Value Categories and References

Swap By Reference

Functions can take references when they need to update caller-owned values directly.

reference parameter A non-const reference parameter allows a function to update the caller's variable.
swap Swapping through reference parameters changes both variables outside the function.

Swap By Reference

first
swap_by_reference.cpp
Replay: real traced execution (multi-file project)
#include <iostream>

void swap_values(int& left, int& right) {
    int saved = left;
    left = right;
    right = saved;
}

int main() {
    int first = 4;
    int second = 9;

    swap_values(first, second);

    std::cout << "first=" << first << std::endl;
    std::cout << "second=" << second << std::endl;
    return 0;
}

#include <iostream>

void swap_values(int& left, int& right) {
    int saved = left;
    left = right;
    right = saved;
}

int main() {
    int first = 1;
    int second = 9;

    swap_values(first, second);

    std::cout << "first=" << first << std::endl;
    std::cout << "second=" << second << std::endl;
    return 0;
}

#include <iostream>

void swap_values(int& left, int& right) {
    int saved = left;
    left = right;
    right = saved;
}

int main() {
    int first = 10;
    int second = 9;

    swap_values(first, second);

    std::cout << "first=" << first << std::endl;
    std::cout << "second=" << second << std::endl;
    return 0;
}

  1. first ← 4, second ← 9

    9int main() {10    int first→ 4 = 4; //@first=1, 1011    int second→ 9 = 9;1213    swap_values(first4, second9);

  2. saved ← 4, left ← 9, right ← 4

    3void swap_values(int& left4, int& right9) {4    int saved→ 4 = left4;5    left→ 9 = right9;6    right→ 4 = saved4;7}

  3. first ← 9, second ← 4

    13    swap_values(first→ 9, second→ 4);1415    std::cout << "first=" << first9 << std::endl;16    std::cout << "second=" << second4 << std::endl;17    return 0;18}
    outputfirst=9
second=4

  1. first ← 1, second ← 9

    9int main() {10    int first→ 1 = 1;11    int second→ 9 = 9;1213    swap_values(first1, second9);

  2. saved ← 1, left ← 9, right ← 1

    3void swap_values(int& left1, int& right9) {4    int saved→ 1 = left1;5    left→ 9 = right9;6    right→ 1 = saved1;7}

  3. first ← 9, second ← 1

    13    swap_values(first→ 9, second→ 1);1415    std::cout << "first=" << first9 << std::endl;16    std::cout << "second=" << second1 << std::endl;17    return 0;18}
    outputfirst=9
second=1

  1. first ← 10, second ← 9

    9int main() {10    int first→ 10 = 10;11    int second→ 9 = 9;1213    swap_values(first10, second9);

  2. saved ← 10, left ← 9, right ← 10

    3void swap_values(int& left10, int& right9) {4    int saved→ 10 = left10;5    left→ 9 = right9;6    right→ 10 = saved10;7}

  3. first ← 9, second ← 10

    13    swap_values(first→ 9, second→ 10);1415    std::cout << "first=" << first9 << std::endl;16    std::cout << "second=" << second10 << std::endl;17    return 0;18}
    outputfirst=9
second=10