String literals, represented by a text surrounded by double quotes like "Hello world" are stored in read-only memory and cannot be
mutated.
Historically, string literals were introduced in C before the keyword const so it was possible to create a mutable pointer to a string
literal:
char * ptr = "Hello world!"; // Noncompliant
To maintain retro-compatibility with the enormous amount of code that had been written without const, pointing to a string literal with a mutable
char pointer remained allowed in C, and although it has been officially removed from C++ since C++11, most compilers still allow it with a
warning.
Because it is a pointer to read-only memory, trying to modify ptr will lead to undefined behavior. And because it is not declared as
const, the type system will not be able to prevent or warn of such modification attempts.
Exceptions
This rule doesn’t raise an issue for explicit conversions to char *.
char * ptr = (char *)"Hello world!"; // Compliant by exception, but you should avoid it.
This can be necessary in some very specific cases where an external API takes a mutable char * with the clear contract that it will
never modify it.
That remains a very dangerous pattern that should be avoided as much as possible.
