Functions

Inline, static, and linkage (overview)

• inline suggests the compiler may substitute the body at the call site (hint, not a guarantee).
• Good for tiny functions; modern compilers often inline without the keyword.
• static on a free function gives **internal linkage** (file-local) in C++.
• Put prototypes in headers, definitions in .cpp (except templates / inline in headers).

Return rules and multiple results

• Non-void functions should return a value on every path (compilers warn otherwise).
• To return multiple values: use references/out-params, a struct/pair/tuple, or a small class.
• return exits the function immediately — code after it in that path does not run.

#include <utility>
using namespace std;

pair<int, int> minMax(int a, int b) {
    if (a < b) return {a, b};
    return {b, a};
}

Real-life function designs

• E-commerce: double calcTotal(items, coupon) returns payable amount; void printInvoice(...) displays.
• Banking: bool withdraw(Account& a, int amount) modifies balance by reference, returns success.
• School: overloaded grade(int marks) vs grade(double gpa) for different inputs.
• Config: connect(host, port = 443) uses a default secure port.

Prototype + definition + call

#include <iostream>
using namespace std;

int square(int n); // prototype

int main() {
    cout << square(6) << "\n";
    return 0;
}

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

void vs returning function

#include <iostream>
using namespace std;

void showMenu() {
    cout << "1. Tea\n2. Coffee\n";
}
int priceOf(int choice) {
    if (choice == 1) return 20;
    if (choice == 2) return 30;
    return 0;
}

int main() {
    showMenu();
    cout << "Coffee costs " << priceOf(2) << "\n";
    return 0;
}

Pass by value vs reference

#include <iostream>
using namespace std;

void addBonusValue(int salary) { salary += 1000; }
void addBonusRef(int& salary) { salary += 1000; }

int main() {
    int pay = 20000;
    addBonusValue(pay);
    cout << pay << "\n"; // 20000
    addBonusRef(pay);
    cout << pay << "\n"; // 21000
    return 0;
}

Scope — local vs global

#include <iostream>
using namespace std;

int tickets = 0; // global counter (demo only)

void book() {
    int seat = 12; // local
    tickets++;
    cout << "Booked seat " << seat << "\n";
}

int main() {
    book();
    book();
    cout << "Total tickets = " << tickets << "\n";
    return 0;
}

Default arguments — shipping

#include <iostream>
#include <string>
using namespace std;

void ship(const string& item, string mode = "Standard") {
    cout << "Shipping " << item << " via " << mode << "\n";
}

int main() {
    ship("Book");
    ship("Phone", "Express");
    return 0;
}

Overloading — area

#include <iostream>
using namespace std;

double area(double r) { return 3.14159 * r * r; }
double area(double l, double w) { return l * w; }

int main() {
    cout << area(5) << "\n";
    cout << area(4, 6) << "\n";
    return 0;
}

Bank withdraw with reference

#include <iostream>
using namespace std;

bool withdraw(int& balance, int amount) {
    if (amount <= 0 || amount > balance) return false;
    balance -= amount;
    return true;
}

int main() {
    int bal = 5000;
    if (withdraw(bal, 1200))
        cout << "OK, left " << bal << "\n";
    else
        cout << "Failed\n";
    return 0;
}