- Dec 12, 2019 In line 9, a variable named obj of type C is declared. However, this declaration raises an error because C is not declared until later in the program, and is not forward-declared. To fix the error, you can either move the entire definition of C before main or else add a forward-declaration for it.
- Variable Declaration in C A variable declaration provides assurance to the compiler that there exists a variable with the given type and name so that the compiler can proceed for further compilation without requiring the complete detail about the variable.
Variables are an extremely core concept to most object orientated programming languages. I like to visualize a variable much like a box. We can put things in the box, we can take things out of the box, and at any point we can see what is inside the box. Each box also has a name to which we can refer to it by, and in C++, each box can only hold a certain type of data.
Jun 25, 2003 Say you want to store data for temporary use in a program. You will have to declare variables to input the data. Luckily, declaring variables in C is very easy. This simple program gets the user's age as input and displays it back to the screen. Here is the source code.
When we create variables we call this the variable declaration, and then when we set them for the first time, we call this the initialization. To declare a variable in C++, we write the function. To declare a basic integer variable called 'age', we could write the following:
From this point we can then refer to the variable by its name, so in this case, we can just write 'age' whenever we want to refer to the variable. To initialise the variable we can write its name, followed by the equals sign, followed by the value we want to set the variable to (followed by a semicolon). The value we set it to can be a constant (a value that doesn't change), or another variable of the same type. An operator is a symbol which has a certain meaning in the programming language, in this case, the equals operator, represented by the = symbol, is an operator which sets whatever is on the left of the operator to whatever is on the right.
The constant value we set the variable to depends on the to 5 with something like the following:
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We can actually combine the variable declaration and initialization into one more-compact line, like the following:
The 'age' variable now contains the number '5', and we can refer to this '5' by writing 'age' anywhere in our program. 8dio solo violin designer 1.0 vst free download. We can also change the value of the variable at any point by using the equals operator as we did for the first initialization:
Although this seems purely for convenience at the moment (as we could just write '5', '3', or '21' in place of 'age'), trust me when I say that these become extremely useful and powerful when you start dealing with dynamic logic and user input (the latter of which we'll be covering later in this tutorial).
Just to give an example of accessing the contents of variables by using their names, we could create a new variable called 'age_two' which is set to the value of 'age', and then we can also try outputting one or both of these variables:
To be clear, all this code should be going into the basic program structure which we learnt how to create in the last tutorial. So we want our 'iostream' include for cout, cin, and some other stuff, we want the std namespace, and we want the majority of our code to be going in our 'main' function. So our full code to demonstrate variables so far, which you can compile and run at any point to test the functionality, is as follows:
Some number variables can handle positive and negative numbers, whereas 'unsigned' number variables can only handle positive numbers, although because of this restriction, can hold larger numbers. You can write the signed or unsigned keywords before the and 'short' - numbers with a decimal place in. Floats are accurate to around 6 or 7 digits and are declared using the float type. Float constants can be defined by simply writing a number with a decimal point followed by the 'f' notation. An example of a simple float declaration and initialization to a float constant is as follows:
Care must be taken, however, with float (and other decimal) operations, as rounding and precision problems to do with how the numbers are stored can trip you up (we don't have infinite memory for recurring decimals like 1/3 for example) -- I recommend reading this article for more information on this if you're interested.
Doubles
The 'double' or 'e'. Character variables are declared by using the char type, and character constants are defined by using single quotes (apostrophes) around the character. An example of character declaration and initialization to a character constant is as follows:
Strings
The lastve talked about string variables in relation to cout before, and as such you should know that string constants are defined by using double quotes. String variables are declared by using the string type, however as strings aren't actually 'primitive' types in C++ (and are instead defined by the standard library of stuff that comes bundled with C++), you are required to #include <string> to use thist strings aren't massively useful, but this is just because we don't really know how to utilize all the functionality of different data-types yet - for example, we don't know how to perform simple mathematics on number types, or how to check the value of booleans to change the logic of the program. All will be revealed in future tutorials.
A C++ program consists of various entities such as variables, functions, types, and namespaces. Each of these entities must be declared before they can be used. A declaration specifies a unique name for the entity, along with information about its type and other characteristics. In C++ the point at which a name is declared is the point at which it becomes visible to the compiler. You cannot refer to a function or class that is declared at some later point in the compilation unit. Variables should be declared as close as possible before the point at which they are used.
The following example shows some declarations:
On line 5, the main function is declared. On line 7, a const variable named pi is declared and initialized. On line 8, an integer i is declared and initialized with the value produced by the function f. The name f is visible to the compiler because of the forward declaration on line 3.
In line 9, a variable named obj of type C is declared. However, this declaration raises an error because C is not declared until later in the program, and is not forward-declared. To fix the error, you can either move the entire definition of C before main or else add a forward-declaration for it. This behavior is different from other languages such as C#, in which functions and classes can be used before their point of declaration in a source file.
In line 10, a variable named str of type std::string is declared. The name std::string is visible because it is introduced in the stringheader file which is merged into the source file in line 1. std is the namespace in which the string class is declared.
In line 11, an error is raised because the name j has not been declared. A declaration must provide a type, unlike other languages such as javaScript. In line 12, the auto keyword is used, which tells the compiler to infer the type of k based on the value that it is initialized with. The compiler in this case chooses int for the type.
Declaration scope
The name that is introduced by a declaration is valid within the scope where the declaration occurs. In the previous example, the variables that are declared inside the main function are local variables. You could declare another variable named i outside of main, at global scope, and it would be a completely separate entity. However, such duplication of names can lead to programmer confusion and errors, and should be avoided. In line 21, the class C is declared in the scope of the namespace N. The use of namespaces helps to avoid name collisions. Most C++ Standard Library names are declared within the std namespace. For more information about how scope rules interact with declarations, see Scope.
Definitions
Variable Declaration Definition
Some entities, including functions, classes, enums, and constant variables, must be defined in addition to being declared. A definition provides the compiler with all the information it needs to generate machine code when the entity is used later in the program. In the previous example, line 3 contains a declaration for the function f but the definition for the function is provided in lines 15 through 18. On line 21, the class C is both declared and defined (although as defined the class doesn't do anything). A constant variable must be defined, in other words assigned a value, in the same statement in which it is declared. A declaration of a built-in type such as int is automatically a definition because the compiler knows how much space to allocate for it.
The following example shows declarations that are also definitions:
Here are some declarations that are not definitions:
Typedefs and using statements
In older versions of C++, the typedef keyword is used to declare a new name that is an alias for another name. For example the type std::string is another name for std::basic_string<char>. It should be obvious why programmers use the typedef name and not the actual name. In modern C++, the using keyword is preferred over typedef, but the idea is the same: a new name is declared for an entity which is already declared and defined.
Static class members
Because static class data members are discrete variables shared by all objects of the class, they must be defined and initialized outside the class definition. (For more information, see Classes.)
extern declarations
A C++ program might contain more than one compilation unit. To declare an entity that is defined in a separate compilation unit, use the extern keyword. The information in the declaration is sufficient for the compiler, but if the definition of the entity cannot be found in the linking step, then the linker will raise an error.
Declaration In C
How To Declare Variable In Dev C++
In this section
Storage classes
const
constexpr
extern
Initializers
Aliases and typedefs
using declaration
volatile
decltype
Attributes in C++
See also
Basic Concepts