You must have known it already, but I can’t stop myself from showing off the interplay of const with pointers in C/C++.
Here goes the code. It’s simple.
void main() {
int arr[]={1,2,3,4}; // data
const int *p1 = &arr[0]; // non-const ptr to const data
int const *p2 = &arr[0]; // non-const ptr to const data
int* const p3 = &arr[0]; // const ptr to non-const data
const int* const p4 = &arr[0]; // const ptr to const data
p1++; // ok
p1[0]++; // compile error: modifying const data
p2++; // ok
p2[0]++; // compile error: modifying const data
p3++; // compile error: modifying const ptr
p3[0]++; // ok
p4++; // compile error: modifying const ptr
p4[0]++; // compile error: modifying const data
}
Now, let’s see what the g++ compiler (C++ 4.8.1) has to say about this code. You can try it yourself at http://ideone.com/AJUBcD.
prog.cpp: In function ‘int main()’:
prog.cpp:11:7: error: increment of read-only location ‘* p1’
p1[0]++; // compile error: modifying const data
^
prog.cpp:14:7: error: increment of read-only location ‘* p2’
p2[0]++; // compile error: modifying const data
^
prog.cpp:16:4: error: increment of read-only variable ‘p3’
p3++; // compile error: modifying const ptr
^
prog.cpp:19:4: error: increment of read-only variable ‘p4’
p4++; // compile error: modifying const ptr
^
prog.cpp:20:7: error: increment of read-only location ‘*(const int*)p4’
p4[0]++; // compile error: modifying const data
^
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