~
/usr/include/c++/6.1.1/ostream:170:7: note:   no known conversion for argument 1 from ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘long unsigned int’
/usr/include/c++/6.1.1/ostream:174:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(bool __n)
       ^~~~~~~~
...

I think the root cause is that issue #6 changed the what() function prototype for the exceptions in hokuyo_exception to be a const function as required by c++11 std::exception::what(). However, the function body is not const, as each of the what() functions modify the member std::stringstream ss variable.

To fix this, it seems like the error description that is being built in what() should be built in the exception constructor, and stored in ss_str. Then the what() function can just return ss_str.c_str(). Doing that could allow you to remove the stringstream ss from the class entirely in favor of a temporary stringstream in each constructor.

[jwakely]

std::exception::what() has always been const in C++, that's not new in C++11. Why is it not const in this code?!