c++ - bad_variant_access not working -


i trying std::bad_variant_access exception when variant not in list .but below code doesn't work returns implicit converted ascii int value

what changes should variant strict on type selection

#include <iostream> #include <variant> using namespace std;  struct printer {     void operator()(int x) {         cout << x << "i"<<endl;      }     void operator()(float x) {         cout << x << "f"<<endl;     }     void operator()(double x)     {         cout << x << "d" << endl;;     } }; int main() {      using my_variant = std::variant<int, float, double>;     my_variant v0('c');      try {         std::visit(printer{}, v0);         }      catch(const std::bad_variant_access& e) {         std::cout << e.what() << '\n';     }      return 0; }

output:

99i

whereas expecting std::bad_variant_access exception

code

std::visit trigger bad_variant_access exception if variant valueless_by_exception (c++17, see n4659 23.7.3.5 [variant.status] )

what means if tried set variant value in fashion throws exception, variant left in "valueless" state, visitation not permitted.

to trigger it, can change code so:

struct s{      operator int() const{throw 42;} };  struct printer{//as before};  int main() {     using my_variant = std::variant<int, float, double>;     my_variant v0{'c'};      try{        v0.emplace<0>(s());     }catch(...){}      try {         std::visit(printer{}, v0);     }     catch(const std::bad_variant_access& e) {         std::cout << e.what() << '\n';     } }

demo

frank answered why construct variant in first place using char (construction chosen via overload).

you can not trigger bad_variant_access attempting first construct variant in fashion throw because [variant.ctor] dictates constructor rethrow exception (in case int).


Comments

Post a Comment

Popular posts from this blog

What is happening when Matlab is starting a "parallel pool"? -

running parallel cpu processes in matlab starts command parpool() according documentation , function: [creates] special job on pool of workers, , [connects] matlab client parallel pool. this function takes bit of time execute, on order of 30 seconds. in other multi-cpu paradigms openmp, parallel execution seems totally transparent -- i've never noticed behavior analogous matlab (granted i'm not experienced parallel programming). so, happening between time parpool() called , when finishes executing? takes long? parallel computing toolbox enables run matlab code in parallel using several different paradigms (e.g. jobs , tasks, parfor , spmd , parfeval , batch processing), , run either locally (parallelised across cores in local machine) or remotely (parallelised across machines in cluster - either 1 own, or 1 in cloud). in of these cases, code run on matlab workers , copies of matlab without interactive desktop. if you're intending run on remo...
Read more

angular - DownloadURL return null in below code -

when change file updated in firebase storage when try downloadurl show null. filechange(event: any) { const imagefolder: string = this.employee.id; const filelist: filelist = event.target.files; const file: file = filelist[0]; const storageref = firebase.storage().ref().child(`${imagefolder}/profile.jpg`).put(file); console.log(storageref.snapshot.downloadurl); } the put() asynchronous. if want url after file uploaded have this: firebase.storage().ref().child(`${imagefolder}/profile.jpg`).put(file).then((snapshot) => { storageref = snapshot.downloadurl: console.log(snapshot.downloadurl); });
Read more

php - Cannot override Laravel Spark authentication with own implementation -

during login process of laravel spark need request token of remote api. api stores exact same users , passwords laravel spark application. therefore need username , non hashed password of user during authentication process. i thought overriding authenticated method solution problem. in routes/web.php override post /login endpoint , pointing endpoint own logincontroller: <?php namespace app\http\controllers\auth; use illuminate\support\facades\log; use illuminate\http\request; use laravel\spark\http\controllers\auth\logincontroller sparklogincontroller; class logincontroller extends sparklogincontroller { /** * create new login controller instance. * * @return void */ public function __construct() { parent::__construct(); } /** * handle successful authentication attempt. * * @param request $request * @param \illuminate\contracts\auth\authenticatable $user * @return response */ public fu...
Read more