A detailed explanation of const usage in C + +

Object-oriented is an important feature of C + +.

But C + + on the basis of a new increase in the number of optimization is also very dazzling on the const directly can replace C in the #define the following points are very important, learning bad consequences are also very serious

Const

1. Qualifier declaration variable can only be read

const int i=5;

int j=0;

......

I=J//illegal, resulting in compilation errors

j=i;//Legal

2. Must initialize

const int i=5;//Legal

const int J;//Illegal, causing compilation errors

3. Referring to the const constant in another connection file

extern const int i;/Legal

extern const int j=10;//illegal, constants cannot be assigned again

4. Easy to type check

Use the Const method to make the compiler more aware of what is being handled.

#define I=10

Const LONG &i=10;/*dapingguo reminder: Because of compiler optimizations, I do not allocate memory when Const long i=10, but have 10 directly into subsequent references so that there are no errors in future code, especially for the purposes of preaching. &i explicitly gives the memory allocation of I. However, once you turn off all optimizations, even a const long i=10 can cause subsequent compilation errors. */

char h=i;//Nothing wrong

Char h=i//compilation warning, which may result in incorrect assignment due to a truncated number.

5. Can avoid unnecessary memory allocation

#define STRING "Abcdefghijklmnn"

const char string[]= "ABCDEFGHIJKLMN";

......

printf (string);//Assigned first memory to STRING

printf (string);//Allocate memory once for string, no longer assigned

......

printf (string);//allocated second memory for STRING

printf (string);

......

Because the const definition constant is given a corresponding memory address from the assembly point of view, rather than giving an immediate number like #define, the const-defined constant has only one copy during the program's operation, while the #define-defined constant has several copies in memory.