Passing `this` to a function as a shared_ptr - c++

Question

I am writing some example code that hopefully captures my current struggle.
Let's assume I have a class for some general shapes Shape
and a nice Function that doubles the perimeter of any shape
float DoublePerimeter (shared_ptr<Shape> shape)
return 2*shape->GetPerimeter();
};
Is it possible to use such a function in a class itself?
class Square : Shape {
float side = 1;
public:
void Square(float aside) : side(aside) {;}
float GetPerimeter(){return 4*side;}
void Computation() { DoublePerimeter (??????);}
};
What can I pass in the ?????? to make this work? I tried using something like
shared_ptr<Shape> share_this(this);
and also tried enable_shared_from_this<> for my class, however the pointer that I pass to the function always returns null on lock. Is this even possible, or is this bad design? Am I forced to make this function a member function?

Answer 1

If you don't want to use enable_shared_from_this, perhaps because your objects are not always owned by a shared pointer, you can always work around it by using a no-op deleter:
void nodelete(void*) {}
void Square::Computation() { DoublePerimeter({this, nodelete}); }
but it's a hack (and a fairly expensive one at that, since you're allocating and deallocating a control block just to con the function you're calling).
A cleaner solution, albeit one that might require more typing, is to separate your free function implementation from the ownership scheme:
float DoublePerimeter(Shape const& shape)
return 2*shape.GetPerimeter();
};
float DoublePerimeter(std::shared_ptr<Shape> shape)
return DoublePerimeter(*shape);
};
void Square::Computation() const { DoublePerimeter(*this); }

Answer 2

I've tried it out. It seems to work for me. See here or below. I fixed some small issues and made Computation return a value for demonstration purpose.
#include <memory>
#include <cassert>
#include <iostream>
class Shape {
public:
virtual float GetPerimeter(){return 0;}
};
float DoublePerimeter (std::shared_ptr<Shape> shape){
return 2*shape->GetPerimeter();
}
class Square : public Shape, public std::enable_shared_from_this<Square> {
float side = 1;
public:
Square(float aside) : side(aside) {}
float GetPerimeter(){return 4*side;}
float Computation() {
return DoublePerimeter (this->shared_from_this());
}
};
int main(){
auto square = std::make_shared<Square>(3.0);
std::cout <<square->Computation() << std::endl;
return 0;
}
edit:
Example for #pptaszni's comment. Changing the main to:
int main(){
Square square{3.0};
std::cout << (&square)->Computation() << std::endl;
return 0;
}
will result in a runtime error:
Program returned: 139
terminate called after throwing an instance of 'std::bad_weak_ptr'
what(): bad_weak_ptr