c - constant pointer vs pointer on a constant value

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Answer 1

char * const a;

means that the pointer is constant and immutable but the pointed data is not.
You could use const_cast(in C++) or c-style cast to cast away the constness in this case as data itself is not constant.

const char * a;

means that the pointed data cannot be written to using the pointer a. Using a const_cast(C++) or c-style cast to cast away the constness in this case causes Undefined Behavior.