c++ - call another constructor from constructor body -

i have object has different constructors. want add constructor based on arbitrary data try process data , call correct constructor.

afaik there 2 ways it:

• static function returns created object

  static *a from_data(/arbitrary data/);

• create private "init" function called constructors

but wondering problems , potential pitfalls of using placement new on using desired constructor. code be:

#include <new>  struct {     /*different constructors*/     a(int i) {};     a(double d) {};      a(/*large set of data*/)     {         /*large set of data gets processed , depending on processing different constructor gets called*/          this->~a(); //deletes current object         if (true) {             new (this) a(1); //reconstructs object calling correct constructor data         } else {             new (this) a(1.0); //reconstructs object calling correct constructor data         }      }; };   int main(int argc, char* argv[]) {     a; } 

you want compile time decision (choice of constructor delegate to) run-time information (result of processing data set).

that means directly want, cannot done.

but can use idea of static member function returns created object, except instead of a* should return a, or else smart pointer.

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re other suggested solution,

” create private "init" function called cosntructors

… doesn't solve stated problem, , it's bad idea.

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re

” problems , potential pitfalls of using placement new on

well, it's not code can rely on. believe it's undefined behavior because @ point object self-destroys it's not initialized. if should turn out it's technically defined case @ hand, it's not code want rely on. , in setting possible maintenance, stumbling on code have replace rather wasting time trying prove ub or not. no™ dark corners of language.