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following code in the picture is a example to illustrate pointer to a constant integer. My question:

  1. Since int w is not declare as a constant integer, why it says that the pointer is pointing to a constant integer?

  2. why we cannot try the last line?

If the pointer to a constant should not be assigned to any other value after assigning to the first value, so what makes the difference between a pointer to a constant and a constant pointer then? ( For my understanding, a constant pointer cannot be changed, but a pointer to constant can be changed...

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  • "It changes the thing in the pointer, not the constant value itself..." is not correct. The previously line (p = &w) assigned a new value to the pointer while the last line (*p = 3) goes and mucks with the object pointed to. Commented Dec 7, 2015 at 2:54
  • What on earth is this mess you're pasting into your question? Commented Dec 7, 2015 at 3:02

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C allows a "pointer to const int" to actually point to a non-const int, as you found out. It doesn't cause any problems, so why not allow it?

The pointer doesn't remember whether it's pointing to a const int or to a normal int, so you always have to treat it as if it's pointing to a const int. That means *p = 3; isn't allowed, because the compiler doesn't know for sure* that *p isn't a const int.

* Modern compilers might well be able to figure this out, but the language says they have to pretend they can't, and they won't always be able to anyway.

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Think of const as a restriction - it prevents you from writing. Then the meaning is clear: const int * adds a restriction that may not exist on the base variable.

C lets you add this kind of restriction freely. The thing you can't do is to remove restrictions - you can't make a int * point at a const int without casting.

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According to what you say, the pointer to a constant should not be assigned to any other value after assigning to the first value. So what makes the difference between a pointer to a constant and a constant pointer then? ( For my understanding, a constant pointer cannot be changed, but a pointer to constant can be changed...
a pointer to constant can be changed (i.e. it points to other variables) but not the value it points to

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