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

Not directly relevant to what you're saying about lambdas, but this might be an idea for using the binary search functions:

#include 
#include 
#include 

struct MS
{
    double aT;
    double bT;
    double cT;
    MS(double a, double b, double c) : aT(a), bT(b), cT(c) {}
};

// template parameter is a data member of MS, of type double
template 
struct Find {
    double d;
    Find(double d) : d(d) {}
};

template 
bool operator<(const Find &lhs, const Find &rhs) {
    return lhs.d < rhs.d;
}
template 
bool operator<(const Find &lhs, const MS &rhs) {
    return lhs.d < rhs.*F;
}
template 
bool operator<(const MS &lhs, const Find &rhs) {
    return lhs.*F < rhs.d;
}

int main() {
    std::cout << (Find<&MS::bT>(1) < Find<&MS::bT>(2)) << "\n";
    std::cout << (Find<&MS::bT>(1) < MS(1,0,0)) << "\n";
    std::cout << (MS(1,0,0) < Find<&MS::bT>(1)) << "\n";

    std::vector vec;
    vec.push_back(MS(1,0,0));
    vec.push_back(MS(0,1,0));
    std::lower_bound(vec.begin(), vec.end(), Find<&MS::bT>(0.5));
    std::upper_bound(vec.begin(), vec.end(), Find<&MS::bT>(0.5));
}

Basically, by using Find as the value, we don't have to supply a comparator, because Find compares to MS using the field that we specify. This is the same kind of thing as the answer you saw over here: how to sort STL vector, but using the value rather than the comparator as in that case. Not sure if it'd be all that great to use, but it might be, since it specifies the value to search for and the field to search in a single short expression.