code:
const char * const key;
There are 2 const in above pointer, I saw things like this the first time.
I know the first const makes the value pointed by the pointer immutable, but did the second const make the pointer itself immutable?
Anyone can help to explain this?
@Update:
And I wrote a program that proved the answer is correct.
#include <stdio.h>
void testNoConstPoiner() {
int i = 10;
int *pi = &i;
(*pi)++;
printf("%d\n", i);
}
void testPreConstPoinerChangePointedValue() {
int i = 10;
const int *pi = &i;
// this line will compile error
// (*pi)++;
printf("%d\n", *pi);
}
void testPreConstPoinerChangePointer() {
int i = 10;
int j = 20;
const int *pi = &i;
pi = &j;
printf("%d\n", *pi);
}
void testAfterConstPoinerChangePointedValue() {
int i = 10;
int * const pi = &i;
(*pi)++;
printf("%d\n", *pi);
}
void testAfterConstPoinerChangePointer() {
int i = 10;
int j = 20;
int * const pi = &i;
// this line will compile error
// pi = &j
printf("%d\n", *pi);
}
void testDoublePoiner() {
int i = 10;
int j = 20;
const int * const pi = &i;
// both of following 2 lines will compile error
// (*pi)++;
// pi = &j
printf("%d\n", *pi);
}
int main(int argc, char * argv[]) {
testNoConstPoiner();
testPreConstPoinerChangePointedValue();
testPreConstPoinerChangePointer();
testAfterConstPoinerChangePointedValue();
testAfterConstPoinerChangePointer();
testDoublePoiner();
}
Uncomment lines in 3 of the functions, will get compile error with tips.
First const tells you can not change *key
, key[i]
etc
Following lines are invalid
*key = 'a';
*(key + 2) = 'b';
key[i] = 'c';
Second const tells that you can not change key
Following lines are invalid
key = newkey;
++key;
Also check how to read this complex declaration
Adding more details.
const char *key
: you can change key but can not change the chars pointed by key.char *const key
: You can not change key but can the chars pointed by keyconst char *const key
: You can not change the key as well as the pointer chars.
const [type]*
means that it's a pointer that does not change the pointed value.
[type]* const
means that the value of the pointer itself cannot change, ie, it keeps pointing to the same value, akin to the Java final
keyword.
来源:https://stackoverflow.com/questions/27074718/c-what-does-this-2-const-mean