If we dont want a variable to change its value then we can use constant qualifier to make that variable to hold a value throughout the program. For example, Enjoy unlimited access on 5500+ Hand Picked Quality Video Courses. Add a new light switch in line with another switch? To make a variable whose value should not be changed. Is there any reason on passenger airliners not to have a physical lock between throttles? Affordable solution to train a team and make them project ready. Here we will see that can we can point another integer variable to the pointer. In general, the const qualifier is used to declare a variable as constant, meaning its value cannot change once the variable has been initialized. Is it appropriate to ignore emails from a student asking obvious questions? How does it really get stored? We use the const qualifier to declare a variable as constant. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. That is, global consts generally get stored in your data segment, function (parameter or declared) consts generally get stored on the stack. It is very likely that qualifiers of the type pointed to by the right. It's in your CPU's register, if you're familiar with that. const int nochange; /* qualifies as being constant */ Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? I only fully understood what. rev2022.12.9.43105. So int *const* ptrArray; declares a pointer to a constant pointer to a (nonconstant) int. The const in C++ is used to define a constant whose value cannot be modified. In function 'main':error: assignment of read-only location '*piIndex'. If you dont want to change the value of a variable. ' const ' type qualifier: 'const' is a type qualifier in 'C' used to enforce read-only features on variables. Automatic local variable may be compiled in static storage if compiler prove that only one instance of it is needed. In the section above we have seen that we can have a const pointer or a const reference that points to a non-const object, the other way around isn't possible. Syntax JavaTpoint offers too many high quality services. So lets us see an example of how we can implement it with the help of following the given examples. This means the common pattern const int x; is actually int const x; in disguise. 'Const' Qualifier in C Two type qualifiers available in C are ' const ' and ' volatile '. Hebrews 1:3 What is the Relationship Between Jesus and The Word of His Power? assigned to in any way during the run Then it's put into addressable space. It qualifies the pointer, not what it points at. the const-qualified version of the You can, however, initialize a const variable. What's the difference between constexpr and const? The highlighted part is an argument declaration. Thus several machines store the constant and jump tables in the text section, which only reads and contains all other executable instructions. using a cast). In programming, also the same case. However, there are several benefits of using const, such as if we have a constant value of the PI, we wouldn't like any part of the program to modify that value. Why do we use a volatile qualifier in C++? But it has a little twist as usual because sometimes we are able to change the values of variables that are declared with const qualifiers. You read modifiers starting from the variable. In your first example the constants are never used so the compiler probably will just ignore them altogether. There can be any types of constants like integer, float, octal, hexadecimal, character constants, etc. What is the difference between const int*, const int * const, and int const *? Constant qualifier is used to declare a variable as contant. modifiable, so a data object that is Const Qualifier in C with Tutorial, C language with programming examples for beginners and professionals covering concepts, c pointers, c structures, c union, c strings etc. If you do alias a pointer marked with restrict, then the result is undefined. The qualifier const can be applied to the declaration of any variable to specify that its value will not be changed ( Which depends upon where const variables are stored, we may change value of const variable by using pointer ). In all your examples all consts may firstly be put static storage (const section) and then easily eliminated. declared with const as a part of its In other words, we can say that the const means not modifiable (cannot assign any value to the object at the run time). I don't know where it is stored, because that's implementation-defined, but your code results in undefined behaviour. Optimizations that are defeated by using the volatile qualifier and can be categorized as follows: Marking union members as restrict will tell the compiler that only some of the options can be accessed in scope, allowing the compiler to optimize the access to members. EDIT: Also see: http://publications.gbdirect.co.uk/c_book/chapter8/const_and_volatile.html. If we dont want a variable to change its value then we can use constant qualifier to make that variable to hold a value throughout the program. Why would Henry want to close the breach? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Memory constants These constants use 'C' qualifier 'const', which indicates that the data cannot be changed. In this example, we will try to change the value of the integer variable using the pointer to constant (piIndex). What is constant qualifier in C? Const or Constant (qualifier in c) The const keyword in a declaration establishes a variable whose value cannot be modified by assignment or by incrementing or decrementing. Putting the address of a The const qualifier explicitly declares a data object as something that cannot be changed. Data types in c | 4 Basic Datatypes of c programming, c/c++ Development Environment Setup Using IDE. Can we change the value of an object defined with const through pointers? So the moral of the story is: Always use -Werror, This is a great answer. const volatile const type qualifier in C The const type qualifier is used to create constant variables. Using const has a very big benefit. When we declare a variable to be volatile, then it tells the compiler that the variable value can be altered at any moment without any action being taken by the code. Let's look at what is meant when const The syntax for declaring a const qualifier is: Use the const type qualifier to qualify an object whose value cannot be changed at any given time. The result is implementation-defined if an attempt is made to change a const. if you want to share const objects in multiple files, you need to add extern before . The syntax to declare ta volatile variable is as follows: An object without the volatile specifier will not tell the compiler to perform the optimizations. We use the const qualifier to declare a variable as constant. The second code shouldn't compile at all. It acts as a contract between a developer and a compiler. Thus compiler treats volatile variables to be special variables and also it cannot assume any value for them. We make use of First and third party cookies to improve our user experience. The objects qualified by the const keyword cannot be modified, i.e., an object declared as a const cannot serve as an operand in an operation that changes its value in further operations. Now consider uint8_t data1 =10 variable definition. There are few rules that can help us decide when to use a const in the C program: Now, let us see some examples in which we will use the const qualifier in our program with a pointer: As you can see in the above declaration that "xData" is pointing to a constant integer variable. The main use of ' const ' is to define constants in your program; so that it's values can't be changed. Find centralized, trusted content and collaborate around the technologies you use most. To learn more, see our tips on writing great answers. Does mutable member disable const optimizations for non-mutable members? The compiler can rely on your promise to optimise the code that's produced - and if you break your promise, it won't necessarily break in an obvious way, it may just produce strange results. Explain Loops - While loop, Do-while loop, for loop - Loops are used to carry out . Unlike with #define, your constant is not necessarily inlined by the compiler. Const qualifier tells the compiler that the value residing in the variable should not be changed. The type of 1 is const int. For example, What warnings are you getting? Now let's see what happens: As you can see in the Output, we cannot change the value of "*pidata". Rather, the compiler will create a symbol corresponding to your const declaration in the object file so that it can be accessed from other code filesremember that const objects have external linkage by default in C (although some compilers will still inline the constant value within the file where it is defined). Now, if you have scenarios where you need to modify the constness, either way, you can use const_cast. However, according to the concept, we will get the compiler error because *piData qualify to constant. Copyright 2011-2021 www.javatpoint.com. What is Constant Qualifier in C? Copyright 2022 Tutorials & Examples All Rights Reserved. It is an optimization hint that no other pointer in the present scope refers to the exact location. It's possible that your compiler is being nice to you and giving you a memory location to modify, but normally that's impossible. 1) Pointer to variable. Variables having volatile qualifier wont be considered for optimization as its value changes that compiler cannot decide what optimizations to perform. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. C program to check whether a given number is odd or even, C Program to Find Roots of a Quadratic Equation. Asking for help, clarification, or responding to other answers. A constant value is an explicit number or character such as 1 or 0.5 or 'c'. 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We can use it when we don't want to change the variable's value after the initialization. const means that something is not To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The compiler can still consider it as a compile time constant for other purposes -- and probably does. Ex. C Program to Swap Two Numbers | 4 Methods. What is the difference between const int*, const int * const, and int const *? ), but this As the name suggests the name constants are given to such variables or values in C/C++ programming language which cannot be modified once they are defined. The compiler determines if the address of the constant is ever needed. pointer to inspect the object, but not http://publications.gbdirect.co.uk/c_book/chapter8/const_and_volatile.html. Learn more. In order to declare a const qualifier, include the keyword const before or after any data type. This means we cannot change the value pointed by the pointer, and we can also not point the pointer to other integer variables. not stored in memory)? Using const has a very big benefit. I imagine you must get some Its totally up to the compiler writer what happens to the const, and it will vary according to the optimisation you request. Volatile Qualifiers. The reason the code snippet that you posted "works" is because the unary operator & can be applied to any lvalue, which includes a const object. Why is the federal judiciary of the United States divided into circuits? 'Const' qualifier will impose a restriction on the variable, such a way that its value can't be changed or modified. Making statements based on opinion; back them up with references or personal experience. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Basically, whether or not S {}.get () can be used is decided by the presence of the . Using const has a very big benefit. The compiler can't consider it a compile time constant (or runtime constant) if it is also declared volatile, however. Type qualifiers are basically the keywords that are used to modify the properties of the existing variables. In order to declare a const qualifier, include the keyword const before or after any data type. We use the const qualifier to declare a variable as constant. The part const int& is the type of the argument variable and var is the name of the variable. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Explanation required about how const modifies the behavior of storage, C: modifying const data in an array using pointer. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? And the compilers I've here agree: gcc gives an error with -pedantic-errors (whose purpose is to turn into error some mandatory diagnostics that historically gcc hasn't considered as error), xlc gives an error as well. dangerous and consequently prohibited permitted; you will be able to use the We use the const qualifier to declare a variable as constant. So you should declare that as a const . Improve INSERT-per-second performance of SQLite. Why is this usage of "I've to work" so awkward? const type into a pointer to the Can virent/viret mean "green" in an adjectival sense? That means that we cannot change the value once the variable has been initialized. That means that we cannot change the value once the variable has been initialized. Volatile prevents compilers to optimize that variable as variables value may change. When you declare (non-extern, non-argument) and initialize variable as const, this means variable is not writable at all. JavaTpoint offers college campus training on Core Java, Advance Java, .Net, Android, Hadoop, PHP, Web Technology and Python. C only. How to convert a std::string to const char* or char*. In this example, we will use the concept of a constant pointer to a constant. The main usage of volatile variables is during thread communication and in the situation where the value of the variable is updated externally by other entities. We use the const qualifier to declare a variable as constant. (although you can get around this by Is there a higher analog of "category with all same side inverses is a groupoid"? Type qualifiers in C: In the C programming language, type qualifiers are the keywords that prepend to the variables to change their accessibility, i.e., we can tell that the type qualifiers are used to change the properties of variables. Therefore we cannot change the value of the pointed integer variable using the pointer (*xData), but we can change the pointer to point to any other integer variable. How to create your own header file? The const qualifier is an instruction to the compiler to reject code that attempts to modify that object directly; attempts to modify the object indirectly (as you do in the second code snippet) results in undefined behavior, meaning any result is possible. The const qualifier is used to declare a variable to be read only (constant), the value may not be changed, and it can be declared using the keyword const. Why does my stock Samsung Galaxy phone/tablet lack some features compared to other Samsung Galaxy models? Now consider uint8_t data1 =10 variable definition. contain an initializer (otherwise, The const type qualifier. In your second example as you use "address off" it must actualy store it somewhere -- probably at the begining of the stack. So compiler is free to place it to read only section. There's something for you to ponder. Think of it like a register variable. compatible types, and the type pointed to by the left has all the Constants are generally not stored anywhere. What is the Difference Between putw and puts Function in c? It will be useful for embedding programming in order to keep the updated value that can be changed from various interrupts. In the case of call by reference, when we don't want to change the value of the passed variable. The const qualifier can be used to qualify any data type, including the single member of a structure or a union. . However, this is not always possible as it depends on where the const variables are stored in memory. Constant changes behavoior of a variable making it not to be modified. The const constant feature only takes effect when performing operations that change itself, so there is no difference between const constants and ordinary variables . unqualified type is much more The keyword const indicates a variable that is read-only (i.e., cannot be changed at run-time). In short, a const-qualified object may be stored in a different area from non-const-qualified objects, but not necessarily. References Or not modifiable, if it protected by MMU or placed in ROM in standalone application. If the address is needed, then the constant is stored as if it were a non-const variable in the current scope (relatively depending upon compiler). Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? What is const qualifier in C++ is a video tutorial for beginners to learn about the important concepts of object oriented programming.Support us on Patreon: . Therefore, all of the usual attributes of variables apply; specifically, it is allocated addressable storage space. That means that we cannot change the value once the variable has been initialized. Now, we will try to change the pointing variable in this example. Though the behavior here is undefined, I suspect that your compiler is detecting this usage and ensuring that your const declaration is given address space, and therefore not inlining it, even within the file it is declared. Here, 'data1' is a variable of type uint8 and that is initialized to 10. If he had met some scary fish, he would immediately return to the surface. PSE Advent Calendar 2022 (Day 11): The other side of Christmas. For example, a const data object cannot appear on the lefthand side of an assignment statement. Difference between #define and const in C. Constant variables are stored in stack section of a c program. Its value is set at initialization. Differences Between putw and putchar in C. Header file creation. Please explain it to me in detail. Say for example: If the answer is Code segment then how does the following code work?? C: Initialize a constant string array dynamically. They are expanded inline. So you should declare that as a const. As you can see that everything is working fine if we don't try to change the value of piData and *piData. const qualifier const is a type modifier used to describe an object that will never change. C not so paranoid as, say, Ada, and all unxepected behaviour is up to programmer, not compiler or RTL. Every constant has some range. However, If a user tries to change the value of the variable (iIndexData1) using the *piData, he will get the compiler error. The above code, when compiled with gcc 4.3.2 at -O1 optimisation or above, results in the output 20 rather than 21. Show more Show more References to const in C++ Neso Academy 8.2K views 1 year ago Pointers and const. In this example, we will try to change the value of *piData. @detly, a const variable can't appear in a place where a constant expression is syntatically required (which indeed is allowed for some of them in C++). If you want a reference: 6.3.16.1 in C90 standard describes when an assignment is possible: both operands are pointers to qualified or unqualified versions of Connect and share knowledge within a single location that is structured and easy to search. Standard does not state, what should happen if you try to write const (it is called "undefined behaviour"), so anything may happen: it may be written, not written, cause exception, hang, or something else you cannot imagine. C++ Programming: const Qualifier in C++ Topics discussed: 1. const Qualifier in C++. It does not indicate a compile-time constant. However, there are several benefits of using const, such as if we have a constant value of the PI, we wouldn't like any part of the program to modify that value. On an ANSI-compliant compiler, the code should produce an error message. The value for the volatile variables is taken through memory instead of register. This shows that it hasn't really "worked" - it just appeared to work. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. You can see that the above give code works perfectly, and we have successfully changed the pointing variable: As you can see in the above declaration, it is described that the constant pointer is pointing to a constant integer variable. One of those will generally be the most sensible solution, but they're not the only solutions: You could also explicitly cast the const away, or use the -Wno-discarded-qualifiers compiler flag to suppress the warning; as the courts have heard a million times, it's only undefined behaviour if you attempt to modify an object defined as const. C const - how is pointer to a const possible if a const is an rvalue (ie. One of the common ways to define constants in C is to use the #define preprocessor directive, as shown below: #define <VAR_NAME> <VALUE> In the above syntax: <VAR_NAME> is a placeholder for the name of the constant. the definition of the object will So now it is possible that, the pointer can point to any other variable because it is usually stored in the R&W area (or read and write memory). const Notes C adopted the const qualifier from C++, but unlike in C++, expressions of const-qualified type in C are not constant expressions; they may not be used as case labels or to initialize static and thread storage duration objects, enumerators, or bit field sizes. same type is both safe and explicitly Every programming language has its own style of declaring the constants, in C++ programming language the symbolic constants can be declared in two ways: Using 'const' keyword Using 'define' directive The 'const' Keyword in C++: The 'const' keyword is used to declare constant identifier. In other words, the qualifiers are the keywords that are applied to the data types or the type modifiers in C. There are three types of type qualifier variables in the C programming language. Central limit theorem replacing radical n with n. Why is apparent power not measured in watts? initialized.Taking the address of a is used. Though it may be modifiable physically (if hardware allows it). Thus it cannot be optimized. No, because when you wrote std::string_view get () const && = delete; you're essentially ( implicitly) saying that S {}.get () should not be compiled. This qualifier will help to prevent accidental changes in the values. All rights reserved. We use 'const' and 'volatile' qualifiers to attribute a feature to a variable. @AProgrammer - indeed, that's a better phrasing. Defined constants These constants use the preprocessor command 'define" with #. A Computer Science portal for geeks. In case of calling a function by passing pointer as its parameter then const is being used to safeguard the variable. For example, if you have a constant value of the value of PI, you wouldn't like any part of the program to modify that value. In order to declare a volatile variable, include the keyword volatile before or after the data type. How to Convert int to char in C Programming? of a program. The const qualifier is an instruction to the compiler to reject code that attempts to modify that object directly; attempts to modify the object indirectly (as you do in the second code snippet) results in undefined behavior, meaning any result is possible. As many stated it is inlined in most cases (if compiler know that to inline), but still retains attributes of variables, such as address (and you can get a pointer), size. For e.g.. We can also use it while mapping the I/O register with the help of pointers in C. After discussing on const qualifier let us discuss in brief about Volatile Qualifiers. *const* ptrArray denotes a pointer to a constant pointer to something. It tells the . In C, are const variables guaranteed to be distinct in memory? When the keyword const is used to qualify a member of any aggregate type, only that member will be qualified. Constant qualifier is used to declare a variable as contant. Const Qualifier in C. The Const Qualifier is used to specify a variable as a constant, which means that its value will not change after it is initialised. The value of variable temp might get altered through digital thermometer attached to the computer. Penrose diagram of hypothetical astrophysical white hole. const type identifier = value. A const qualifier isn't a request to have the variable placed in a particular kind of memory - it's a promise from you to the compiler, that you won't modify that variable by any means. Describe Selection - if and switch using C++ - if is decision making control and its general for is:. Do non-Segwit nodes reject Segwit transactions with invalid signature? Normally, constness applies to the left, but in this case, it is the left most token of the type. By using this website, you agree with our Cookies Policy. Developed by JavaTpoint. The type of 0.5 is const double. So you should declare that as a const. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Share Improve this answer Follow answered Nov 26, 2010 at 0:05 John Bode 116k 18 115 191 Using const has a very big benefit. Constant changes behavoior of a variable making it not to be modified. ptrArray is a variable. 2 Answers. Here, 'data1' is a variable of type uint8 and . However, there are some advantages to using const, such as the fact that if the PI has a constant value, we don't want any part of the program to change it. When they are used as array sizes, the resulting arrays are VLAs. 'const' type qualifier: 'const' is a type qualifier in 'C' used to enforce read-only features on variables. For example, if you have a constant value of the value of PI, you wouldn't like any part of the program to modify that value. if you were accessing a hardware port Const Qualifier in C In general, the const qualifier is used to declare a variable as constant, meaning its value cannot change once the variable has been initialized. Adding/Removing const qualifier with const_cast. only to read from it, then it would be at a fixed memory address and promised Because the const object cannot be modified once it is created, the const object must be initialized. declared to be const but not . But if you use a reference int& value or a pointer int* value it makes sense to declare it const as otherwise a caller might assume that value will be modified. The new type of qualifier introduced to C99 can be applied to pointer declarations. Once the const object is defined, it cannot be assigned a new value, so it must be initialized. Not the answer you're looking for? And in the program, you can modify the value of that variable, which is allowed. Difference between const int*, const int * const, and int const * in C/C++? So you should declare that as a const. As a result, we may use the const . const qualifier is used to specify the variable that can't be change throughout the program. You cannot use const data objects in expressions requiring a modifiable lvalue. Variable with const qualifier is often named in uppercase. Difference between const int*, const int * const, and int const * in C, Difference between const char* p, char * const p, and const char * const p in C, Explain the constant type qualifier in C language. For example, if you have a constant value of the value of PI, you wouldn't like any part of the program to modify that value. It's really quite simple: Therefore we can declare it as a const using the const qualifier. Share The reason for having this type qualifier is mainly to do with the problems that are encountered in real-time or embedded systems programming using C. volatile is a qualifier that is applied to a variable when it is declared. execvp execvp const What is const double in C? If const is specified when declaring an aggregate type, all the aggregate type members are treated as objects qualified with const. That means once a value is assigned to a constant variable, that value is fixed and cannot be changed throughout the program. It's in your CPU's register until you need its address. Agree Note, local pointers may also be eliminated if their locations can be computed in compile time. The syntax is as follows . Mail us on [emailprotected], to get more information about given services. If all const reads and pointers to it are eliminated, storage for const will also be eliminated by compiler (if it is static or local) or linker (if it is global). modify it. Objects declared with const-qualified types may be placed in read-only memory by the compiler, and if the address of a const object is never taken in a program, it may not be stored at all. To declare a volatile variable, the keyword . The rubber protection cover does not pass through the hole in the rim. Lets us see what happens: In the Output of both above given programs, you can see that in case of constant pointer to constant, we cannot change the constant value and neither we can point the constant pointer to another variable. Const Qualifier in C Difficulty Level : Medium Last Updated : 30 Jul, 2021 Read Discuss Practice Video Courses The qualifier const can be applied to the declaration of any variable to specify that its value will not be changed ( Which depends upon where const variables are stored, we may change the value of const variable by using pointer ). What is the difference between const and readonly in C#? 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This qualifier will help to prevent accidental changes in the values. For example, if you have a constant value of the value of PI, you wouldn't like any part of the program to modify that value. They are fixed values in a program. However, when we declare an identifier as constant using the const qualifier, it depends on the implementation (or on the compiler) where the "constant variable" will go or stored in the process control block. When a variable is created with const keyword, the value of that variable can't be changed once it is defined. Something can be done or not a fit? That means that we cannot change the value once the variable has been initialized. Where does a const variable gets stored exactly and how does it behaviour change? For example, #define PI 3.1415. It's recommended that you name constants in the uppercase, as it helps differentiate them from other variables defined in the program. It indicates that the compiler can apply any optimizations depending on the program context and compiler optimization levels. data object of a type which isn't The real question is initialization - if you need its address and is therefore actually allocated, then where is it initialized? Also writes to local variables may be eliminated if they're not read after it, so your snippet may have no code at all. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, I can assure you that your 2nd code segment is, I believe most modern compilers would give a warning when passing through the line "p=&j" warning about Different const qualifiers, because you are assigning a pointer to const int to a pointer to int. This code works fine How it is possible to change a read only memory? The following are some examples. example: const int bufsize = 512; once defined, its value cannot be changed, and by default, it is only valid in the file. Here's where the const declaration in C++ rears its head. Please mail your requirement at [emailprotected] Duration: 1 week to 2 week. since you can't assign to it, how type specification must not be In this exceptional case, it applies to the right instead. If it is not, it is (usually) input inline into the code segment because that's (usually) faster than referencing memory. would it ever get a value? const and putting it into a pointer to They stored like any other variables, but may be replaced with inline by optimizer. As C is designed to replace assembly language instructions, and for writing OS kernal and device driver type code it lets you do all sorts of things and assumes you know what you are doing when you start messing with pointers. For example, if you have a constant value of the value of PI, you wouldn't like any part of the program to modify that value. As const is not modifiable it also compiled in static storage, but may be eliminated as stated above. Constant values cannot be changed; that is, they cannot appear on the left-hand side of an assignment statement. Syntax of const object in C: To make a variable constant, you only need to add the const qualifier at the time of variable declaration, see the below-mentioned statement: const <data_type> <var_name> = <value>; e.g, In General, the const qualifier only gives only an indication to the compiler that the value of qualify object cannot be changed. The const keyword specifies that a variable's value is constant and tells the compiler to prevent the programmer from modifying it. Ready to optimize your JavaScript with Rust? Thanks for contributing an answer to Stack Overflow! That means that we cannot change the value once the variable has been initialized. In your case this is useless as int value makes a copy anyway. If you qualify an argument as being constant, it shows the caller that you won't change that argument inside the function. So you should declare that as a const. But then later in the code you're writing f (S {}.get ()); // should compile which contradicts the above code. Include stdio.h header file for outputting the result.Declare a constant variable with const qualifier and initialize it with a value.Output the value stored in that variable. is not always the case. Meaning of 'const' last in a function declaration of a class? 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