C++ lambdas for std::sort and std::lower_bound/equal_range on a struct element in a sorted vector of structs

Question

I have a std::vector of this struct:

struct MS
{        
  double aT;
  double bT;
  double cT;
};

which I want to use std::sort on aswell

Answer 1

I had the same problem for std::equal_range and came up with an alternative solution.

I have a collection of pointers to objects sorted on a type field. I need to find the find the range of objects for a given type.

const auto range = std::equal_range (next, blocks.end(), nullptr,
    [type] (Object* o1, Object* o2)
{
    return (o1 ? o1->Type() : type) < (o2 ? o2->Type() : type);
});

Although it is less efficient than a dedicated predicate as it introduces an unnecessary nullptr test for each object in my collection, it does provide an interesting alternative.

As an aside, when I do use a class as in your example, I tend to do the following. As well as being shorter, this allows me to add additional types with only 1 function per type rather then 4 operators per type.

class MSbTLess 
{
private:
    static inline const double& value (const MS& val)
    {
        return val.bT;
    }

    static inline const double& value (const double& val)
    {
        return val;
    }

public:
    template 
    bool operator() (const T1& lhs, const T2& rhs) const
    {
        return value (t1) < value (t2);
    }
};