Where should you declare variables in C++
Definition Variable is the place where a variable is created, allocated memory.
A variable in C++ is a name for a piece of memory that can be used to store information.
There are many types of variables, which determines the size and layout of the variable's memory.
Initialization means assigning a value to the variable.
Declaration is a place where the type of variable is stated, but no storage is allocated.
Variables can be declared many times, but defined only once. Memory space is not allocated for a variable while declaration. It happens only on variable definition.
A variable is a name of a memory location. It is used to store data. Its value can be changed, and it can be reused many times. It is a way to represent memory location through a symbol so that it can be easily identified.
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| Best C++ Variables Declaration Local Global Variables Method |
Variable Naming Convention in C++
We can use any combination of letters and numbers for Variable and function names, but it must start with a letter.We can use underscore (_) as a letter in variable name and can begin with an underscore But Identifiers beginning with an underscore are reserved, And identifiers beginning with an underscore followed by a lower case letter are reserved for file scope identifiers Therefore using underscore as starting letter is not desirable.
XYZ and xyz are different identifiers because upper and lower case letters are treated as different identifiers.
There are five types of Declaration
1. Local variable
2. Global variable
3. Static variable
4. Automatic variable
5. External variable
Rules for defining variables
1. A variable can have alphabets, digits and underscore
2. A variable name can start with alphabet and
underscore only. It can't start with digit.
3. No white space is allowed within variable name.
4. A variable name must not be any reserved word or
keyword e.g. char, float etc.
How do you know if a variable is valid?
int xy; int ab; int a30; int x_y; int a1; float xy; float ab; float a1; |
What are invalid variable?
int 4; int x y; int 1xy; |
What is variable syntax in C++?
Data_Type variable_name = value; |
Here data_Type is a(integer, float, char) and variable_name(like a, b, c any more variable name) but variable name not define keyword name.
What is initializing a variable in C++?
int id; float marks; char grade; |
What is variable in C++ with example?
#include<iostream.h> #include<coino.h> void main() { clrscr(); { int id=8556; float marks=87.67; char grade='A'; cout<<"student id:<<id<<endl; cout<<"student marks:<<marks<<endl; cout<<"student grade:<<grade; getch(); } **************OUTPUT*************** student id:8556 student marks:87.67 student grade:A |
What is an auto variable in C++?
Initialization done each time block is entered. external and static variables cannot be initialized with a value that is not known until run-time; the initializer must be a constant expression.
A variable that has not been assigned a value is called an uninitialized variable. Uninitialized variables are very dangerous because they cause intermittent problems. This can make them very hard to debug.
It can be Inside a function or a block which is called local variables, In the definition of function parameters which is called formal parameters or Outside all functions which is called global variables.
Difference between Local and Global Variables in C++
Global variables: Global variable are declared outside any functions, usually at top of program. they can be used by later blocks of code
#include<iostream.h> #include<conio.h> int a=10; void add() { cout<<"Value of a:"<<a; } void main() { add(); getch(); } |
Local variables: Variables that are declared inside a function or block are local variables. The scope of local variables will be within the function only. These variables are declared within the function and can't be accessed outside the function.
#include<stdio.h> #include<conio.h> void add() { int a=10; cout<<"Value of a:"<<a; } void main() { add(); getch(); } |
Automatic: The default class. Automatic variables are local to their block. Their storage space is reclaimed on exit from the block.
#include<iostream.h> #include<conio.h> void data() { int a=100; auto int b=10; cout<<"value of a:"<<a<<endl; cout<<"value of b:"<<b; } int main() { data(); data(); getch(); } |
register: If possible, the variable will be stored in a processor register. May give faster access to the variable. If register storage is not possible, then the variable will be of automatic class.
Use of the register class is not recommended, as the compiler should be able to make better judgement about which variables to hold in registers, in fact injudicious use of register variables may slow down the program.
static: On exit from a block, static variables are not reclaimed. They keep their value. On re-entry to the block, the variable will have its old value.
#include<iostream.h> #include<conio.h> void data() { static int i=0; cout<<i<<endl; i++; } void main() { data(); data(); data(); data(); getch(); } |
extern: You can use an external variable. Using an external variable, a global variable can share a variable across many C source files. You must use the extern keyword to declare an external variable.
Allows access to external variables. An external variable is either a global variable or a variable defined in another source file. External variables are defined outside any function.
c file name edata.h extern int x=100; edata.c save file #include "edata.h" #include <stdio.h> void printdata() { cout<<"Global variable:"<<x; } void main() { clrscr(); printfdata(); getch(); } |
Note: Variables passed to a function and modified by way of a pointer are not external variables.
static external: External variables can be accessed by any function in any source file which make up the final program. Static external variables can only be accessed by functions in the same file as the variable declaration.
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