c++ - In which cases is it unsafe to (const) reference a return value? -

-

i tend use following notation lot:

const datum& d = a.calc(); // continue use d

this works when result of calc on stack, see http://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/. though compilers optimize here, feels nice explicitly avoid temporary objects.

today realized, content of d became invalid after data written member of a. in particular case function returned reference member, unrelated write however. this:

const datum& d = a.get(); // ... operation ... a.somedata = datum2(); // d invalid.

again, somedata has nothing d or get() here.

now ask myself:

  1. which side-effects lead invalidation?
  2. is bad-practice assign return values const reference? (especially when don't know interior of function)

my application single-threaded except qt gui-thread.

which side-effects lead invalidation?

holding reference internal state of class (i.e., versus extending lifetime of temporary) , calling non-const member functions may potentially invalidate reference.

is bad-practice assign return values const reference? (especially when don't know interior of function)

i bad practice hold reference internal state of class instance, mutating class instance, , continuing use original reference (unless documented non-const functions don't invalidate reference)


Comments

Post a Comment

Popular posts from this blog

html - Firefox flex bug applied to buttons? -

-
i'm trying apply flex button on firefox i'm having problem on chrome doesn't happens. this css: .button { display: inline-flex; align-items: center; } as can see in fiddle, "caret" word moved new line, happens if element <button> tag, <div> works expected. http://codepen.io/anon/pen/xblmgx is firefox bug or there css i'm missing? nevermind, looks "mr firefox" doesn't idea... <button> not implementable (by browsers) in pure css, bit of black box, perspective of css. means don't react in same way e.g. <div> would. this isn't specific flexbox -- e.g. don't render scrollbars if put "overflow:scroll" on button, , don't render table if put "display:table" on it. stepping further, isn't specific <button> . consider <fieldset> , <table> have special rendering behavior: data:text/html,<fieldset style="d...
Read more

html - Missing border-right in select on Firefox -

-
Image
when put select div width , padding-left, options have no border on right. more, happend if select have width 118px , parent have 15px of padding left! have idea, going on? my code: <!doctype html> <html> <head lang="en"> </head> <body> <div style="width: 1000px; padding-left: 15px;"> <select style="width: 118px;"> <option>a</option> <option>b</option> <option>c</option> </select> </div> </body> </html> and result: here plunker: click it happend on firefox on windows. tested on 40.0.2 version -webkit-box-sizing: border-box; -moz-box-sizing: border-box; -ms-box-sizing: border-box; box-sizing: border-box; source: https://stackoverflow.com/a/11390432/4431269 source of problem: https://bugzilla.mozilla.org/show_bug.cgi?id=924068 and use of google ! edit: (source of problem, explained) after rea...
Read more

python - build a suggestions list using fuzzywuzzy -

-
i building suggestion engine database. i'm using fuzzywuzzy module match user input string existing database shown below. if request.method == "get": display_dict = {} user_input = request.get["user_input"] # user input match_dict = {} food_items_obj = food_items.objects.all() # fetch objects table items in food_items_obj : match = fuzz.ratio(user_input.lower(),items.name.lower()) # try match user input existing names match_dict[match] = items.name # put 1 dictionary matched_dict = ordereddict(sorted(match_dict.items(),reverse = true)) # sort dictionary if max(matched_dict.keys()) == 100: # if exact match put in dict , return display_dict[100] = matched_dict[100] else : # if not found try best matching words.. ###########this part need help! ############# k,v in matched_dict.items(): if user_input in v : display_dict[k]...
Read more