Question

Suppose I want to use std::lower_bound on a std::vector of pointers like this:

struct X {
    int x;
    double y;
};

// overloads for normal comparisons
bool operator< (int left, const X& right) { return left < right.x; }
bool operator< (const X& left, int right) { return left.x < right; }

std::vector<X*> v;

int searchValue = 5;
std::vector<X*>::iterator it = std::lower_bound(v.begin(), v.end(), searchValue,
    ? // what the heck do I put here?
);

Would I use boost::bind or boost::lambda here, and if so, how?

I would think it would be like this:

std::lower_bound(v.begin(), v.end(), searchValue, searchValue < *_1);

However I get an illegal indirection error on this.

Answer 1

Got it after some trial and error. Boost.Bind won’t work here and the compiler will get confused on with placeholder to use. Boost.Bind uses _1 in an anonymous root namespace. Boost.Lambda uses it under the namespace boost::lambda::_1.

Also, I had the order wrong with the comparisons.

#include <boost/lambda/lambda.hpp>

// ...

std::vector<X*>::iterator it = std::lower_bound(v.begin(), v.end(), searchValue,
    *boost::lambda::_1 < searchValue
);