Can't convert from 'unsigned short ** ' to 'const unsigned short ** '? Why not?

Code:

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cannot convert parameter 2 from 'unsigned short ** ' to 'const unsigned short ** '
Conversion loses qualifiers

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I've seen this error a few times and it kind of baffles me.
I know you need a const_cast to remove constness, but if I'm reading this correctly, this error says it can't add constness.
I've passed non-const parameters to functions that take const parameters more times than I can count without problems. Why would it complain about this only sometimes? I thought constness can always be implicitely added, but you have to explicitely remove it?

You're not adding constness. You're trying to convert a pointer to one type (unsigned short*) to a pointer to a different, unrelated type (const unsigned short *). Adding const is only allowed at the outermost level and an unbroken chain going inward. The reason for this restriction is that, if you could cast int** to const int**, you could subvert const-correctness without a cast:

Code:

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const int ci = 0;
int *pi; // Valid - uninitialized pointer to int
int **ppi = π // Valid - ppi points at pi
const int **ppci = ppi; // Valid? ppci now points at pi
const int *pci = &ci; // Valid - pci is const and may point at the const int
*ppci = pci; // Valid - *ppci is "const int *", which you can assign to
// But uh, oh - pi points to ci now!
*pi = 100; // Modify ci???

// Since an unbroken chain is allowed, this is actually valid:
const int *const *pcpci = ppi;
*pcpci = π // Invalid. *pcpi is "const int *const" and cannot be assigned to.

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Quote:

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Originally Posted by CornedBee

You're not adding constness. You're trying to convert a pointer to one type (unsigned short*) to a pointer to a different, unrelated type (const unsigned short *). Adding const is only allowed at the outermost level and an unbroken chain going inward. The reason for this restriction is that, if you could cast int** to const int**, you could subvert const-correctness without a cast:

Code:

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const int ci = 0;
int *pi; // Valid - uninitialized pointer to int
int **ppi = π // Valid - ppi points at pi
const int **ppci = ppi; // Valid? ppci now points at pi
const int *pci = &ci; // Valid - pci is const and may point at the const int
*ppci = pci; // Valid - *ppci is "const int *", which you can assign to
// But uh, oh - pi points to ci now!
*pi = 100; // Modify ci???

// Since an unbroken chain is allowed, this is actually valid:
const int *const *pcpci = ppi;
*pcpci = π // Invalid. *pcpi is "const int *const" and cannot be assigned to.

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Ah, OK. I guess that makes sense. I'll probably forget this again later, but at least I know for now. ;)
Thanks.