CPP Modern C++ Features (C++17/20) 2

Question 1

What does the C++17 structured binding `auto [x, y] = std::make_pair(1, 2.0);` deduce for `x` and `y`?

Accepted Answer

x is int, y is double

Answer

Structured bindings decompose the pair preserving each element's original type, so x is int and y is double.

Question 2

Which C++17 feature allows `if (auto it = map.find(key); it != map.end())` syntax?

Accepted Answer

If statement with initializer

Answer

C++17 added an optional init-statement to if and switch, scoping the declared variable to the conditional block.

Question 3

What is the result of the fold expression `(args + ...)` when args is an empty pack?

Accepted Answer

Compilation error for operator+

Answer

Unary fold over + with an empty pack is ill-formed; only &&, ||, and comma operator have identity elements defined for empty packs.

Question 4

What does `std::optional<int> opt;` evaluate to when used in a boolean context?

Accepted Answer

false, because it holds no value

Answer

A default-constructed std::optional is disengaged, and its bool conversion returns false.

Question 5

Which C++17 class template deduction guide allows `std::vector v = {1, 2, 3};` without specifying `<int>`?

Accepted Answer

Class Template Argument Deduction (CTAD)

Answer

CTAD lets the compiler deduce template arguments from constructor call arguments, eliminating the need for explicit angle-bracket types.

(CPP) C++ Programming Professional Certificate Practice Test