Reproduced const differences in C and C + +

Reprinted from Csdn

http://blog.csdn.net/huangshanchun/article/details/41215065

const differences in C and C + +

Test environment: Windows 7 vs2010

Using the const modifier variable means that it is read-only, but the const in C is quite different from the const in C + +.

C Language Const is "counterfeit", see the following C program:

[CPP]View Plaincopy

2. #include <stdio.h>
4. #include <stdlib.h>
8. int main ()
10. {
12. const INT a = 5;
14. int *p = NULL;
18. printf ("A before modification:%d \ n", a);
20. p = (int *) &a;
22. *p = 6;
26. printf ("a modified:%d \ n", a);
28. System ("pause");
30. return 0;
32. }

Test results:

The value of a in the program can be modified, although we cannot directly modify, but can be obtained by obtaining the address of a to indirectly modify.

The same program we implemented in C + +:

[CPP]View Plaincopy

2. #include <iostream>
4. Using namespace std;
8. int main ()
10. {
12. const INT a = 5;
14. int *p = NULL;
18. cout<<"a modified before:" <<a<<endl;
20. p = (int *) &a;
22. *p = 6;
26. cout<<"a modified:" <<a<<endl;
28. System ("pause");
30. return 0;
32. }

Test results:

C + + and C do not work the same, and the C + + compiler handles const constants by placing constants in the symbol table when a constant declaration is encountered, and when a constant is found in the compilation, it is replaced directly by the values in the symbol table.

Question: How to interpret the address, the compilation process if you find that the const using extern or the & operator, the corresponding constant allocation of storage space (in order to be compatible with C). Although the C + + compiler may allocate space for Const constants (for & Operations), it does not use the values in its storage space.

Conclusion:

const constant in C: It is a read-only variable, has its own storage space, can be indirectly modified by the address of its value.

Const constants in C + +: They are placed in the symbol table, may allocate storage space, may not allocate storage space, and a temporary memory space is allocated when the & symbol is used.

Reproduced const differences in C and C + +