Bjarne gave a mnemonic method in his The C++ Programming Language:Read a declaration from right to left.。
char * const cp; ( * 读成 pointer to ) cp is a const pointer to char const char * p; p is a pointer to const char; char const * p;
Same as above. Because there is no const* operator in C++, const can only belong to the preceding type.
The C++ standard stipulates that placing the const keyword before the type or before the variable name is equivalent.
const int n=5; //same as below int const m=10; const int *p; //same as below const (int) * p int const *q; // (int) const *p char ** p1; // pointer to pointer to char const char **p2; // pointer to pointer to const char char * const * p3; // pointer to const pointer to char const char * const * p4; // pointer to const pointer to const char char ** const p5; // const pointer to pointer to char const char ** const p6; // const pointer to pointer to const char char * const * const p7; // const pointer to const pointer to char const char * const * const p8; // const pointer to const pointer to const char
Speaking of which, we can look at a previous Google written test question:
const char *p="hello"; foo(&p); // 函数foo(const char **pp)下面说法正确的是［］
- A. The function foo() cannot change the content of the string pointed to by p.
- B. The function foo() cannot make the pointer p point to the address generated by malloc.
- C. The function foo() can make p point to a new string constant.
- D. The function foo() can assign NULL to p.
As for the answer to this question, opinions vary. Regarding the above question, we can use the following program to test:
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
void foo(const char **pp)
{
// *pp=NULL;
// *pp="Hello world!";
*pp = (char *) malloc(10);
snprintf(*pp, 10, "hi google!");
// (*pp)[1] = 'x';
}
int
main()
{
const char *p="hello";
printf("before foo %s/n",p);
foo(&p);
printf("after foo %s/n",p);
p[1] = 'x';
return;
}
The conclusions are as follows:
- In the foo function, p in the main function can be made to point to a new string constant.
- In the foo function, p in the main function can be made to point to NULL.
- In the foo function, p in the main function can be made to point to a memory block generated by malloc, and it can be freed with free in main, but there will be a warning. However, note that even if p is made to point to a memory block generated by malloc in foo, you still cannot use a statement like p
- In foo, you cannot use a statement like (*pp)
Therefore, it seems that gcc only restricts it based on the literal meaning of const. That is, for a pointer like const char*p, no matter whether p later actually points to memory from malloc or constant memory, you cannot use a statement like p