Why am I getting an error converting a ‘float**’ to ‘const float**’?

See Why am I getting an error converting a Foo** → const Foo**?

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

The reference goes on to give an example of how such an implicit conversion could allow me one to modify a const object without a cast.