gcc linker errors when using boost to_lower & trim - c++

Question

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).