Using std::function to write a function that returns a function

Question

Our C++ professor told us to practise the use of std::function, lambda and pointers to functions. He asked us to:

Write the function std::function<bool(float)> interval(float low, float high). The function should accept two values low and high and should return a function. The function returned will check if it's parameter is inside or outside the interval (low,high) and will return true or false accordingly.

I tackled the problem with this code.

The function declaration:

std::function<bool(float)> interval (float low,float high){
 return [low,high](float f){ return (f > low) && (f < high); };
}

How is it used in the main:

std::function<bool(float)> interval (float low,float high);
bool res = interval(10, 20)(150);
std::cout << res;

And it works. But I am not sure how the solution is elegant, or "best practise".

In particular, this call bool res = interval(10, 20)(150); is what scares me. It is really awful to call it like this. How would you have done the excersize? Do you find my solution clear?

• Josiah: "Is it best practice" isn't really a useful question for homework problems. The task is to use the technique. If you had something that you called like interval(10, 20, 150) that would be wrong.
• Josiah: On the other hand, it may help you to understand the value of this if you split up the call. For example, you could have auto isLegalSpeed = interval(0, 60); and on another line if isLegalSpeed(currentSpeed) which would be a lot more readable than if interval(0, 60, currentSpeed)
• Josiah: Even better, because these lambdas are objects, you can put them in data structures like maps and arrays. For example, you might have something like speedCheck[highway] = interval(50, 70); and speedCheck[city] = interval(10, 30); That allows for very powerful constructions indeed: if you had a list of speed camera readings from a list of roads, you could then test speedCheck[roadType](speed) for each reading instead of the much more verbose if roadType == city {...

Answer 1

This looks like it almost conforms to the spec (I think those tests should be >= and <= given that's an inclusive interval). You might want to consider what happens if a NaN value is passed in - since they are neither less-than nor greater-than actual numbers, we'll return false, which seems reasonable. It's probably not a good idea to create an interval with NaN as start or end of the range, though!

Though it wasn't your choice, I would question the use of float rather than double here (actually, in real code, I'd probably make it a template function so it can work with any type that has a partial ordering).

If you don't like the chained call syntax, you can separate out using a variable, and that's likely to be a more common case than building and immediately using just once as shown in the question:

auto is_valid = interval(10, 20);
for (auto v: values) {
 if (!is_valid(v)) {
 std::cerr << v << " out of range\n";
 }
}