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Declaring an integer constant size by
const int size = 50;
makes it clear that you cannot change the value of size.
But it is less obvious what a constant pointer means. In
int x = 25; const int* p = &x;it is not immediately clear whether the const designator indicates that you cannot change p or that you cannot change *p or both.
const int* p = &x;
says that the program cannot use variable p to change *p.
In the following, the first three lines are allowed but the fourth is not
because it tries to change *p.
const int x = 25, y = 100; const int* p = &x; p = &y; *p = 3;
There is nothing wrong with creating a const pointer that points to a non-const variable. For example,
int x = 25; const int* p = &x; x = 3;is fine. Variable x itself is not constant, and you are free to change it. But if you refer to x through p then you cannot change it. It is all about how you refer to a variable.
C++ code
int* p = ...; const int* q = p;is allowed. It copies the pointer from p, where it has no restrictions, into q, where it has a const restriction. It is okay to add restrictions. But you cannot remove a restriction. C++ code
const int* p = ...;
int* q = p;
*q = 0;
tries to change *p by storing the pointer that is in p into
another variable that does not have a const
restriction. That is not allowed.
Suppose you want to use a pointer variable as a control variable in a loop. A first attempt might look something like this, where irrelevant details have been replaced by dots.
void loop(const int* ptr)
{
int* p = ptr;
while(...)
{
...
p = ...
}
}
But that is not allowed because it copies the value
of const pointer ptr into nonconst
variable p. That is easy to fix:
make p const.
void loop(const int* ptr) { const int* p = ptr; while(...) { ... p = ... } }
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