What are top-level const qualifiers?

Question

What does const at "top level" qualifier mean in C++?

And what are other levels?

For example:

int const *i; int *const i; int const *const i; 

Answer 1

A top-level const qualifier affects the object itself. Others are only relevant with pointers and references. They do not make the object const, and only prevent modification through a path using the pointer or reference. Thus:

char x; char const* p = &x; 

This is not a top-level const, and none of the objects are immutable. The expression *p cannot be used to modify x, but other expressions can be; x is not const. For that matter

*const_cast<char*>( p ) = 't' 

is legal and well defined.

But

char const x = 't'; char const* p = &x; 

This time, there is a top-level const on x, so x is immutable. No expression is allowed to change it (even if const_cast is used). The compiler may put x in read-only memory, and it may assume that the value of x never changes, regardless of what other code may do.

To give the pointer top-level const, you'd write:

char x = 't'; char *const p = &x; 

In this case, p will point to x forever; any attempt to change this is undefined behavior (and the compiler may put p in read-only memory, or assume that *p refers to x, regardless of any other code).

Answer 2

int *const i puts const at the top-level, whereas int const *i does not.

The first says that the pointer i itself is immutable, whereas the second says that the memory the pointer points to is immutable.

Whenever const appears immediately before or after the type of the identifier, that is considered a top-level qualifier.