gcc linker errors when using boost to_lower & trim

Question

Hi

I'm trying to use the boost library in my code but get the following linker errors under Sparc Solaris platform.

The problem code can essentially be summarised to:

#include <boost/algorithm/string.hpp>

std::string xparam;

... 
xparam = boost::to_lower(xparam);

The linker error is:

LdapClient.cc:349: no match for `std::string& = void' operator
/opt/gcc-3.2.3/include/c++/3.2.3/bits/basic_string.h:338: candidates are: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]
/opt/gcc-3.2.3/include/c++/3.2.3/bits/basic_string.h:341:                 std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const _CharT*) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]
/opt/gcc-3.2.3/include/c++/3.2.3/bits/basic_string.h:344:                 std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(_CharT) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]
gmake: *** [LdapClient.o] Error 1

Any ideas?

Answer 1

boost::to_lower does not return a copy of the string, it operates on the variable passed into the function. For some examples, read this.

So no need to reassign:

boost::to_lower(xparam);

You will get an error because you are trying to assign the string to the value void.

If you want to make a copy of it, use the copy version:

std::string xparamLowered = boost::to_lower_copy(xparam);

Answer 2

boost::to_lower modifies the string in-place, it does not return a new string. This is sufficient:

boost::to_lower(xparam);

And your code doesn't compile because the return type of to_lower is void (as the error message says).