Are 'const foo**' and 'foo** const' the same thing?

Question

I know that for a type foo, the function declaration for bar

void bar(const foo*) and void bar(foo* const)

are the same thing. But that about double indirection? That is, are

void bar(const foo**) and void bar(foo** const)

also the same thing? If not, then what do I need to do to void bar(foo** const) to put the const first?

user_12002570 · Answer

Here we consider

void bar(const foo*); #1
void bar(foo* const)  #2

In #1 we've a pointer to a const foo while in #2 we've a const pointer to a nonconst foo.

You could use std::is_same to confirm that they're different:

std::cout << std::is_same::value << ' ';   //false

Here we consider

void bar(const foo**); #3
void bar(foo** const); #4

In #3 we've a pointer to a pointer to a const foo while in #4 we've a const pointer to a nonconst pointer to a nonconst foo.

std::cout << std::is_same::value << ' '; //false

Moreover, note that const foo * is the same as foo const * but this case you don't have in your given examples.

std::cout << std::is_same::value << ' '; //true

Are 'const foo' and 'foo const' the same thing?