Databases
StackOverflow2013
Postsdbo
POLogical AND + assignment in c++, safe?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POLogical AND + assignment in c++, safe?
    primarykey
    data
    text
    <p>I just learned this great pattern (from javascript actually) and I would like to apply it to my c++ code.</p> <p>To explain the pattern, let's say I am representing a string as a linked list of these:</p> <pre><code>struct link_char; struct link_char { link_char * next; char code; }; </code></pre> <p>Note that the last character of any link_char string will always have code==0. This property means that I can check for a value in the string, while using &amp;&amp; short-circuiting to prevent NULL pointer access.</p> <pre><code>bool equals_hello( const link_char * first_char ) { const link_char * c = first_char; return c-&gt;code=='h' &amp;&amp; (c=c-&gt;next)-&gt;code=='e' &amp;&amp; (c=c-&gt;next)-&gt;code=='l' &amp;&amp; (c=c-&gt;next)-&gt;code=='l' // if string == "hel", we short-circuit here &amp;&amp; (c=c-&gt;next)-&gt;code=='o'; } </code></pre> <p>My question is about safety, not readability. I know the short-circuiting will work as long as &amp;&amp; is not overloaded. But will the assignment operations happen in the right order, or is it implementation defined?</p> <p>The above example is explicit about where reads/writes can happen, but I would also like to use this pattern in situations where there can be side-effects. For example:</p> <pre><code>// think of these as a bunch of HRESULT type functions // a return value of 0 means SUCCESS // a return value of non-zero yields an Error Message int err; ( !(err=initialize()) &amp;&amp; !(err=create_window()) &amp;&amp; !(err=run_app() ) || handle_error(err); </code></pre> <p>Will these kinds of operations work as intended cross-platform? I've read that "if you read a variable twice in an expression where you also write it, the result is undefined". But intuitively I feel like the short-circuiting guarantees the order, does it not?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. UScmeub
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POLogical AND + assignment in c++, safe?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POLogical AND + assignment in c++, safe?
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POLogical AND + assignment in c++, safe?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COI am assuming it's okay because `&&` is a sequence point. But I may be mistaken.
      singulars
      1. POLogical AND + assignment in c++, safe?
      1. USGWW
    2. CORe: Your comment on readability; I wouldn't check in such code unless I wanted to comment each instance of it with a link to this question :)
      singulars
      1. POLogical AND + assignment in c++, safe?
      1. USMerlyn Morgan-Graham
    3. COAlso, your first example won't check for `NULL` before dereferencing `code`.
      singulars
      1. POLogical AND + assignment in c++, safe?
      1. USMerlyn Morgan-Graham
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload