How to filter characters from a string with C++/Boost

Question

Hello,

This seems like such a basic question, so I apologize if it's already been answered somewhere (my searching didn't turn up anything).

I just want to filter a string object so that it contains only alphanumeric and space characters.

Here's what I tried:

#include "boost/algorithm/string/erase.hpp"
#include "boost/algorithm/string/classification.hpp"

std::wstring oldStr = "Bla=bla =&*\nSampleSampleSample ";
std::wstring newStr = boost::erase_all_copy(oldStr, !(boost::is_alnum() || 
                                                      boost::is_space()));

But the compiler is not at all happy with that -- it seems that I can only put a string in the second argument of erase_all_copy and not this is_alnum() stuff.

Is there some obvious solution I'm missing here?

Answer 1

It's been years since I've used boost, but perhaps you could use erase_all_regex_copy() instead of erase_all_copy()? It might be a bit of a performance hit, but it may be your only choice aside from iterating over each element and checking manually. If you're not familiar with regular expressions, the expression you'd use in this case would be something like "[^a-zA-Z0-9 ]+".

For completeness' sake, some sample code:

#include "boost/regex.hpp"
#include "boost/algorithm/string/regex.hpp"

std::wstring oldStr = "Bla=bla =&*\nSampleSampleSample ";
std::wstring newStr = boost::erase_all_regex_copy(oldStr, boost::regex("[^a-zA-Z0-9 ]+"));

Answer 2

With the std algorithms and Boost.Bind:

std::wstring s = ...
std::wstring new_s;
std::locale loc;
std::remove_copy_if(s.begin(), s.end(), std::back_inserter(new_s), 
    !(boost::bind(&std::isalnum<wchar_t>, _1, loc)||
      boost::bind(&std::isspace<wchar_t>, _1, loc)
));