Default constructor weirdness in member initialization list

[mhvaughn]

struct S1
{
int i;
};

struct S2 {
S1 s;
// version 1
S2() {} ;
// version 2
S2() : s() {} ;
};

int main()
{
S2 s;
printf( "%d\n", s.s.i );
return 0;
}

----------

With version 1 of the S2 constructor the program will spew random
values as expected.
With version 2 of the S2 constructor the program only spews zeros.
Does explicitly calling the default constructor in the member
initialization list somehow cause the "default" constructor for the
int in S1 to be called?

[devp]

On Feb 1, 5:18 am, [e-mail] wrote:

struct S1
{
int i;

};

struct S2 {
S1 s;
// version 1
S2() {} ;
// version 2
S2() : s() {} ;

};

int main()
{
S2 s;
printf( "%d\n", s.s.i );
return 0;

}

----------

With version 1 of the S2 constructor the program will spew random
values as expected.
With version 2 of the S2 constructor the program only spews zeros.
Does explicitly calling the default constructor in the member
initialization list somehow cause the "default" constructor for the
int in S1 to be called?

Hi
I tried the program with g++ it works same both ways, as should. Which
compiler did you use?

cheers
projyal

[mhvaughn]

On Jan 31, 5:22 pm, devp <[e-mail]:

On Feb 1, 5:18 am, [e-mail] wrote:

struct S1
{
int i;

};

struct S2 {
S1 s;
// version 1
S2() {} ;
// version 2
S2() : s() {} ;

};

int main()
{
S2 s;
printf( "%d\n", s.s.i );
return 0;

}

----------

With version 1 of the S2 constructor the program will spew random
values as expected.
With version 2 of the S2 constructor the program only spews zeros.
Does explicitly calling the default constructor in the member
initialization list somehow cause the "default" constructor for the
int in S1 to be called?

Hi
I tried the program with g++ it works same both ways, as should. Which
compiler did you use?

[615]$g++ -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --enable-languages=c,c+
+,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-
system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-
threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/
4.1.3 --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu
--enable-libstdcxx-debug --enable-mpfr --enable-checking=release
x86_64-linux-gnu
Thread model: posix
gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)

>
cheers
projyal

[Andrey Tarasevich]

Alf P. Steinbach wrote:

...

>In C++ there's no syntax and way in general to "explicitly call" a
constructor.

I'm sorry, that's incorrect.

There's no dedicated C++ syntax for constructor calls. What the syntax
means depends on context. Whether to say that an explicit constructor
call is an explicit constructor call is a terminologial issue.

I disagree. Constructors are functions. They are special functions, but
functions nevertheless. For this reason I'd expect the "explicit
constructor call", if it existed, to be have the function call
semantics. However, if I'm not mistaken, there's no way to call
constructor in C++ by using function call semantics. Constructors calls
in C++ are "side effects" of the more generic process called
"initialization". It is is invoked by various language constructs
having... well, initialization semantics, which is essentially different
from function call semantics. This is why I say that there's no way to
"call" a constructor in C++.

The language's creator calls an explicit constructor call an explicit
constructor call.

Where? In TC++PL? If yes, then sorry, but that book is not canonical in
the context of ANSI standard C++ discussion. It is intentionally
simplified to be more accessible for newcomers. And the price of that
simplification is that sometimes it makes statements, which are simply
incorrect from the standard C++ point of view. (For example, it states
that scalar types have constructors, if I remember correctly).

--
Best regards,
Andrey Tarasevich