c++11 - C++ 11 - Is moving non local variable safe? -


this question has answer here:

say have function goes like:

void a_fct( std::vector<double> &some_vector ) {   std::vector<double> a_temporary_vector = some_vector;    ... stuff involving a_temporary_vector ...    some_vector = a_temporary_vector; }

which of course bit silly, intended bringing following general questions:

  • it seems me 1 should rather move a_temporary_vector some_vector here, goes out of scope. safe move some_vector a_temporary_vector rather copying ?

  • imagine somewhere else in code, have pointer vector given argument here. pointer still pointing if move some_vector a_temporary_vector ?

in both cases, can expect true stl implementation ?

the standard (in 17.6.5.15) says:

objects of types defined in c++ standard library may moved (12.8). move operations may explicitly specified or implicitly generated. unless otherwise specified, such moved-from objects shall placed in valid unspecified state.

thus, some_vector required have (unspecified but) valid state after being moved from. you're allowed move temporary , move again temporary some_vector.

std::vector<double> a_temporary_vector = std::move(some_vector); // stuff on a_temporary_vector not some_vector some_vector = std::move(a_temporary_vector);

pointers , references some_vector still valid pointers or references content of some_vector not (as expect when passing object function taking non-constant reference).

note: use swap in case instead if you're not sure whether move or not.

void foo( std::vector<double> &some_vector ) {   // swap contents of some_vector temporary   std::vector<double> a_temporary_vector;   a_temporary_vector.swap(some_vector);    // operate on temporary    // swap content some_vector   a_temporary_vector.swap(some_vector);   some_vector = a_temporary_vector; }

Comments

Post a Comment

Popular posts from this blog

python - Installing PyDev in eclipse is failed -

unable read repository @ http://pydev.sf.net/updates/content.xml. sun.security.validator.validatorexception: pkix path building failed: sun.security.provider.certpath.suncertpathbuilderexception: unable find valid certification path requested target i installing pydev in eclipse using python, doesn't install successfully. gives above error. happened? how solve error? i had error when trying install on laptop behind corporate firewall. couldn't tell setup of firewall, not sure of root cause. i got passed error installing pydev eclipse when out behind firewall. another way got around have eclipse on non-firewalled machine, install pydev eclipse, , copy whole eclipse directory non-firewalled machine firewalled machine. there better way, , way fix properly, didn't have time work out firewall doing. this on windows 7 machine (yeah know) not work *nix versions installed through package manager yum or apt.
Read more

PHP OOP-based login system -

lets building oop-based user authentication system, , incorporate following principles: direct injection, inheritance, encapsulation, polymorphism , single responsibility principle. my background in programming has relied on procedural programming, , thus, finding difficult put these practices correct use. assume have these classes: class config { public function set($key, $value); public function get($key, $default = null); } class user { public function __construct(pdo $dbh, $id = null); public function setprofile(profile $profile); } class auth { public function __construct(config $config); public function login($username, $password, $keeploggedin = true); public function isloggedin(); public function getloggedinuser(); public function logout(); public function register(array $data); } class session { public function start($sessionname = null); public function write($key, $value); public function read($key, $default =...
Read more

c# - Nested Internal Class with Readonly Hashtable throws Null ref exception.. on assignment -

in internal game class , have both a) defined nested internal gamertags class , b) defined gamertags[] _array variable. in internal gamertags class (the nested class, simplicity left out) have readonly hashtable . after initialization, still null ref exception when try assign value. i'm not changing pointer value only , need add initialization hastable? internal class gamertags { private readonly hashtable _privatereadonlyhashtable; private string _json; private string _hash; internal idictionary privatereadonlyhashtable { { return this._privatereadonlyhashtable; } } internal string hash { { this.ensurehash(); return this._hash; } } internal object this[string key] { { return this._privatereadonlyhashtable[k...
Read more