C++ Functions

A function is a named block of code which you can call many times.

#include <iostream>

void greet() {
    std::cout << "Hello from a function!\n";
}

int add(int a, int b) {
    return a + b;
}

int main() {
    greet();
    std::cout << add(5, 7) << '\n';
}

Declaration and definition

A function must be known before you call it. When the definition comes later, write a declaration (prototype) first. Prototypes are normally in header files.

#include <iostream>

int square(int n);                 // declaration

int main() {
    std::cout << square(9) << '\n';
}

int square(int n) {                // definition
    return n * n;
}

Pass by value

The function gets a copy. Changing the parameter does not change the original (we will use references for that).

#include <iostream>

void change(int x) {
    x = 100;
    std::cout << "inside: " << x << '\n';
}

int main() {
    int n = 5;
    change(n);
    std::cout << "outside: " << n << '\n';
}

Return values and multiple results

#include <iostream>
#include <string>
#include <tuple>
#include <utility>

std::pair<int, int> min_max(int a, int b) {
    return {a < b ? a : b, a < b ? b : a};
}

auto describe(int n) {                         // the return type is found from the code
    return std::to_string(n) + (n % 2 == 0 ? " is even" : " is odd");
}

int main() {
    auto [low, high] = min_max(9, 4);          // structured bindings (C++17)
    std::cout << low << ' ' << high << '\n';
    std::cout << describe(7) << '\n';
}

Special function types

• inline: a hint to put the code in place of the call.
• constexpr: can run at compile time.
• [[nodiscard]]: the compiler warns when the result is ignored.

#include <iostream>

constexpr int factorial(int n) {
    return n <= 1 ? 1 : n * factorial(n - 1);
}

int main() {
    constexpr int f5 = factorial(5);              // calculated while compiling
    static_assert(f5 == 120);
    std::cout << f5 << '\n';
}

Rules

• Give a function one clear job and a verb name: calculate_total, is_valid.
• A function with a return type must return a value on every path (the compiler warns).
• main is special: it can not be called by you, and it returns 0 if you forget.

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