c++ - Using a pointer in the same statement of its declaration -


i came across code pointer used on same line of declaration. essential sscce it:

#include "stdafx.h" #include <iostream>  struct c  {     uint32_t a; };  int main() {     c* pc = (c*) malloc(sizeof(*pc));     // <---- ???     pc->a = 42;      std::cout << pc << std::endl;     std::cout << pc->a << std::endl;      free(pc); }

when try similar uint32 (insert before free()):

uint32_t = + pc->a; std::cout << << std::endl;

then either nothing printed statement, or while debugging random value stored in a , vs2015 gives me runtime warning. errorlevel after execution 3. know can't work.

why can use pointer? legal? why isn't compiler complaining such statements? statement split multiple statements behind scenes?

uint32_t = + pc->a; gives bad results because using value of a before initialized undefined behavior. more on see why 'int = i;' legal?

c* pc = (c*) malloc(sizeof(*pc)); on other hand not using value of pc in initialization. sizeof, when applied named variable, gives size of type. unevaluated operand means not evaluated. not using value of *pc instead getting size of pointer points c.

one reason want if changed type of pc not have change sizeof part. if had

c* pc = (c*) malloc(sizeof(c));

then need remember change sizeof(c) part when changing type of pc.

of course of can avoided using std::unique_ptr like

auto pc = std::make_unique<c>();                            ^ here place have specify type

Comments

Post a Comment

Popular posts from this blog

inversion of control - Autofac named registration constructor injection -

does autofac support specifying registration name in components' constructors? example: ninject's namedattribute . you need use autofac.extras.attributed package on top achieve this so let's have 1 interface , 2 classes: public interface ihello { string sayhello(); } public class englishhello : ihello { public string sayhello() { return "hello"; } } public class frenchhello : ihello { public string sayhello() { return "bonjour"; } } then have consumer class, in want select instance injected: public class helloconsumer { private readonly ihello helloservice; public helloconsumer([withkey("en")] ihello helloservice) { if (helloservice == null) { throw new argumentnullexception("helloservice"); } this.helloservice = helloservice; } public string sayhello() { return this.helloservice.sayhello()...
Read more

verilog - Systemverilog dynamic casting issues -

i've code snippet following in testbench function void write_to_port( my_data_type_base data ); my_data_type_extended data_ext; if(!$cast(data_ext, data)); `uvm_error(get_type_name(), "failed cast"); `uvm_info(get_name(), $psprintf("data_ext :\n%s", data_ext.sprint()), uvm_medium) // write data_ext out port.... endfunction when run it, i'm getting uvm_error "failed cast." i'm not quire sure why $cast not returning 1. can see, i'm printing out extended class data item after casting uvm_info. can see it's being cast properly. if don't use $cast if condition, don't runtime error. isn't coding practice use if dynamic cast check if $cast returning 1 ? what might reason behind cast not returning 1 in above case? i think semicolon on line 'if' not belong? i think consumes if statement, , uvm_error executes regardless of how if evaluates: if(!$cast(data_ext, data)); <- no semico...
Read more

ios - Change Storyboard View using Seague -

i have following code checks connection server, , depending on want change views if no connection found. nsstring *connect = [nsstring stringwithcontentsofurl:[nsurl urlwithstring:@"http://myserver.com"] encoding:nsutf8stringencoding error:nil]; if (connect == null) { [self performseguewithidentifier:@"network_failure" sender:self]; } else { // other code here } my problem not change views. have added storyboard segue between 2 views identifier of network_failure . not have navigation controller or that, problem? thanks help! no not problem. first of all, in objectivec exists nil object oriented null . must use that. so: if(connect == nil) or better: if(!connect) so due fact nil not same of null , probably code does't enter in if scope . you have been able discover breakpoint. anyway: be sure set trigger segue (line) 2 view controller , , name network_failure (that not best name. should instead view1toview2 ) as, m...
Read more