Help to explain this error?

[Bo Yang]

The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }
~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;
return 0 ;
}

And the g++ give the error:
no matching call to : Test::Test()

I know this is because of my don't provide the Test::Test() function.

But when I change the array initialization as below:

Test array[10] = { Test(1) };
g++ say:
no matching call to : Test::Test(Test)

I can't understand where I call Test::Test(Test)?

[mlimber]

Bo Yang wrote:

The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }
~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;
return 0 ;
}

And the g++ give the error:
no matching call to : Test::Test()

I know this is because of my don't provide the Test::Test() function.

But when I change the array initialization as below:

Test array[10] = { Test(1) };
g++ say:
no matching call to : Test::Test(Test)

I can't understand where I call Test::Test(Test)?

You're creating a temporary Test(1) which is then used to construct the
first element in your array via the copy constructor. However, you have
provided a copy constructor from reference to a non-const object of
type Test, and since you can't bind temporaries to non-const references
and since the presence of that function means that the compiler won't
generate the implicit copy constructor from a reference to a const
object, you get the second error. Make the parameter in your copy
constructor const and you'll be back to your original error since
you're still trying to default-initialize the other nine elements of
the array.

BTW, you could eliminate some unnecessary code:

Test array[] = { 1, 2 }; // Invokes Test::Test(int) for each element

Cheers! --M

[Rolf Magnus]

Bo Yang wrote:

The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }
~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;
return 0 ;
}

And the g++ give the error:
no matching call to : Test::Test()

I know this is because of my don't provide the Test::Test() function.

But when I change the array initialization as below:

Test array[10] = { Test(1) };
g++ say:
no matching call to : Test::Test(Test)

I can't understand where I call Test::Test(Test)?

You create a temporary Test object with 'Test(1)'. This object has to be
copied into the array. For that, the compiler doesn't find a suitable copy
constructor.

[Victor Bazarov]

Bo Yang wrote:

The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }
~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;
return 0 ;
}

And the g++ give the error:
no matching call to : Test::Test()

I know this is because of my don't provide the Test::Test() function.

But when I change the array initialization as below:

Test array[10] = { Test(1) };
g++ say:
no matching call to : Test::Test(Test)

I can't understand where I call Test::Test(Test)?

Test array[10] = { Test(1) };
^^^^^^^
This is a temporary object. The first element of the array is copy-
constructed from it. Since your copy-constructor does not have 'const'
in the argument type, it cannot be used. In order to copy-construct
from a temporary, the copy constructor should be 'Test(Test const&)'.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Rud1ger Sch1erz]

Bo Yang <st******@mail.nankai.edu.cnwrites:

The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }

^^^^^
Test( const Test & test ) { std::cout << "Copy" << std::endl; }

~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;

Test array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0 } ;

return 0 ;
}

And the g++ give the error:
no matching call to : Test::Test()

I know this is because of my don't provide the Test::Test() function.

Yep.

But when I change the array initialization as below:

Test array[10] = { Test(1) };
g++ say:
no matching call to : Test::Test(Test)

You must provide intialization/construction for every object in the
array.

Cheers,
Rudiger

[Bo Yang]

Rud1ger Sch1erz :

Bo Yang <st******@mail.nankai.edu.cnwrites:

>The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }

^^^^^
Test( const Test & test ) { std::cout << "Copy" << std::endl; }

> ~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;

Test array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0 } ;

> return 0 ;
}

I have change the copy constructor to the right form.
I think the output must to be:

Test Constructor!
Copy!
Destroy!

But, to my surprise, there is no Copy! output.
Why ?

>>
And the g++ give the error:
no matching call to : Test::Test()

I know this is because of my don't provide the Test::Test() function.

Yep.

>But when I change the array initialization as below:

Test array[10] = { Test(1) };
g++ say:
no matching call to : Test::Test(Test)

You must provide intialization/construction for every object in the
array.

Cheers,
Rudiger

[Rolf Magnus]

Bo Yang wrote:

Rud1ger Sch1erz :

>Bo Yang <st******@mail.nankai.edu.cnwrites:

>>The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }

^^^^^
Test( const Test & test ) { std::cout << "Copy" <<
std::endl; }

>>~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;

Test array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0 } ;

>>return 0 ;
}

I have change the copy constructor to the right form.
I think the output must to be:

Test Constructor!
Copy!
Destroy!

But, to my surprise, there is no Copy! output.
Why ?

Because the compiler might - as an optimization - remove the actual copy
operation and directly initialize the array members. However, a suitable
copy constructor must, even if not used, still be available, because the
compiler's optimization behavior doesn't influence the correctness of your
code.

[terminator]

Rolf Magnus wrote:

Bo Yang wrote:

Rud1ger Sch1erz :

Bo Yang <st******@mail.nankai.edu.cnwrites:

The test code is as below:

#include <iostream>
class Test{
public:
Test(int i ) { std::cout << "Test Constructor!" << std::endl ; }
Test( Test & test ) { std::cout << "Copy" << std::endl; }
^^^^^
Test( const Test & test ) { std::cout << "Copy" <<
std::endl; }

~Test() { std::cout << "Destroy!" << std::endl ; };
};
int main( int argc , char ** argv ){
Test array[10] = { } ;

Test array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0 } ;

return 0 ;
}

I have change the copy constructor to the right form.
I think the output must to be:

Test Constructor!
Copy!
Destroy!

But, to my surprise, there is no Copy! output.
Why ?

Because the compiler might - as an optimization - remove the actual copy
operation and directly initialize the array members. However, a suitable
copy constructor must, even if not used, still be available, because the
compiler's optimization behavior doesn't influence the correctness of your
code.

this code:

class C{
public:
int i;
C(){puts("default");i=1;};
C(const C& cc){puts("copy");i=2*cc.i;};
};

int _tmain(int argc, _TCHAR* argv[])
{
C cc[5]={C()};
printf("%d\n",cc[3]);
getch();
return 0;
}

outputs:

default
default
default
default
default
1 1 1 1 1
this has nothing to do with optimization.
elements are not copied .ohterwise last line
of output should have been 2 2 2 2 2.

u actually tell the compiler how to initialize
array elements in a code similar to above.
that is u tell the compiler which constructor
with which arguments should be invoked to initialize
the array.