When I should add the word "class" creating pointer in C++?

Question

In what cases it could be invalid to write Type *p = nullptr;

whereas only class Type *p = nullptr; is satisfying?

463035818_is_not_an_ai · Accepted Answer · 2021-07-14T16:01:31.373

7

You need it when the type Type is shadowed by the name of a variable:

class Type{};

int main() {
    int Type = 42;

    //Type * p = nullptr; // error: 'p' was not declared in this scope
    class Type* p = nullptr;
}

However, as pointed out by Ayxan Haqverdili, ::Type* p = nullptr; works as well and has the added benefit of also working with type aliases.

edited Jul 14 '21 at 16:01

answered Jul 14 '21 at 15:23

463035818_is_not_an_ai

109,796
11
89
185

Like in order to specify that it must be a pointer to object of a class but not to a plain type? – Max Popov Jul 14 '21 at 15:36
1

This I didn't know. – Sreeraj Chundayil Jul 14 '21 at 15:36
1

@MaxPopovsky `Type * p` means: multiply the `int` called `Type` with some `p`, but there is no `p` in scope. And next error would be that you cannot assign `nullptr` to the result of that multiplication – 463035818_is_not_an_ai Jul 14 '21 at 15:38
@MaxPopovsky actually I have no clue how to do it when `class Type{};` is replaced with `using Type = int;` – 463035818_is_not_an_ai Jul 14 '21 at 15:45
2

@463035818 you could do `::Type`. – Aykhan Hagverdili Jul 14 '21 at 15:55
1

@AyxanHaqverdili you just saved me from posting a very silly question :) – 463035818_is_not_an_ai Jul 14 '21 at 16:00

score 2 · Answer 2 · answered Jul 14 '21 at 15:45

In addition to another answer, an elaborated type specifier forward declares a name, so you can write:

// class Type1 {};
Type1 *p = nullptr;         // error: 'Type1' was not declared in this scope

// class Type2 {};
class Type2 *p = nullptr;   // compiles

This could also be useful when you declare functions in headers:

void foo(class Type*);   // no separate forward declaration of Type is needed

When I should add the word "class" creating pointer in C++?

2 Answers2