Because converting Foo** → const Foo** would be invalid and dangerous.

C++ allows the (safe) conversion Foo* → Foo const*, but gives an error if you try to implicitly convert Foo** → const Foo**.

The rationale for why that error is a good thing is given below. But first, here is the most common solution: simply change const Foo** to const Foo* const*:

  1. class Foo { /* ... */ };
  3. void f(const Foo** p);
  4. void g(const Foo* const* p);
  6. int main()
  7. {
  8. Foo** p = /*...*/;
  9. // ...
  10. f(p); // ERROR: it's illegal and immoral to convert Foo** to const Foo**
  11. g(p); // Okay: it's legal and moral to convert Foo** to const Foo* const*
  12. // ...
  13. }

The reason the conversion from Foo** → const Foo** is dangerous is that it would let you silently and accidentally modify a const Foo object without a cast:

  1. class Foo {
  2. public:
  3. void modify(); // make some modification to the this object
  4. };
  6. int main()
  7. {
  8. const Foo x;
  9. Foo* p;
  10. const Foo** q = &p; // q now points to p; this is (fortunately!) an error
  11. *q = &x; // p now points to x
  12. p->modify(); // Ouch: modifies a const Foo!!
  13. // ...
  14. }

If the q = &p line were legal, q would be pointing at p. The next line, *q = &x, changes p itself (since *q is p) to point at x. That would be a bad thing, since we would have lost the const qualifier: p is a Foo* but x is a const Foo. The p->modify() line exploits p’s ability to modify its referent, which is the real problem, since we ended up modifying a const Foo.

By way of analogy, if you hide a criminal under a lawful disguise, he can then exploit the trust given to that disguise. That’s bad.