How to call template constructor?

SAn

Here is the class with template constructor:

class C
{
private:
	int x;
public:
	template<int X> C(void):x(X){;}	
};

How to call this constructor? This does not work:

C c<10>;
C<10> c;

Braunstein

AFAIK it is not possible. But you can do something like this

class C
{
private:
    int x;
    C(int X) : x(X) {}
public:
    template<int X> static C getC() { return C(X); }
};

int main()
{
    C elem(C::getC<2>());
    return 0;
}

LordJaxom

That's right, constructors can't ever be called with template parameters. The only way of invoking such a constructor is by letting the compiler deduce the template arguments from the constructor arguments. This also means that a template constructor with no parameters can never be called at all.

SAn

Ok.

The question is: How I can pass a compile-time constant to the constructor?
The constant should still be the compile-time constant inside the constructor.

SAn schrieb:

Ok.

The question is: How I can pass a compile-time constant to the constructor?
The constant should still be the compile-time constant inside the constructor.

also erstmal solltest du Deutsch lernen

SAn

^^^^^^, I decided to use German C++ forum for two reasons:

It was at the first place in Google search (query "C++ Forum");
I think German people are very "disciplined", so they should write the most correct (according to ANSI standard) C++ programs.

I do not know German language (I am from Russia). I decided that English will be OK. I can help beginner programmers here (I have perfect automatic translator from German to Russian), but I do no know if they know English.

I have been to Germany. There are beautiful places there:
http://keep4u.ru/imgs/b/071213/eb/ebd7c154670bae5fdf.jpg
http://www.photosight.ru/photo.php?photoid=2072575&ref=author
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LordJaxom

Don't bother about the unregistered "Troll"

You could pass the constant via an empty pseudo-object that is an argument to the template-constructor (so the compiler can deduce the template argument from the real argument):

template< int N > struct value_t {};

class C
{
public:
    template< int N >
    C( value_t< N > )
    {
        // access N
    }
};

C c1( value_t< 12 >() );
C c2( value_t< 100 >() );

EDIT: Some optimizations.

SAn

LordJaxom, thanks a lot. This works for me.

Konrad Rudolph

Hi,

I was intrigued by the question so I tried to get the code to work. Obviously, I failed. However, I'd like to know *why* I failed.

Consider the following class and constructor:

struct C {
    int x;
    C() : x() { }
};

Now, the following instantiation works on both MSVC 9 as well as GCC 4.2 and as far as I understand the standard, it's legal:

int main() {
    C c = C::C();
}

Analogously, I believe the following code should work; that is, I see no reason (in the standard or elsewhere) that it differs from the one above:

struct C {
    int x;
    template <int X> C() : x(X) { }
};

int main() {
    C c = C::C<10>();
}

However, the code fails in both compilers. Here's what they say.

GCC: 'C' is not a template. No matching function for call to 'C::C()'
MSVC: 'C::{ctor}': cannot take address of this member function

Can anyone explain?

Konrad Rudolph schrieb:

Now, the following instantiation works on both MSVC 9 as well as GCC 4.2 and as far as I understand the standard, it's legal:
int main() {
    C c = C::C();
}

In german for Konrad:
Bei Sachen ob illegal oder nicht würde ich immer den Comeau zu rate ziehen: http://www.comeaucomputing.com/tryitout/ Einfach Code unten rein und "Compile".

"ComeauTest.c", line 7: error: a constructor or destructor may not have its address
          taken
      C c = C::C();

Standard 12.1.12 schrieb:

No return type (not even void) shall be specified for a constructor. A return statement in the body of a constructor
shall not specify a return value. The address of a constructor shall not be taken.

Im Falle C c = C(); erzeugst du ja im schlimmsten Fall ein temporäres Objekt das danach in c kopiert wird.
Wenn du schreibst C::C() bezeichnet man das glaube ich als einen "qualifizierten" Aufruf, d.h. du versuchst die Funktion( den Konstruktor ) direkt aufzurufen, was natürlich fehlschlägt.

Zudem definiert man ja mit dem Ausdruck C::C() den C'tor außerhalb der Klasse, kann sein, dass das damit auch was zu tun hat.

[ Selbstgestrickte Theorie -> camper verbessert das schon ]

hustbaer

Selbstgestrickte Theorie -> camper verbessert das schon

ROFL

BTW: Wir sollten camper den Ehrentitel "C++ Standard Bot" geben (analog zu "Bible Bot" und ähnlichen Einrichtungen)

LordJaxom

I have once learned that in C++ it is impossible to call any constructor directly, except via placement-new. Therefore, as I understand the standard, the first example should be illegal as well.

I think the quoted section of the standard confirms this, since you need an address for any function call (and since a constructor doesn't have a return value, it's result couldn't be used for an initialization anyways).

Konrad Rudolph

LordJaxom schrieb:

I have once learned that in C++ it is impossible to call any constructor directly, except via placement-new. Therefore, as I understand the standard, the first example should be illegal as well.

Yes, of course. KasF gave the correct explanation. Thanks to you both.