C++ assign a logical operator function to a variable - c++

Question

I am trying to assign a logical operator function to a variable but am unable to get it to work. I am using:
function<bool(double,double)> opFunct = less<bool>();
double highorlow = pOut.high;
if(pCheck){
opFunct = greater<bool>();
highorlow = pOut.low;
}
if(opFunct(highorlow,pStay.middle){
//never gets done
}
The problem with this code is no matter what the highorlow,pStay.middle doubles are, it returns false.
What am I missing?
Thanks

Answer 1

Short version:
less<bool> compares bools. Use less<double> to compare doubles (also in greater<>).
Long version:
This is an interesting question. Specifically, how come the following line compiles?
function<bool(double, double)> opFunct = less<bool>();
After all std::less<bool>:: operator() looks like bool(bool, bool), why does it match bool(double, double)?
Well that's because the check that std::function's constructor performs is simply whether less<bool>() can be invoked as bool(double, double), and yes it can! double is implicitly-convertible to bool, 0.0 becomes false and any other value true.
That obviously won't produce the expected result because e.g. opFunct(1.0, 2.0) will invoke less(true, true) which will return false.
The fix is to change bool to double
function<bool(double, double)> opFunct = less<double>();
And also here
opFunct = greater<double>();
But wait, std::function overhead aside1, depending on how the rest of your code looks like, the fragment shown can potentially be simplified to just:
if (pCheck ? pStay.middle < pOut.low : pOut.high < pStay.middle) {
// . . .
}
Or maybe even
if (pStay.middle < pOut.low || pOut.high < pStay.middle) {
// both checks at once . . .
}
1std::function comes at a cost of some 48-96 bytes of memory and an extra indirection or two. Compare the generated code for version with std::function vs. version without std::function.