working process of this pointer in c++

Copyright 2012 2022 BeginnersBook . }; NULL. Note that no destructor is needed because C++ can clean up integer member variables for us. Consequently, this allows us to do the following: We have effectively condensed three lines into one expression! Lets take a look at this in more detail. All rights reserved. Although this is acceptable coding practice, we find using the m_ prefix on all member variable names provides a better solution by preventing duplicate names altogether! So, we assign the this variable to these variables for the easy calling of the variable in any class instance. It is worth noting that C++ offers another option for changing pointers and variables in general that is: references. via void pointers or template-types (not just. return *this; The modified code snippet then becomes: It is arguably much easier to observe that changing x has no effect on the values in v in this version of the example. When you declare and initialize a pointer, e.g. Attempting to assign x to p directly as shown in the commented code would cause a compilation error because p and x have different types. If the member function is declared with a cv-qualifier sequence cv, the type of this is cv X* (pointer to identically cv-qualified X). By signing up, you agree to our Terms of Use and Privacy Policy. class loc_mem_same Then, Locate and open your .cpp file on the desktop. The result is illustrated in Fig. Here is the new version of Calc with chainable functions: Note that add(), sub() and mult() are now returning *this. Since 9 is greater than 8, we move on to the right node (data = 10). } 9. You can read this value, increment it, decrement it, add another value to it etc. Developed by JavaTpoint. But what if the right pointer of the node (8) was pointing to NULL? This file should be saved and then closed. Five In-built string functions in C++ that you should know, String Data Structure Practice Problems: How to verify a character after a letter in a given string, Back to Basics: Programmings Storage Container. A node in a binary search tree is often represented as follows: A few variants are tree node structures that: Lets see how we can insert a new element into a binary search tree. Now we will write a code in that file in which we will create a class with three functions that will be interlinked with one another, and the output will be dependent on each other. Please mail your requirement at [emailprotected] Duration: 1 week to 2 week. Whenever we call a non-static member function of a class on some objects, a this pointer get created and address of that object get stored in it and passed as a hidden argument to that function and in function body we can use this pointer as a local variable. Open the terminal and get to the desktop directory by writing cd Desktop and then make a .cpp file by writing and executing the command touch with the filename and extension of .cpp. p is an int*; a pointer to a slot capable of holding other integers addresses. In other words, the this pointer cannot be aliased in a constructor: It is possible to execute delete this;, if the program can guarantee that the object was allocated by new, however, this renders every pointer to the deallocated object invalid, including the this pointer itself: after delete this; returns, such member function cannot refer to a member of a class (since this involves an implicit dereference of this) and no other member function may be called. Contrast this with the subtlety of the same issue when we were dealing with pointers. Second, it can sometimes be useful to have a class member function return the object it was working with as a return value. In this article, we learned about the concept and uses of the this pointer in the C++ programming language. Inside setID(), the this pointer holds the address of object simple. Therefore, inside a member function, this may be used to refer to the invoking object. Another example of using this pointer is to return the reference of current object so that you can chain function calls, this way you can call all the functions for the current object in one go. Since the function call now has an added argument, the member function definition needs to be modified to accept (and use) this argument as a parameter. However, there are a few occasions where doing so can be useful: First, if you have a constructor (or member function) that has a parameter with the same name as a member variable, you can disambiguate them by using this: Note that our constructor is taking a parameter of the same name as a member variable. cout << "The float value is = " << f << endl; A pointer to an integer is defined as follows: whereas a reference to an integer is defined as: 2. Not that it makes dealing with pointers less tricky, but this realization may make us slightly better at avoiding some of their common pitfalls. Please, do feel free to leave your comments, thoughts or questions. We know the search is over if x becomes NULL. On the other hand, calling a function that expects a reference looks exactly the same from a syntax perspective like calling a function that copies the parameters by value. 6. We check if the value were inserting (9) is less than the current node (8), which evaluates to false and we proceed to execute the code in the else block. During construction of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified. Thus, when this points to the address of simple, this->m_id will resolve to simple.m_id. { cout << "The character value is = " << c << endl; We have explored the pass-by-value semantics in C/C++ and discussed alternatives to use in case we would like to change variables or pointers passed to functions. In C++, this pointer is mainly used for accessing or referring the current instance variable of a class, this pointer is also used for passing the parameters which are current objects to any other method in the program, and lastly, this pointer is used for declaring the indexers in C++. public: Copyright 2022 Educative, Inc. All rights reserved. Now we will write a code in that file in which we will be using the same name for local and member variables of a class and a method, respectively, so that we can inherit the attributes of the private value to the local variable and assign or edit different values to the variable in the end. The answer is yes, but this initalizes p, a copy of v[i], as opposed to initializing v[i] itself. Let us try the following example to understand the concept of this pointer , When the above code is compiled and executed, it produces the following result , We make use of cookies to improve our user experience. The following behavior-changing defect reports were applied retroactively to previously published C++ standards. public: the value at address 500, also known as our integer variable x. float f = 2.05890; 12, we first conduct a search for it. Assuming we would like to insert the value 9 into the tree shown in Fig. Now the .cpp file should be saved and then closed. { Some programmers prefer using pointers when passing variables to functions that could change their values because having to use a pointer or the address-of operator as in the previous example gives a strong reminder to the programmer that the function theyre calling has the ability to change the value to which the pointer is pointing. But before we go any further, lets try to imagine how variables are represented in memory. We also assume were about to execute the first iteration of the for loop (i = 0): The code checks if p is NULL, which is false, and moves on to check v[1]. What if we change the contents of the slot j is pointing to (#500) though? One of the most useful data structures that are based on trees is Binary Search Trees. New programmers are sometimes confused about how many this pointers exist. Love podcasts or audiobooks? In general, we can define this in C++ as a pointer that is passed as an argument to nonstatic member functions where to access the object having a single copy of member functions the compiler provides this hidden pointer implicitly by using the keyword this followed by the function name because static member function does not use this pointer. But what happens if we pass a pointer to a function? ref_func &seti(int p) So far, pointers do not look any different. In this case, we use the this pointer to come out of this problem. When a non-static class member is used in any of the contexts where the this keyword is allowed (non-static member function bodies, member initializer lists, default member initializers), the implicit this-> is automatically added before the name, resulting in a member access expression (which, if the member is a virtual member function, results in a virtual function call). Because this is just a function parameter, it doesnt add any memory usage to your class (just to the member function call, since that parameter needs to be passed to the function and stored in memory). Copyright 2011-2021 www.javatpoint.com. We cannot have p point to x by just assigning x to p directly. We dont know if its the right place to insert our value yet. ref_func &setc(char r) { Lets try to visualize what happened. When the parameter name is the same as the private variables name, we use the this keyword inside the method to distinguish between the two when assigning values. } int main() Learn more. cout << "The character value is = " << c << endl; were using a pointer to pointer. In this post, I present an attempt to explain some of the intricate details of dealing with pointers in simple terms. When compiled, the compiler converts simple.setID(2); into the following: Note that this is now just a standard function call, and the object simple (which was formerly an object prefix) is now passed by address as an argument to the function. The this pointer will act as an indexer when assigned to any current class instance variable. Like pointers, you can use references to change the variables at which theyre pointing. #include This procedure can be written recursively as follows: One nice thing about this algorithm is that whenever we fail to find any given value, we end up landing at the pointer that should have been pointing to that value had it been in the tree. The this pointer will be beneficial as it will ease transferring their objects to one another. In such case if you want to assign the local variable value to the data members then you wont be able to do until unless you use this pointer, because the compiler wont know that you are referring to objects data members unless you use this pointer. How do we do that? But why does it sound all confusing when we start using pointers? x to r, without any special operators. We will discuss some of them in detail. this always points to the object being operated on. Finally, calc.mult(4) multiplies m_value by 4 and returns calc, which isnt used further, and is thus ignored. This difference also explains why we have special syntax to change the contents of the slot/variable a pointer is pointing to, e.g. 9, we assume that v contains three pointers: v[0] points to slot #500, while v[1] and v[2] both point to NULL. You can think of a binary search tree as a sorted array. Lets see what happens when we pass variables to other functions. You can use a pointer slot to modify the contents of another slot. Think with me: what should the type of j be in this case? The code in the else block calls the same function recursively assigning the value it returns to, Without thinking about any further recursive calls, what does the second version of insert() do to, Now regardless of what the code in the if-else block spanning lines 16 throughout 13 does, we can easily see that the code returns, This means that executing line #22 in the first copy of. We recommend that you avoid doing so, as it tends to make your code less readable for little benefit. Lets say we write the following code: When process() is called, the code copies the contents of the slot/variable x into a new slot n that process() uses. Also, note it when we are using this pointer in the constructor we need to be careful as the objects may not be formed yet. private: The following is a simple class that holds an integer and provides a constructor and access functions. c = r; In this article, the usage of the this pointer is fully described, and the relevance of its use is demonstrated by examples in run-time circumstances in Ubuntu 20.04 environment in extensive detail. There can be 3 main usage of this keyword in C++. Another easy way to achieve the same effect is to have process() return the new address and assign it directly to p. In fact, this is exactly how operator new works in C++; it is just a function that allocates a block of memory on the Operating Systems heap/free-store and returns its address. Well, j is definitely a pointer, and its a pointer that is capable of pointing to stuff like p. What is p? For example Myclass:: myStaticMethod(int num); To test if this pointer is available to static method, write this method to above class Myclass. Here we discuss the introduction and working of this pointer in C++ with examples respectively. Pointer slots are similar to non-pointer slots in the sense that both are copied when theyre passed to functions. void name_var (int i, float f,char c) 504, to it. To focus: sub questionsQ-when does this pointer in C++ get created?Answer: when a non-static member function of a class gets called.Q- what is a type of this pointer in C++?Answer : The type of this pointer is the type of object it is pointing to. In this article, we conclude that this pointer in C++ is like a hidden pointer where the compiler implicitly declares this pointer with the keyword and is followed by the function name. 4. this->f = f; In member function setMyValues() we have two local variables having same name as data members name. As a result, the this pointer is also used to refer to the calling object within a member function. Unlike pointers, references cannot be NULL. It holds the objects address that is being constructed during that call. When you declare and initialize a simple variable, e.g. This article does not require any special background, only preliminary knowledge of elementary C/C++ concepts like variables, pointers, and functions. Fig. Arent we calling new and assigning its return value to p? In this article, we saw how this pointer is used for accessing the local variables values even when the data member names and local variables names are the same using the this-> pointer and we also how we can refer to the latest value assigned to the variable or current object of the class using this* pointer. This is most often used when overloading operators for your classes (something well talk about more in chapter 14). We create a pointer to int-pointers pp and have it point to the address of, The code proceeds to call new int and store the result in, allow data of various types to be stored in a node, e.g. return *this; The first use of the this pointer is to send the current object to another method as a parameter. Thus, in the body of function setID(), m_id (which is a class member variable) has been converted to this->m_id. { Open the terminal again and compile the file with this command g++ along with your file name and extension. }. Theres just one more detail to take care of. OK. But x is just a copy of that left pointer and having it point to the new node or anywhere else for that matter does not affect the original pointer. we keep copying the address of every node we visit to it. private:
Example 1 fix v.1: Change the original pointers, not their copies. Take a look at the following line of code from the example above: Although the call to function setID() looks like it only has one argument, it actually has two! This way we know for sure that the values we assign will make it to the original pointers directly, not copies thereof. char c; Its worth noting that this is a const pointer -- you can change the value of the underlying object it points to, but you can not make it point to something else! We have seen how we could change a variable or pointer by passing a pointer to it. Well, lets change our function process() to have it expect a pointer: Now, process() expects a pointer, main() declares a pointer p that points to x, and calls process() with p. What happens when we invoke process() with a pointer turns out to be exactly the same as what happened with non-pointer variables the contents of slot p are copied into a new slot j that is then passed to process(). We end up with a red-arrow, i.e. In C++, this pointer can be used for calling all the functions for the present instance at once by just returning the reference of the current object and we can continue to call the functions using these current objects. This explains why we have to use the address-of operator &, to tell the compiler that we want to store the address of slot x, i.e. But thats only half of the answer. This keyword is only accessible within the nonstatic member functions of a class/struct/union type. Lets have our pointer p point to something, like the integer x we introduced earlier. char c = 'E'; Again, it is better to visualize this in order to understand it. Although you cannot have a reference-to-reference, it is definitely valid to have a reference-to-pointer.

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