How to use void* as a single variable holder? (Ex. void* raw=SomeClass() )

Question

I am trying to make void* to hold a value (to avoid default constructor calling).

I want to:-

• copy K to void* e.g. K k1; --> void*

Answer 1

What you are trying to do doesn't make sense. A void * is used to hold an arbitrary type of pointer, not an arbitrary type of other object. If you want to use storage for an arbitrary object type, use a char[].

Other problems with your code include that you need to ensure correct alignment of the raw storage, use reinterpret_cast to a reference rather than static_cast to a non-reference, your in-place destructor call syntax is wrong, and that you don't construct the K object in the "raw" storage. Here's a corrected version:

#include <string>
#include <iostream>

class K{
    public: std::string yes="yes"   ;
};

int main() {
    //objective:  k1->raw->k2  , then delete "raw"
    alignas(alignof(K)) char raw[sizeof(K)];        //<--- try to avoid heap allocation
    K k1;
    new (reinterpret_cast<K *>(&raw)) K(k1);     //<--- compile now succeeds :)
    K k2= reinterpret_cast<K &>(raw);
    std::cout << k2.yes << std::endl;           //test
    reinterpret_cast<K&>(raw).K::~K();  // call destructor
    return 0;
}