CODEWITHSUNDEEP
c++
P0W
P0W

Reputation: 47784

Why does the compiler allow this?

The following compiles successfully:

struct XX {
  int element;
};

int main ()
{
  XX  XX, prev_XX  ;
  prev_XX = XX;
  XX = prev_XX;  //  ??? NO error: expected unqualified-id before '=' token
}

How does compiler know whether it's a type or a variable?

Edit :

An error for this : (from sftrabbit's example)

int main ()
{
    XX XX, prev_XX;
    XX XX2; // error: expected ';' before 'XX2'
}

Upvotes: 1

Views: 111

Answers (4)

Joseph Mansfield
Joseph Mansfield

Reputation: 110648

The variable name hides the struct name (§3.3.10):

A name can be hidden by an explicit declaration of that same name in a nested declarative region or derived class (10.2).

If you need to refer to the struct name specifically, you need to use an elaborated type specifier, struct XX (§3.4.4):

An elaborated-type-specifier (7.1.6.3) may be used to refer to a previously declared class-name or enum-name even though the name has been hidden by a non-type declaration (3.3.10).

So the compiler knows that XX refers to the variable here because that's all it can correspond to. It won't even consider the possibility of it referring to the struct. In fact, let's see what happens if we try to use it as a type:

struct XX {
    int elemnent;
};

int main ()
{
    XX XX, prev_XX;
    XX XX2; // Can we continue to use XX as a type?
}

We actually get an error:

error: expected ‘;’ before ‘XX2’

So instead, we have to do struct XX XX2;, which gives no error.

Upvotes: 10

Matthieu M.
Matthieu M.

Reputation: 299760

This is a subtle issue harking back to the days of C.

When you declare XX XX; the second XX (variable) shadows the first XX (type). You can still access the type by using struct XX to name it.

struct XX {};

int main() {
    XX XX;

    XX other; // error: expected ‘;’ before ‘other’

    struct XX another; // OK

    another = XX; // OK

    return 0;
}

You can check it out here.

Upvotes: 1

Vlad from Moscow
Vlad from Moscow

Reputation: 310930

The name of the object hides the name of the structure. If you want to specify the structure you should use elaborated name for example

XX XX;
struct XX YY;

It was introduced in C++ because in C structure names and variables names belong to different name spaces due to the fact that in C you must always specify keyword struct with structures. In C++ it is allowed to use structure names without keyword struct but variable names hides structure names if they coinside.

Upvotes: 1

Ed Heal
Ed Heal

Reputation: 59997

It is due to the syntax of the expression.

Just like some words can be either a noun or verb - depends on the location in the sentence

Upvotes: 1

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