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A do-while loop in C++ is a post-test control flow statement that guarantees the execution of its enclosed block of code at least once. Unlike a standard while loop, which evaluates its condition prior to execution (pre-test), the do-while loop executes the loop body first and evaluates its boolean expression at the end of the iteration.

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Syntax

do {
    // Loop body: Statements to execute
} while (condition);
Critical Syntax Note: The while (condition) statement at the end of the loop must be explicitly terminated with a semicolon (;). Omitting this is a common compilation error.

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Execution Mechanics

  1. Execution: The program enters the do block and executes the statements sequentially.
  2. Evaluation: After the block executes, the condition is evaluated. This condition must yield a boolean value (true or false) or a type convertible to a boolean (where non-zero is true and zero is false).
  3. Branching:
    • If the condition evaluates to true, control flow jumps back to the beginning of the do block for the next iteration.
    • If the condition evaluates to false, the loop terminates, and control flow proceeds to the next statement immediately following the do-while construct.

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Variable Scope

Variables declared within the do block are strictly local to that block. They are destroyed at the end of the iteration and cannot be accessed within the while condition. Any variable evaluated in the condition must be declared in an outer scope prior to the do statement. 
// Incorrect Scope
do {
    int x = 5;
    --x;
} while (x > 0); // ERROR: 'x' is out of scope

// Correct Scope
int x = 5;
do {
    --x;
} while (x > 0); // Valid

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Loop Control Statements

  • break: Immediately terminates the do-while loop. Control flow jumps to the statement following the loop, bypassing the condition evaluation entirely.
  • continue: Skips the remaining statements in the current iteration of the do block and jumps directly to the while (condition) evaluation to determine if the next iteration should occur.

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Mechanical Example

The following code demonstrates the guaranteed single execution and subsequent condition evaluation: 
#include <iostream>

int main() {
    int counter = 5;

    do {
        std::cout << "Counter value: " << counter << '\n';
        --counter;
    } while (counter > 5); 

    // Output will be "Counter value: 5" exactly once, 
    // because the condition (4 > 5) evaluates to false after the first pass.

    return 0;
}
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