C++17 and C++20

Learn new features of C++17 and C++20.

Introduction to modern C++

C++17 and C++20 add stronger, safer tools to the language. C++17 brings structured bindings, if constexpr, std::optional and std::variant. C++20 adds concepts, ranges and modules on top.

These releases clean up legacy APIs and tighten compile-time checks. Bugs are caught at compile time; run-time cost is often zero. Modern C++ reduces raw pointer use and makes template error messages readable.

Safe returns with std::optional

A function may or may not produce a value. std::optional<T> models this in a type-safe way. When present the value is read with *opt or opt.value(); when absent std::nullopt is returned.

Example: std::optional<int> half_even(int x) returns x/2 for even numbers and std::nullopt for odd ones. The caller checks with if (opt). This is safer than sentinel values like -1.

Exercise half_even with std::optional

Write std::optional<int> half_even(int x): return x / 2 for even numbers and std::nullopt for odd ones. In main, call half_even(8), check with if (r) and print *r via std::cout.

Show example solution

#include <iostream>
#include <optional>

std::optional<int> half_even(int x) {
    if (x % 2 == 0) return x / 2;
    return std::nullopt;
}

int main() {
    auto r = half_even(8);
    if (r) std::cout << *r;
    return 0;
}

Test yourself

Which feature was added in C++20?
- Concepts
- auto type deduction
- Lambda expressions
- Range-based for loop
Show answer

Concepts — Concepts were added in C++20 to constrain templates.

Complete the empty return

std::optional<int> half_even(int x) {
    if (x % 2 == 0) return x / 2;
    return ____;
}

Show answer

std::optional<int> half_even(int x) {
    if (x % 2 == 0) return x / 2;
    return std::nullopt;
}

Match each feature to its version

structured bindings

Multi unpack (C++17)
if constexpr

Compile branch (C++17)
concepts

Constraint (C++20)
ranges

View chain (C++20)

Next lesson: Structured bindings

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