Create conditional an object on stack? - c++

Question

What is the easiest way to create an object on stack based on a certain condition within the current scope?
Assume my current code is:
Base obj();
obj.print();
Now I want to change the code to 'plugin' a different derived class e.g. :
bool doBase=true;
doBase ? Base obj() : Derived obj();
obj.print();
But this of course does not work?
I know how to get this working, e.g. with an if, however my print in this example is in real world say 10-50 lines so this result in code duplication. Another approach is dynamically creating the objects, but I am curious there is a way to fix this without dynamically creating the objects.
bool doBase=true;
if (doBase) {
Base obj();
obj.print();
} else {
Derived obj();
obj.print(); // CODE DUPLICATION
}
Full snippet:
#include <iostream>
class Base {
virtual void print() { std::cout << "In base"<<std::endl; }
};
class Derived : public Base {
virtual void print() override { std::cout << "In derived"<<std::endl; }
};
int main() {
bool doBase=true;
doBase ? Base obj() : Derived obj();
obj.print();
doBase=false;
doBase ? Base obj2() : Derived obj2();
obj2.print();
}

Answer 1

You could use lambdas to wrap the repetitive code, eg:
#include <iostream>
class Base {
public:
virtual void print() { std::cout << "In base"<<std::endl; }
};
class Derived : public Base {
public:
virtual void print() override { std::cout << "In derived"<<std::endl; }
};
int main() {
auto print = [](Base &&obj){ obj.print(); };
auto doIt = [&print](bool doBase) { doBase ? print(Base{}) : print(Derived{}); };
doIt(true);
doIt(false);
}
Online Demo
Alternatively, use std::variant, eg:
#include <iostream>
#include <variant>
class Base {
public:
virtual void print() { std::cout << "In base"<<std::endl; }
};
class Derived : public Base {
public:
virtual void print() override { std::cout << "In derived"<<std::endl; }
};
int main() {
auto print = [](Base& obj){ obj.print(); };
auto doIt = [&print](bool doBase) {
std::variant<Base, Derived> v;
if (doBase) {
v = Base{};
} else {
v = Derived{};
}
std::visit(print, v);
};
doIt(true);
doIt(false);
return 0;
}
Online Demo