C++17 std::optional error: expected primary-expression before 'auto'

Question

I was experimenting with C++17 feature std::optional

The optional return type is std::optional>. I call the sum_p

Answer 1

if(auto Pair = sum_pair(vec, sum) )
    std::cout << "Resulting indexes are: " << Pair->first << " " << Pair->second << std::endl;
else
    std::cout << "Nothing found!\n";

this is valid C++. You are allowed to put a declaration in the opening condition of an if clause.

(auto Pair = sum_pair(vec, sum) )?
    std::cout << "Resulting indexes are: " << Pair->first << " " << Pair->second << std::endl
:
    std::cout << "Nothing found!\n";

this is not valid C++. Declarations are not expressions. There are places where expressions are allowed, but declararions are not. The left hand side of ?, the trinary operator, is one of them.