c++ - Do polymorphic C-style casts have any overhead? -

-

  • does casting pointer instance of dervived class instances base class have run-time overhead in c++, or resolved @ compile-time?

  • if have any, has computed in casting operation?

example:

class foo; class bar : public foo;  bar* y = new bar; foo* x = (foo*) y;

(i know should use c++-style casts , answer same them)

yes, though it's negligible.

in case, c-style cast interpreted static_cast, may incur pointer adjustment.

struct base {};  struct derived: base { virtual ~derived(); } // note: introduced virtual method  int main() {   derived derived;   base* b = &derived;   derived* d = (derived*) b;    void* pb = b;   void* pd = d;    printf("%p %p", pb, pd);    return 0; }

this overhead happens whenever base subobject not aligned @ same address derived object, happens when:

  • introducing first virtual method
  • using multi-inheritance

of course, pointer adjustment counted negligible, , compiler shall smart enough eliminate if it's unnecessary anyway (an adjustment of 0).

note: implementation dependent, not mandated standard


Comments

Post a Comment

Popular posts from this blog

linux - Mailx and Gmail nss config dir -

-
i trying send mail linux command line using mailx command. can send local domain no problem want set mail send gmail account receive mail sent gmail account. after configuring mail.rc so: account gmail { set smtp=smtps://smtp.gmail.com:587 set smtp-auth=login set smtp-auth-user=username@gmail.com set smtp-auth-password=password set ssl-verify=ignore } i error: resolving host smtp.gmail.com . . . done. connecting 74.125.25.109 . . . connected. missing "nss-config-dir" variable. "/home/username/dead.letter" 11/354 . . . message not sent. after looking "nss-config-dir" here , located certn.db , keyn.db files , added mail.rc so: account gmail { set smtp=smtps://smtp.gmail.com:587 set smtp-auth=login set smtp-auth-user=username@gmail.com set smtp-auth-password=password set ssl-verify=ignore set nss-config-dir=/home/user/.mozilla/firefox/location.default } now when try send mail using command: echo "sent gmail account" | mailx -v -a
Read more

c# - Is it possible to remove an existing registration from Autofac container builder? -

-
something along lines: builder.registertype<mytype>().as<itype>(); builder.registertype<mytype2>().as<itype>(); builder.deregistertype<mytype>().as<itype>() var container = builder.build(); var types = container.resolve<ienumerable<itype>>(); assert.istrue(types.count == 1); assert.istrue(types[0].gettype == typeof(mytype2)); scenario: go through bunch of assemblies , go register types want make sure have 1 implementation of given type. need before create container. track on own nice if autofac me bit. this cannot done directly using containerbuilder , unless start on new one. mind you, having first built container should able construct new container filtering away unwanted types , reusing registrations first container. this: ... var container = builder.build(); builder = new containerbuilder(); var components = container.componentregistry.registrations .where(cr => cr.activator.limittype != typ
Read more

php - Mysql PK and FK char(36) vs int(10) -

-
i have read mysql can handle indexes , searches better if tables keys integers rather char(36) uuid's. is worth converting int(10)? note: use 1 server , have have plans use multiple databases concurrently therefore removing 1 of big reasons using uuid. our tables innodb if makes difference. thanks in advance. usually rdbms can handle integer keys pk more efficient, other datatypes. reason how build index column, yes: long dont require string (or other typed) keys, should use integers. however: char(36) , int(10) far away being equal, because int(10) much smaller, char(36). dont know, if require many different keys, should keep in mind. update, complete last paragraph: int(10) 32-bit, char(36) 36 - ^byte^ (= 288-bit). not mean, int consumes less space, means, char(36) has 4 times more different keys.
Read more