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

Control Flow

Conditional Statements: if, else, switch

Introduction to Conditional Statements

Conditional statements allow programs to execute different blocks of code based on boolean expressions. They form the foundation of decision-making in programming. Both C++ and Rust provide similar constructs but with important syntactic and semantic differences reflecting their language philosophies.

if and else Statements

C++

• The if statement evaluates a condition; if the condition is true (non-zero), the following block executes; otherwise, an optional else block executes (cppreference).

• Syntax:

  if (condition) {
      // code if condition is true
  } else {
      // code if condition is false
  }
  

• Conditions must be convertible to bool; implicit conversion from integral or pointer types is allowed. Zero or nullptr evaluates as false, non-zero as true.

• Nested if-else and else if chains are common.

• Since C++17, if statements can include an initializer, introducing a new variable with limited scope:

  if (int x = foo(); x > 0) {
      // use x here
  }
  

• This pattern enhances code clarity and limits variable scope to the if block (cppreference if statement).

Rust

• In Rust, if is an expression and must evaluate to a boolean (bool) — no implicit conversion from integers or other types is allowed (Rust Reference).

• Syntax:

  if condition {
      // code if true
  } else {
      // code if false
  }
  

• Because if is an expression, it returns a value, allowing:

  let x = if condition { 5 } else { 10 };
  

• There is no traditional ternary operator in Rust (?: in C++); instead, if expressions serve this role.

• Rust requires explicit boolean conditions, enhancing type safety and reducing bugs.

The switch Statement

C++

• The switch statement allows multi-way branching based on integral or enumeration types (cppreference).

• Syntax:

  switch (expression) {
      case value1:
          // code
          break;
      case value2:
          // code
          break;
      default:
          // code
  }
  

• Important features:

  • switch works only with integral, enumeration, or constexpr values convertible to integral types.

  • Each case label must be a compile-time constant.

  • Fallthrough between cases occurs unless explicitly broken with break.

  • Since C++17, [[fallthrough]]; attribute can document intentional fallthrough.

• switch provides efficient jump table or binary search implementations by compilers.

Rust

• Rust does not have a switch statement. Instead, it provides a more powerful match expression (Rust Reference).

• match allows pattern matching on values of many types, not just integers.

• Syntax:

  match value {
      pattern1 => { /* code */ },
      pattern2 => { /* code */ },
      _ => { /* default case */ },
  }
  

• Features:

  • Exhaustiveness checking: all possible cases must be handled or covered by a wildcard _.

  • Patterns can be literals, ranges, enums, or destructured data.

  • match is an expression and returns a value.

  • Prevents bugs common in switch, such as missing cases or accidental fallthrough.

Summary of Differences

Practical Notes and Best Practices

• Use if and else for simple conditional branches in both languages.

• Prefer switch in C++ for multiple discrete integer or enum cases with care for break statements.

• Use Rust’s match for powerful, safe, and exhaustive multi-way branching that can destructure complex data types.

• Exploit C++17's if initializer to limit scope of variables used in conditions.

• Rust’s strict boolean conditions and exhaustive matching reduce runtime errors and improve code safety.

References

1. C++ if statement — cppreference https://en.cppreference.com/w/cpp/language/if

2. C++ switch statement — cppreference https://en.cppreference.com/w/cpp/language/switch

3. ISO C++17 Standard (for if initializer and [[fallthrough]]) https://isocpp.org/std/the-standard

4. Rust if expression — Rust Reference https://doc.rust-lang.org/reference/expressions/if-expr.html

5. Rust match expression — Rust Reference https://doc.rust-lang.org/reference/expressions/match-expr.html

6. Rust Book: Control Flow https://doc.rust-lang.org/book/ch03-05-control-flow.html