#17 Modern C++ and Rust Programming – A Comparative Educational Guide from Concepts to Applications

Functions and Scoping

Parameters and References

Function parameters define how data is passed to functions. Both C++ and Rust provide sophisticated mechanisms for passing arguments, including by value, by reference, and using pointers or borrowing. Understanding these mechanisms is crucial for writing efficient, safe, and idiomatic code.

Parameters in C++

Passing by Value

• Passing parameters by value means the function receives a copy of the argument. Modifications inside the function do not affect the original.

• Efficient for small data types (e.g., fundamental types like int, float) but can be costly for large objects due to copying.

• Example:

  void foo(int x) {
      x = 5;  // modifies local copy only
  }
  

• Reference: https://en.cppreference.com/w/cpp/language/function

Passing by Reference

• C++ supports references, allowing functions to access the original variable without copying.

• Syntax uses the ampersand & in the parameter declaration:

  void foo(int& x) {
      x = 5;  // modifies the original variable
  }
  

• Passing by reference avoids copying overhead, enables modification of arguments, and supports more complex data types efficiently.

• const references (const T&) allow passing large objects without copying while preventing modification, improving safety and performance.

• Since C++11, rvalue references (T&&) enable move semantics, allowing efficient transfer of resources instead of copying, crucial for performance optimization (ISO C++11 standard).

• References must be initialized and cannot be null, reducing errors common with pointers.

• References support binding to lvalues, const rvalues, and move semantics, forming the foundation of modern C++ performance paradigms.

• References can be qualified with & (lvalue reference) or && (rvalue reference), with distinct semantics.

• Reference: https://en.cppreference.com/w/cpp/language/reference

Pointers vs. References

• Pointers can be null, support pointer arithmetic, and are more flexible but require manual management.

• References are safer, simpler aliases to existing variables without nullability or arithmetic.

Parameters and References in Rust

Passing by Value

• Rust passes variables by value by default, moving ownership to the function parameter.

• Moving ownership transfers the resource, preventing data races and ensuring memory safety without a garbage collector (Rust Book).

• For Copy types (simple scalars like integers), the data is copied rather than moved.

• Example:

  fn foo(x: i32) {
      // x is a copy of the argument
  }
  

Passing by Reference (Borrowing)

• Rust uses borrowing to pass references without transferring ownership.

• References are declared with & for immutable borrowing or &mut for mutable borrowing.

• Borrowing enforces Rust’s ownership and borrowing rules at compile time, preventing data races and dangling pointers.

• Example:

  fn foo(x: &i32) {
      println!("{}", x);  // immutable borrow
  }
  
  fn bar(x: &mut i32) {
      *x += 1;  // mutable borrow
  }
  

• References in Rust must always be valid (non-null), enforced by the compiler.

• The borrow checker ensures that at any time, there is either one mutable reference or any number of immutable references, preventing undefined behavior.

• References do not require explicit deallocation.

• Reference: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html

Ownership and Lifetimes

• Rust parameters integrate with lifetimes, which specify how long a reference is valid.

• Functions can accept references with explicit or elided lifetimes, ensuring safe access without data races or dangling references (Rust Reference Lifetimes).

Comparison of C++ References and Rust Borrowing

Modern Practices

• C++20 and later encourage extensive use of references and move semantics for performance.

• Rust’s ownership and borrowing model represent a paradigm shift emphasizing memory safety without runtime overhead, influencing new language designs.

References and Further Reading

1. C++ Function Parameters and References — cppreference https://en.cppreference.com/w/cpp/language/function https://en.cppreference.com/w/cpp/language/reference

2. ISO C++ Standard (C++11 and later) on References and Move Semantics https://isocpp.org/std/the-standard

3. The Rust Programming Language — Ownership and Borrowing https://doc.rust-lang.org/book/ch04-01-what-is-ownership.html https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html

4. Rust Reference — Lifetimes https://doc.rust-lang.org/reference/lifetimes.html

5. Rustonomicon — Detailed Rust references and unsafe code insights https://doc.rust-lang.org/nomicon/references.html

Conclusion

Understanding function parameters and references is foundational to mastering both C++ and Rust. While C++ offers flexible but potentially unsafe references, Rust enforces strict ownership and borrowing rules to guarantee memory safety without sacrificing performance. This section prepares readers to write efficient and safe functions in both languages, appreciating their respective paradigms.