Uninitialized constant objects

[=?ISO-8859-1?Q?Ney_Andr=E9_de_Mello_Zunino?=]

Hello.

Based on this simple test program:

class Test {};

int main()
{
Test t;
const Test u;
}

Why does the definition of 'u' fail? What if all that's wanted is to
have a default constructed Test object? Must one explicitly write so? E.g.:

const Test u = Test();

What's the rationale for such restriction? And, lastly, why does
defining a default constructor solve the issue? I.e.:

class Test
{
public:
Test() {}
};

int main()
{
Test t;
const Test u;
}

Now the compiler accepts it just fine. Why? Pardon me if the answer to
these questions should be obvious. I don't presently find them to be so.

Thank you,

 

[bkirungein]

which compiler you are using..?
I tried it on VC++ compiler it is not giving any error in both the
cases

since constructor is called during object creation, it doesn't matter
wether the object is const or not

by declaring a object as const the only restrictions that you have on
it is
-- you can only call const methods of that object( ex: methods like
void print() const )
-- you cannot modify any member variables

 

[Karl Heinz Buchegger]

Ney André de Mello Zunino wrote:
[snip]

What's the rationale for such restriction? And, lastly, why does
defining a default constructor solve the issue? I.e.:

The code you posted is not your real code.
The code you posted should compile fine without a problem,
since if you don't define any constructor on your own, the compiler
will create a default constructor for you.

So please post your real problem code.

 

[Max M.]

class Test {};

int main()
{
Test t;
const Test u;
}

Why does the definition of 'u' fail?

[8.5]
9. If no initializer is specified for an object, and the object is of
(possibly cv-qualified) non-POD class type (or array thereof), the object
shall be default-initialized; if the object is of const-qualified type,
the underlying class type shall have a user-declared default constructor.
Otherwise, if no initializer is specified for an object, the object and
its subobjects, if any, have an indeterminate initial value 90) ; if the
object or any of its subobjects are of const-qualified type, the program
is ill-formed.

Your object is of const POD type, then the last sentence of the paragraph
applies.

MM

 

[Alf P. Steinbach]

* Karl Heinz Buchegger:

Ney André de Mello Zunino wrote:
[snip]

What's the rationale for such restriction? And, lastly, why does
defining a default constructor solve the issue? I.e.:

The code you posted is not your real code.
The code you posted should compile fine without a problem,
since if you don't define any constructor on your own, the compiler
will create a default constructor for you.

So please post your real problem code.

Sorry.

Check out the standard's text quoted by "Max M." in this thread;
some explicit definition of the value is necessary for a const
object.

Unfortunately Visual C++ erronously compiles the code, at least as
I remember it it does.

 

[Karl Heinz Buchegger]

* Karl Heinz Buchegger:

Ney André de Mello Zunino wrote:
[snip]

What's the rationale for such restriction? And, lastly, why does
defining a default constructor solve the issue? I.e.:

The code you posted is not your real code.
The code you posted should compile fine without a problem,
since if you don't define any constructor on your own, the compiler
will create a default constructor for you.

So please post your real problem code.

Sorry.

Check out the standard's text quoted by "Max M." in this thread;
some explicit definition of the value is necessary for a const
object.

I saw it.
I didn't know that.
I learned.