Is there ever a need for a "do {...} while ( )" loop?

Bjarne Stroustrup (C++ creator) once said that he avoids "do/while" loops, and prefers to write the code in terms of a "while" loop instead. [See quote below.]

Since hearing this, I have found this to be true. What are your thoughts? Is there an example where a "do/while" is much cleaner and easier to understand than if you used a "while" instead?

In response to some of the answers: yes, I understand the technical difference between "do/while" and "while". This is a deeper question about readability and structuring code involving loops.

Let me ask another way: suppose you were forbidden from using "do/while" - is there a realistic example where this would give you no choice but to write unclean code using "while"?

From "The C++ Programming Language", 6.3.3:

In my experience, the do-statement is a source of errors and confusion. The reason is that its body is always executed once before the condition is evaluated. However, for the body to work correctly, something very much like the condition must hold even the first time through. More often than I would have guessed, I have found that condition not to hold as expected either when the program was first written and tested, or later after the code preceding it has been modified. I also prefer the condition "up front where I can see it." Consequently, I tend to avoid do-statements. -Bjarne

Avoiding the do/while loop is a recommendation included in the C++ Core Guidelines as ES.75, avoid do-statements.

Solution 1:

Yes I agree that do while loops can be rewritten to a while loop, however I disagree that always using a while loop is better. do while always get run at least once and that is a very useful property (most typical example being input checking (from keyboard))

#include <stdio.h>

int main() {
    char c;

    do {
        printf("enter a number");
        scanf("%c", &c);

    } while (c < '0' ||  c > '9'); 
}

This can of course be rewritten to a while loop, but this is usually viewed as a much more elegant solution.

Solution 2:

do-while is a loop with a post-condition. You need it in cases when the loop body is to be executed at least once. This is necessary for code which needs some action before the loop condition can be sensibly evaluated. With while loop you would have to call the initialization code from two sites, with do-while you can only call it from one site.

Another example is when you already have a valid object when the first iteration is to be started, so you don't want to execute anything (loop condition evaluation included) before the first iteration starts. An example is with FindFirstFile/FindNextFile Win32 functions: you call FindFirstFile which either returns an error or a search handle to the first file, then you call FindNextFile until it returns an error.

Pseudocode:

Handle handle;
Params params;
if( ( handle = FindFirstFile( params ) ) != Error ) {
   do {
      process( params ); //process found file
   } while( ( handle = FindNextFile( params ) ) != Error ) );
}