Const twice in one method parameter?

Question

I'm fairly new to C++ and trying to understand some code I'm looking at:

bool ClassName::ClassMethod(const STRUCT_THING* const parameterName) {}

What is the purpose of the second "const" in the argument? How does it differ from just const STRUCT_THING* parameterName?

Thanks!

Answer 1

That means it is a const pointer to a const variable.

See the following examples:

int x = 5;                // non-const int
int* y = &x;              // non-const pointer to non-const int

int const a = 3;          // const int
int* const b = &a;        // const pointer to non-const int
int const* const c = &a;  // const pointer to const int

So you can see that two things have the potential to be mutable, the variable and the pointer. Either of these two can be const.

A const variable works just as you'd imagine:

int foo = 10;
foo += 5;     // Okay!

int const bar = 5;
bar += 3;     // Not okay! Should result in a compiler warning (at least)

A const pointer works the same way:

int foo = 10;
int bar = 5;

int* a = &foo;
a = &bar;  // Okay!

int* const b = &foo;
b = &bar;  // Not okay! Should also result in a compiler warning.