References: another name for a variable

The problem references solve

In the last module you saw that C++ copies arguments by default (pass by value). That is a problem in two cases. First, when a function SHOULD change the caller's variable, such as adding money to a wallet. Second, when the argument is big, such as a list of 1,00,000 orders; copying it on every call wastes time and memory. References solve both.

What a reference is

int marks = 80;
int& ref = marks;   // ref is another name for marks

ref = 95;
std::cout << marks << '\n';   // 95

Rules of references: a reference must be initialised when it is created (int& r; is an error), and it can never be changed to refer to a different variable later. Writing r = other; copies other's value INTO the variable r refers to.

Pass by reference

void addBonus(int& marks) {
    marks += 5;
}

void swapValues(int& a, int& b) {
    int temp = a;
    a = b;
    b = temp;
}

int main() {
    int m = 80;
    addBonus(m);
    std::cout << m << '\n';          // 85

    int x = 1, y = 2;
    swapValues(x, y);
    std::cout << x << ' ' << y;      // 2 1
}

Pass by const reference

#include <string>
#include <vector>

double average(const std::vector<int>& marks) {
    double sum = 0;
    for (int m : marks) sum += m;
    return marks.empty() ? 0 : sum / marks.size();
}

void greet(const std::string& name) {
    std::cout << "Namaste, " << name << '\n';
    // name += "!";   // error: name is const
}

References in range-for loops

std::vector<int> prices = {100, 200, 300};
for (int p : prices) p += 10;        // changes copies only
for (int& p : prices) p += 10;       // changes the real items: 110 210 310
for (const auto& p : prices) std::cout << p << ' ';

Common mistakes

• Forgetting the & in a function that should change its argument, then wondering why nothing changes.
• Passing a big std::vector or std::string by value in a loop that runs many times, which makes the program slow.
• Trying to pass a literal to a non-const reference: addBonus(80) fails, because 80 is not a variable. A const reference can bind to a temporary value, so greet("Asha") works.