Question

Let’s say I have

struct Value { int foo(); };
size_t *begin = ...,
       *end   = ...;

If I want to sort a bunch of Value indices in C++03, I have to write something tedious like this:

struct Comparator
{
    Value *data;
    Comparator(Value *data) : data(data) { }
    bool operator()(size_t a, size_t b)
    { return data[a].foo() < data[b].foo(); }
};
sort(begin, end, Comparator(data));

Is there any way to write this more neatly, preferably in 1 line, with Boost (perhaps with Boost.Lambda)?

Answer 1

Using Boost.Phoenix (which is the preferred lambda library of Boost):

#include <boost/phoenix/phoenix.hpp>

{
    // for bind and ref
    using namespace boost::phoenix;

    using namespace boost::phoenix::placeholders;
    std::sort(begin, end
              , bind(&Value::foo, ref(data)[arg1]) < bind(&Value::foo, ref(data)[arg2]) );
}

An alternative is to use LocalFunction which essentially lets you do what you’re doing (passing a local type to std::sort, which is not allowed for C++03) in a portable fashion. Usage should be (code is untested for this one though):

#include <boost/local_function.hpp>

{
    int BOOST_LOCAL_FUNCTION(const bind& data, std::size_t lhs, std::size_t rhs)
    {
        return data[lhs].foo() < data[rhs].foo();
    } BOOST_LOCAL_FUNCTION_NAME(comparator)

    std::sort(begin, end, comparator);
}