Optional results and interface contracts

The idea

This chapter assumes functions, vectors, and references. Sometimes a valid computation has no answer: an empty collection has no largest element. Returning zero would confuse absence with a genuine maximum of zero, and throwing may be unnecessary for an expected empty case. C++17 optional<T> holds either a T value or no value. The result type makes that possibility visible to every caller.

Check an optional before dereferencing it, or use value_or when a fallback makes sense for the caller’s task. Keep the search function’s contract separate from display policy: the function reports absence, while the interface may show a message or substitute a default. This separation makes boundary cases testable and prevents a convenient display value from leaking into later calculations as though it were real data.

#include <iostream>
#include <optional>
#include <vector>
std::optional<int> firstPositive(const std::vector<int>& values){
 for(int x:values) if(x>0) return x;
 return std::nullopt;
}
int main(){auto answer=firstPositive({-2,0,6}); std::cout<<answer.value_or(-1)<<"\n";}

6

The function returns at the first positive value. An empty or nonpositive-only vector produces nullopt rather than an invented match.