How to negate a predicate function using operator ! in C++?

Question

I want to erase all the elements that do not satisfy a criterion. For example: delete all the characters in a string that are not digit. My solution using boost::is_digit wo

Answer 1

You should be able to use std::not1.

std::unary_negate<my_is_digit> operator!( const my_is_digit& x )
{
    return std::not1( x );
}

For this to work you have to #include <functional> and derive your my_is_digit functor from the utility class std::unary_function< char, bool >. This is purely a typedef helper and adds no runtime overhead to your functor.

---

Complete working example:

#include <string>
#include <algorithm>
#include <functional>
#include <iostream>
#include <ostream>

struct my_is_digit : std::unary_function<char, bool>
{
    bool operator()(char c) const
    {
        return c >= '0' && c <= '9';
    }
};

std::unary_negate<my_is_digit> operator!( const my_is_digit& x )
{
    return std::not1( x );
}

int main() {
    std::string s( "1a2b3c4d" );
    s.erase( std::remove_if( s.begin(), s.end(), !my_is_digit() ), s.end() );
    std::cout << s << std::endl;
    return 0;
}

Answer 2

How could I implement such a ! like boost::is_digit()

...presumably you could look at the code that forms the is_digit implementation? You will see predicate_facade and the relevant code:

template<typename PredT>
inline detail::pred_notF<PredT>
operator!( const predicate_facade<PredT>& Pred )
{
// Doing the static_cast with the pointer instead of the reference
// is a workaround for some compilers which have problems with
// static_cast's of template references, i.e. CW8. /grafik/
return detail::pred_notF<PredT>(*static_cast<const PredT*>(&Pred)); 
}