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C++ Tutorial - Lesson 24: This Pointer

Introduction, Overview

by John Kopp

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Introduction
Welcome to EasyCPlusPlus.com's free tutorial on C++ programming. This lesson covers the purpose and use of the "this" pointer. In addition to the explicit parameters in their argument lists, every class member function (method) receives an additional hidden parameter, the "this" pointer. The "this" pointer addresses the object on which the method was called. There are several cases when the "this" pointer is absolutely required to implement desired functionality within class methods.

Purpose
To understand why the "this" pointer is necessary and useful let's review how class members and methods are stored. Each object maintains its own set of data members, but all objects of a class share a single set of methods. This is, a single copy of each method exists within the machine code that is the output of compilation and linking. A natural question is then if only a single copy of each method exists, and its used by multiple objects, how are the proper data members accessed and updated. The compiler uses the "this" pointer to internally reference the data members of a particular object. Suppose, we have an Employee class that contains a salary member and a setSalary method to update it. Now, suppose that two Employees are instantiated.

class Employee {
public:
    ....
    void setSalary(double sal)
    {
        salary = sal;
    }
private:
    ....
    double salary;
    ....
}

int main()
{
    ....
    Employee programmer;
    Employee janitor;

    janitor.setSalary(60000.0);
    programmer.setSalary(40000.0);
    ....
}

If only one setSalary method exists within the binary that is running, how is the correct Employee's salary updated? The compiler uses the "this" pointer to correctly identify the object and its members. During compilation, the compiler inserts code for the "this" pointer into the function. The setSalary method that actually runs is similar to the following pseudo-code.

void setSalary(Employee *this, float sal)
{
    this->salary = sal;
}

The correct object is identified via the "this" pointer. So what, you say. Interesting, maybe. But if this implicit use of the "this" pointer were all there was, this topic would mainly be of interest to compiler designers. As we will see, explicit use of the "this" pointer also has great utility.

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