c++ - What did the author mean with his comment about the input of a user-defined type? -


this example extracted section 10.3.3 input of user-defined types book "the c++ programming language" second edition, b. stroustrup. code old still compiles minor changes. example:

#include <istream> #include <complex>  using namespace std;  istream& operator>>(istream& s, complex<double>& a) {     // input formats complex; "f" indicates float:     //     //   f     //   (f)     //   (f, f)      double re = 0, im = 0;     char c = 0;      s >> c;     if( c == '(' ) {         s >> re >> c;         if( c == ',' ) s >> im >> c;         if( c != ')' ) s.clear(ios::badbit);  // set state     }     else {         s.putback(c);         s >> re;     }      if( s ) = complex<double>(re, im);     return s; }

despite scarcity of error-handling code, handle kinds of errors. the local variable c initilized avoid having value accidentally '(' after failed operation. final check of stream state ensures value of argument a changed if went well.

i failed understand phrase emphasized above.

if s >> c fails c not written to.

if c uninitialized , remains uninitialized @ point of test if( c == '(' ). reading uninitialized char causes undefined behaviour.

the author talking possible way undefined behaviour might manifest itself.

the suggested fix of char c = 0; relies on fact thats.putback(c); nothing if s not good(). ok, although imho clearer write:

char c; s >> c;  if ( !s )     return s;

then reading code can see behaves in case of error; instead of having thread way through flow of function , check no other operations unexpected.


Comments

Post a Comment

Popular posts from this blog

angularjs - ADAL JS Angular- WebAPI add a new role claim to the token -

i have created angular spa webapi backend using adal js authentication.as there no roles in ad, need manually add role claims in order give users access different api controllers. the roles stored in database. expecting inject claim through call webapi after authentication ad. webapi code might this. identity.addclaim(new claim("role", "user")); var ticket = new authenticationticket(identity, props); var accesstoken = startup.oauthbeareroptions.accesstokenformat.protect(ticket); is possible replace adal idtoken new token? is viable solution or there other better way handle this? as initial token generated azuread, possible edit token add new claim? appreciated. graph api supporting group claims. see here: http://justazure.com/azure-active-directory-part-4-group-claims/ if @ examples on page, users assigned groups , app can check group in claims. in current version of portal, need app manifest , modify it.
Read more

php - CakePHP HttpSockets send array of paramms -

i problem when make request (i can request). $link = 'http://uri/path/?param1=1&sid=2&sid=3&sid=4' $http = $httpsocket = new httpsocket(); $res = $httpsocket->get($link); if check request pr($http); i see : [line] => /url/path/?param1=1&sid%5b0%5d=2&sid%5b1%5d=3&sid%5b2%5d=4 http/1.1 and i'm getting empty response because server doesn't know parameters sid%5b0%5d=2 when try send parameters array : $data = array('param1' => '1', 'sid' => array('2','3', '4')); i see same changes %5b0%5d - additional indexes. how fix it? unfortunately , can not change server side, if send request in browser, i'll normal response in json format. when make request, can parameters this: $param1 = $_get['param1']; $sid= $_get['sid']; tip: use diferente names parameters. have multiples sid. use sid1, sid2, sid3... update: can parameters this:
Read more

node.js - Using Node without global install -

i'm working on few node & electron (atom shell) projects , i'm curious how them work without user having install node globally. i've seen few applications including node directory in project contains node.exe binary. how go setting node project use binary? edit: thought should add, i've searched everywhere answer this, unrelated results.
Read more