Where do we commonly perform validity checking in our user-defined classes c++ code example

Example: new class * [] c++

/*
 Keyword "this"
You can use keyword "this" to refer to this instance inside a class definition.

One of the main usage of keyword this is to resolve ambiguity between the names of 
data member and function parameter. For example:
*/
class Circle {
private:
   double radius;                 // Member variable called "radius"
   ......
public:
   void setRadius(double radius) { // Function's argument also called "radius"
      this->radius = radius;
         // "this.radius" refers to this instance's member variable
         // "radius" resolved to the function's argument.
   }
   ......
}