How to simplify this code where a variable is declared after an user input in c++?

I wrote the following code :

#include <iostream>
using namespace std;

class Shape {
    public : float lgth;
    public : void getLgth() { cin >> lgth;}
};

class Square : public Shape {
    public : float calcArea() { return lgth * lgth; }
};

class Circle : public Shape {
    public : float calcArea() { return 3.14 * lgth * lgth; }
};

int main() {
    char shape_type;

    cout <<"Shape Type (c/s) ? ";
    cin >> shape_type;
    if (shape_type == 'c') {
      Circle my_shape;
      cout << "Shape Length ? ";
      my_shape.getLgth();
      cout << "Shape Area : " << my_shape.calcArea() << endl;
    }
    else if (shape_type == 's') {
      Square my_shape;
      cout << "Shape Length ? ";
      my_shape.getLgth();
      cout << "Shape Area : " << my_shape.calcArea() << endl;
    }

    return 0;
}

As the type of shape is not known at the beginning of the execution (it is asked to user), the variable my_shape is declared in an if then else statement. After this variable is only known in this scope (I understand this point). So, this part of code :

  cout << "Shape Length ? ";
  my_shape.getLgth();
  cout << "Shape Area : " << my_shape.calcArea() << endl;

must be repeated twice in this example.(But more if many shape classes exist).

I am a beginner in C++ but I supposed it is not the best way to do it. Any suggestions would be welcome. Thanks

>Solution :

With template, you might factorize code:

template <typename TShape>
void do_job()
{
    TShape my_shape;
    std::cout << "Shape Length ?";
    my_shape.getLgth();
    std::cout << "Shape Area : " << my_shape.calcArea() << std::endl;
}

And in your main:

if (shape_type == 'c') {
    do_job<Circle>();
} else if (shape_type == 's') {
    do_job<Square>();
}

or with more types

switch (shape_type) {
    case 'c': do_job<Circle>(); break;
    case 's': do_job<Square>(); break;
// ...
}