const与指针

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;



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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 ) = 'x' is legal and well defined.

But

char const x = 'x';
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 = 'x';
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).