Question

I have two constructors for an objects, which use two different sets of data.

myclass myobject(set 1);
myclass myobject(set 2);

set 1 and set 2 are of different types.

I want to create an object of myclass called myobject either with constructor 1 or constructor 2 depending on the set of arguments are available at this point and then use myobject outside of the if statement.

if(set1)
  myclass myobject(set 1);
else
  myclass myobject(set 2);
myobject.operation();

Can anyone tell me the good way to do this?

Answer 1

If default construction and copy (or move) assignment are cheap, just use:

myclass myobject;
if (set1)
    myobject = myclass(1, 2);
else
    myobject = myclass(3);
myobject.operation();

If it is acceptable to allocate things on the heap, use a pointer:

std::unique_ptr<myclass> myobject;
// const std::auto_ptr<myclass> in C++03 (it's fine as long as you keep it unique)
if (set1)
    myobject.reset(new myclass(1, 2));
else
    myobject.reset(new myclass(3));
myobject->operation();

If you must allocate it on stack, I don’t see anyway except using placement new (or changing the constructor to accept both possibilities).

#include <new>

char myobject_buffer[sizeof(myclass)];
myclass* ptr;
if (set1)
    ptr = new (myobject_buffer) myclass(1, 2);
else
    ptr = new (myobject_buffer) myclass(3);
ptr->operation();
...
ptr->~myclass();  // <-- use RAII to call this for exception safety