local, global, static, extern…😎😎
When a program run to “int iCount = 0;” then an area in RAM will be allocated for this variable.
When talk about variables, there 2 things we should know :
- Scope of variable : global, local,…
- The lifetime of the variable : when the variable is destroyed or when the allocated memory in RAM of variable is free
Overview
- If you don’t initialize the value of variable, it’ll have a dummy value. So it’s a good practice to initialize the value of variable when you declare it.
- Types of initializations
1. Local variable
- Variable which is defined inside a block.
- It has block scope. Which mean, we can access this variable outside the block.
- It has dynamic time which means it’s created when we defined, and destroyed when we exit the block contains it.
- We could have variables with the same name in different blocks. When in a nested-block, there is a variable with the same name, then the program will hide the variable outiside. We could do it but it’s not recommended, it’s better we we name it differently.
- We should define a variable in the nearest place we use it.
2. Global variable
- Variable which is defined outside a block
- It has global scope, file scope or global namespace scope.
- It has static time which means it’s created when a program starts, and destroyed when a program ends.
- It’s defined in the beginning of the file, below “#include”.
- We should prevent using global variable, since it’s risky especially in big and complex program. We should prefer using local variable. It’s risky because everywhere could call and change global variable , so it’s hard to manage it. We could use global constant to walk around it, since the constant value is unchanged.
- The global variable will reduce the flexibility of your sub functions. i.e : we can’t reused sub functions in other programs which don’t have such that global variable.
- We can use “::” to point global variable when we have duplicated variables.
3. Static variable
- When we use “static” keyword with local variable, then it’ll become static variable.
- Static variable is a combination of global and local variable.
- Static variable is created only 1 time, at the 1st time we call it, and alive until a program ends. (global)
- Static variable can be accessed only in the block where define it. (local)
Demo :
Here we can see that the static variable s_iCount is not destroyed and continue increasing in next calls.
- There are 2 types of static variable : internal linkage and external linkage.
- When we should use static variable ? When we want to store the value of variable during a program. i.e : a func to generate unique id which is kept increasing during a program.
