Const keyword

const Decorated read-only variable

A lot of people think that being const The modified value is a constant . This is not accurate , To be precise, it should be a read-only variable , Its value cannot be used at compile time , Because the compiler doesn't know what it stores at compile time .

const The original purpose of the launch , Just to replace precompiled instructions , Eliminate its shortcomings , While inheriting its advantages . Let's look at it and define The difference between macros .（ Many people mistakenly think define Is the key word ,define Is the compiler's precompiled instructions , It is implemented by the compiler , No C The content of language .）

Definition const A read-only variable , Have no variability .

const int Max=100;
int Array[Max];

stay Visual C++6.0 Create... Respectively .c Document and .cpp File test . You'll find out .c In file , The compiler will prompt an error , And in the .cpp The file runs smoothly . Why? ？ We know that defining an array must specify the number of its elements . This is also confirmed from the side in C In language ,const Embellished Max It's still a variable , It's just a read-only property ;

const Modified read-only variables must be initialized at the same time as they are defined , Why? ？
answer ： Because the defined variables are modified into read-only variables , After definition, it cannot be modified , Therefore, the definition must be initialized .

case Whether the statement can be followed by const Modified read-only variables ？
answer ： no way ,const It is a read-only variable after modification, but it is still a variable in essence , and case The following statement needs integer 、 Constant of character type 、 Constant expression

Save a space , Avoid unnecessary memory allocation , Improve efficiency at the same time

Compilers are usually not normal const Read only variable allocates storage space , Instead, they are saved in the symbol table , This makes it a compile time value , No storage and read operations , Make it efficient .

#define M 3 // Macro constants 
const int N=5;		// At this time, the  N  Put it in memory 
......
int i=N;			// This is the case  N  Allocate memory , No more assignments in the future ！
int I=M;			// Macro replacement during precompiling , Allocate memory 
int j=N;			// No memory allocation 
int J=M;			// Replace the macro again , Allocate memory again ！

const Defined read-only variables from an assembly point of view , Only the corresponding memory address is given , Rather than image #define The same is given for the immediate number , therefore ,const There is only one copy of the defined read-only variable during the running of the program （ Because it's a global read-only variable , Store in static area ）, and #define The macro constant defined has several copies in memory .#define Macros are replaced during the precompile phase , and const The modified read-only variable determines its value at compile time .#define Macro has no type , and const The modified read-only variable has a specific type .

Modify general variables

A general constant is a simple type of read-only variable . This read-only variable is defined , Modifier const Can be used before a type specifier , It can also be used after a type specifier . for example ：

int	const	i=2;	
 or 	
const	int	i=2;

Modify the array

Define or describe a read-only array in the following format ：

int const a[5]={
    1, 2, 3, 4, 5};
 or 
const int a[5]={
    1, 2, 3, 4, 5};

Modify a pointer

const int *p;				// p  variable ,p  The object pointed to is immutable 
int const *p;				// p  variable ,p  The object pointed to is immutable 
int *const p;				// p  immutable ,p  Mutable object to point to 
const int *const p; 		// The pointer  p  and  p  The objects pointed to are immutable 

Ignore type name first （ The compiler also ignores type names when parsing ）, Let's see const Which is close to .“ Near water first ”, Decorate who is close to you .

const int *p;			//const  modification *p,p  Is a pointer ,*p  Is the object that the pointer points to , immutable 
int const *p;			//const  modification *p,p  Is a pointer ,*p  Is the object that the pointer points to , immutable 
int *const p;			//const  modification  p,p  immutable ,p  Mutable object to point to 
const int *const p; 	// Previous  const  modification *p, After a  const  modification  p, The pointer  p  and  p  The objects pointed to are immutable 

Modify the parameters of the function

const Modifiers can also modify the parameters of a function , Use... When you don't want this parameter value to be accidentally changed in the function body . for example ：

void Fun(const int i);

Tell compiler i The in the function body cannot be changed , Thus, some unintentional or wrong modifications of the user are prevented .

Modify the return value of the function

const Modifiers can also modify the return value of a function , Return value cannot be changed . for example ：

const int Fun (void);

Reference... In another connection file const A read-only variable ：

extern const int i;				// Correct statement 
extern const int j=10; 			// error ！ The value of a read-only variable cannot be changed .

Note that this is a declaration, not a definition