Template parameter type constructor method call

[AlexanderVX]

How do I write a constructor mehtod call in this case

/*-----------*/
template<typena me Tclass CObjectPoolImpl
{
public:

void smth(T* pObj)
{
if (pObj)
pObj->T::T(); // an attempt to call a constructor method of
class T
}

};

CObjectPoolImpl <mynamespace::C MyTypeCF;
CMyType mt;

CF.smth(&mt);
/*-----------*/

MS Visual C++ 7.1:
error C2039: 'T' : is not a member of 'mynamespace::C MyType'

There is a reason not to write 'new T' and not to explicitly write
constructor method name (type name). Default constructor method for
CMyType exists.

[Victor Bazarov]

Re: Template parameter type constructor method call

AlexanderVX wrote:

How do I write a constructor mehtod call in this case

There is no "constructo r method call" in C++. What is it you're
trying to accomplish?

>
/*-----------*/
template<typena me Tclass CObjectPoolImpl
{
public:
>
void smth(T* pObj)
{
if (pObj)
pObj->T::T(); // an attempt to call a constructor method of
class T

I am guessing here, but you might want to do

new (pObj) T();

}
>
};
>
CObjectPoolImpl <mynamespace::C MyTypeCF;
CMyType mt;
>
CF.smth(&mt);

This will very likely cause the constructor to be called twice for the
same object. That has undefined behaviour, AFAICT. Why do you think
you need it? 'mt' is already a fully constructed object.

/*-----------*/
>
MS Visual C++ 7.1:
error C2039: 'T' : is not a member of 'mynamespace::C MyType'
>
There is a reason not to write 'new T' and not to explicitly write
constructor method name (type name). Default constructor method for
CMyType exists.

Read about "placement new".

V
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[Kai-Uwe Bux]

Re: Template parameter type constructor method call

AlexanderVX wrote:

How do I write a constructor mehtod call in this case
>
/*-----------*/
template<typena me Tclass CObjectPoolImpl
{
public:
>
void smth(T* pObj)
{
if (pObj)
pObj->T::T(); // an attempt to call a constructor method of class T

new ( (void*) pObj ) T ();

or:

std::allocator< T>().construct ( pObj, T() );

}
>
};
>
CObjectPoolImpl <mynamespace::C MyTypeCF;
CMyType mt;
>
CF.smth(&mt);
/*-----------*/
>
MS Visual C++ 7.1:
error C2039: 'T' : is not a member of 'mynamespace::C MyType'
>
There is a reason not to write 'new T' and not to explicitly write
constructor method name (type name). Default constructor method for
CMyType exists.

Note: probably, someonewill tell you that you cannot call a constructor and
that all that you can do is cause the implementation to invoke/call it.
That slogan refers to three facts: (a) constructor calls are not function
calls, (b) the standard, somewhat cryptically, states that constructors
have no names (hence they are not found through name lookup), and (c) the
standard uses the passive voice in talking about constructors being called.

Personally, I find it rather confusing to underscore the difference between
functions calls and constructor calls by taking the linguistic license to
say that we call functions but not taking the license to say that we call
constructors. (You take a linguistic license in either case since the
standard also uses the passive voice when talking about a function being
called: the standard is just not concerned with what programmers do. Also,
a function call according to the standard is just a certain form of
expression: all you do is to write such an expression in your program and
that causes the implementation to invoke the function in the course of
evaluating the expression.)


Best

Kai-Uwe Bux

[AlexanderVX]

Re: Template parameter type constructor method call

This will very likely cause the constructor to be called twice for the

same object. That has undefined behaviour, AFAICT. Why do you think
you need it? 'mt' is already a fully constructed object.

You are right. This is just a sample. And the question was not about
it...

Read about "placement new".

I knew, but this kind of solution was not on the top of my head...

[AlexanderVX]

Re: Template parameter type constructor method call

Kai-Uwe Bux, your philosophic remark helps understanding. Good I am not
on technical interview.

[AlexanderVX]

Re: Template parameter type constructor method call

And I did it:
pObj = new ( (void*) pObj ) T ();

Guys, "placement new" solution results in

error C2059: syntax error : '('

I have an MFC project with a lot of headers included... What can cause
this?

[Victor Bazarov]

Re: Template parameter type constructor method call

AlexanderVX wrote:

And I did it:
pObj = new ( (void*) pObj ) T ();
>
Guys, "placement new" solution results in
>
error C2059: syntax error : '('
>
I have an MFC project with a lot of headers included... What can cause
this?

Probably not including <memory>

V
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[AlexanderVX]

Re: Template parameter type constructor method call

Probably not including <memory>

I am not quite sure why I should have included that STL header... BTW,
this no-good project include stuff even not allowing me to use none of
STL....

If I include <memoryor <listor whatever STL:

C:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\includ e\xutility(341) : error C2084: function
'std::_Scalar_p tr_iterator_tag std::_Ptr_cat(c onst std::_Bool
*,std::_Bool *)' already has a body
C:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\includ e\xutility(251) : see previous definition of '_Ptr_cat'
C:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\includ e\xutility(335) : error C2084: function
'std::_Scalar_p tr_iterator_tag std::_Ptr_cat(s td::_Bool *,std::_Bool
*)' already has a body
C:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\includ e\xutility(245) : see previous definition of '_Ptr_cat'
C:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\includ e\xutility(168) : error C2766: explicit specialization;
'std::iterator_ traits<std::_Bo ol>' has already been defined

And I am confused because it is not obvious what's causing it...

But now my question is why I cannot compile such a simple thing... This
requires include <memoryor something?

pObj = new ( (void*) pObj ) T ();

[Kai-Uwe Bux]

Re: Template parameter type constructor method call

AlexanderVX wrote:

And I did it:
pObj = new ( (void*) pObj ) T ();

[snip]

Nope. Just

new ( (void*) pObj ) T ();

This is not allocating memory for you. The pointer pObj must already point
to the memory location where you want that object to be constructed.


Best

Kai-Uwe Bux

[AlexanderVX]

Re: Template parameter type constructor method call

Kai-Uwe Bux, thanks, it removes unwanted assignment but compilation
error does not go away...

[Frederick Gotham]

Re: Template parameter type constructor method call

Victor Bazarov posted:

Probably not including <memory>

You sure you don't mean <new>?

--

Frederick Gotham

[Kai-Uwe Bux]

Re: Template parameter type constructor method call

AlexanderVX wrote:

Kai-Uwe Bux, thanks, it removes unwanted assignment but compilation
error does not go away...

Well, the line is legal. I would guess that there is a syntax error
somewhere close. Please post a minimal complete example that shows the
problem.


Best

Kai-Uwe Bux

[Kai-Uwe Bux]

Re: Template parameter type constructor method call

Frederick Gotham wrote:

Victor Bazarov posted:
>

>Probably not including <memory>

>
You sure you don't mean <new>?

Do we need any headers for placement new? I thought, <newjust provides
allocation and deallocation functions. Placement new shouldn't need those,
or would it?


Best

Kai-Uwe Bux

[Alf P. Steinbach]

Re: Template parameter type constructor method call

* Kai-Uwe Bux:

Frederick Gotham wrote:
>

>Victor Bazarov posted:
>>

>>Probably not including <memory>

>You sure you don't mean <new>?

>
Do we need any headers for placement new? I thought, <newjust provides
allocation and deallocation functions. Placement new shouldn't need those,
or would it?

There are infinitely many different placement new's, but /the/ placement
new, the one that constructs in place, requires the header <new>. It's
also a good idea to use '::'. This is because it's a library feature,
not a language feature (although those are strongly connected here).

But, but, but... I hear you say, the <newheader also defines the
standard global allocation function invoked by ordinary 'new', and we
don't need to include no flickin' <newheader to use ordinary 'new'?

Well, that's because the ordinary 'new' is made easy to use, while the
placement form is made less easy to use. It's specified by §5.3.4/11.
If the placement new syntax is used (and only then), overload resolution
is performed to determine the allocation function to call, which
allocation function, in the case of /the/ placement new, is supplied by
the <newheader.

Cheers,

- Alf

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[Victor Bazarov]

Re: Template parameter type constructor method call

Kai-Uwe Bux wrote:

Frederick Gotham wrote:
>

>Victor Bazarov posted:
>>

>>Probably not including <memory>

>>
>You sure you don't mean <new>?

>
Do we need any headers for placement new? I thought, <newjust
provides allocation and deallocation functions. Placement new
shouldn't need those, or would it?

He's right, <newis what's needed, not <memory>. Placement new
is declared in it. See 18.4 of the Standard.

V
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