CODEWITHSUNDEEP
c++language-lawyer
user2889159
user2889159

Reputation:

Declaration and definition in c++

I thought that every declaration is definition because there was the following quote from standard:

A declaration is a definition unless %restrictions%.

But my assumption is not true. Actually, applying ODR we have that the following program

class A;
class A;

int main(){ }

is ill-formed. But it is not true. I can't to find part of standard which permit to redeclare class type in the declarative region.

Upvotes: 0

Views: 145

Answers (4)

4pie0
4pie0

Reputation: 29754

The paragraph § 3.1.2 states that

A declaration is a definition unless it declares a function without specifying the function’s body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification 25 (7.5) and neither an initializer nor a function- body, it declares a static data member in a class definition (9.2, 9.4), it is a class name declaration (9.1), it is an opaque-enum-declaration (7.2), it is a template-parameter (14.1), it is a parameter-declaration (8.3.5) in a function declarator that is not the declarator of a function-definition, or it is a typedef declaration (7.1.3), an alias-declaration (7.1.3), a using-declaration (7.3.3), a static_assert-declaration (Clause 7), an attribute- declaration (Clause 7), an empty-declaration (Clause 7), or a using-directive (7.3.4).

Here

class A;
class A;

int main(){ }

it is a class name declaration.

In a statement

A declaration is a definition unless %restrictions%.

the %restrictions%. part is important.

I thought that every declaration is definition

Let's prove that this is not true by a contradiction. So assume this is true. Then because we can have many redeclaration and every of this declarations is definition - we can have many redefinitions, right? But C++ Standard n3337 § 3.2/1 says

No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template.

what contradicts our assumption and thus this is not true that every declaration is definition.

Upvotes: 1

Mike Seymour
Mike Seymour

Reputation: 254751

Your quote (from C++11 3.1/2) answers the general question: "unless %restrictions%" means that not every declaration is a definition. It's only a definition if none of those restrictions apply.

If you read those restrictions, you'll find

it is a class name declaration

which answers your specific question. class A; is a class name declaration, but not a definition.

I can't to find part of standard which permit to redeclare class type in the declarative region.

In general, you can declare an entity multiple times in the same declarative region, per C++11 3.3.1/4

Given a set of declarations in a single declarative region, each of which specifies the same unqualified name, they shall all refer to the same entity, or [other cases not relevant here]

Upvotes: 1

Michael
Michael

Reputation: 968

Consider the case of:

Header1.h:

class A;

void function1(A * a);

Header2.h:

class A;

void function2(A * a);

main.cpp

#include "Header1.h"
#include "Header2.h"

#include "A.h" // header file defining A

int main()
{

}

what this really looks like to the compiler is:

class A;

void function1(A * a);

class A;

void function2(A * a);


class A { /* definition from A.h */ };


int main()
{

}

Upvotes: 0

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171177

Yes, "a declaration is a definition unless %restrictions%" is true. Have you read the restrictions? One of them is:

it is a class name declaration

So class A; is not a definition because it is covered by one of the restrictions.

Just to clarify, quoting C++11, [basic.def]§2

Upvotes: 3

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