Modern C++ Features (C++17/20) Flashcards

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

Answer: x is int, y is double

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

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

Answer: If statement with initializer

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

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

Answer: Compilation error for operator+

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

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

Answer: false, because it holds no value

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

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

Answer: Class Template Argument Deduction (CTAD)

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

What guarantee does C++17 provide about the evaluation of `f() + g()` regarding the order of `f` and `g`?

Answer: No guaranteed order between f() and g()

C++17 strengthened sequencing rules but still leaves the relative order of operand evaluation for + unspecified.

In C++17, `if constexpr (condition)` differs from regular `if` in that:

Answer: The discarded branch is not instantiated in templates

if constexpr discards the non-taken branch at compile time within templates, preventing instantiation of otherwise ill-formed code.