The calculator_plugins.h file is present in the chapter_5_codes/pluginlib_calculator/include/pluginlib_calculator folder and the main purpose of this file is to define complete functions of the calculator plugins, which are named as Add, Sub, Mul, and Div. Here is the explanation of this code:

#include <pluginlib_calculator/calculator_base.h> 
#include <cmath> 
 
namespace calculator_plugins  
{ 
  class Add : public calculator_base::calc_functions 
  { 

This header file includes the calculator_base.h for accessing the basic functionalities of a calculator. Each plugin is defined as a class and it inherits the calc_functions class from the calculator_base.h class:

    public: 
  Add() 
  { 
    number1_ = 0; 
    number2_ = 0; 
  } 
 
  void get_numbers(double number1, double number2) 
  { 
    try{ 
 
      number1_ = number1; 
      number2_ = number2; 
       } 
 
    catch(int e) 
    { 
    std::cerr<<"Exception while inputting numbers"<<std::endl; 
    } 
 
  }   
 
  double operation() 
  { 
    return(number1_+number2_); 
  } 
 
    private: 
      double number1_; 
      double number2_; 
}; 

In this code, we can see definitions of inherited get_numbers() and operations() functions. The get_number() retrieves two number inputs and operations() performs the desired operation. In this case, it performs additional operations.
We can see all other plugin definitions inside this header file.