Initialize an array of classes?

[Bob Altman]

Hi all,

I have a class that contains a member variable that is an array of class
instances:

class MyClass {
private:
SomeClass m_someClass;
SomeClass m_arrayOfClasse s[2];
};

Now, the SomeClass class doesn't have a default (paramterless) constructor,
so I need to call its constructor in my initialization list of the MyClass
constructor, kind of like this:

// Constructor
MyClass::MyClas s(void) :
// This compiles ok
m_someClass( constructor_arg uments ),

// This doesn't compile
m_arrayOfClasse s[0]( constructor_arg uments ),
m_arrayOfClasse s[1]( constructor_arg uments )
{}

How do I initialize the array of class instances?

TIA - Bob

[Bob Altman]

Re: Initialize an array of classes?

Here is a complete C++ console app example that demonstrates what I'm trying
to do. The challenge is to change InnerValue to an array and figure out how
to call the constructor for each element of the array.

TIA - Bob

#include "stdafx.h"
#include <iostream>
using namespace std;

class CInner {
public:
// Constructor
CInner(int x) : Value(x) {}

// Member variable
int Value;
};

class COuter {
public:
CInner InnerValue;

// Constructor
COuter() :
InnerValue(2)
{}
};

int _tmain(int argc, _TCHAR* argv[])
{
COuter x;
cout << x.InnerValue.Va lue << endl;
return 0;
}

[Bob Altman]

Re: Initialize an array of classes?

Well, I found a way to get kind of what I'm after. Instead of trying to
have an array of class instances, I can create an array of pointers to class
instances. That way I can use the "new" operator in the body of the
constructor to initialize the array. Of course, this means that I need to
go through my code and change all of the relevant "." operators to "->"
operators, but that's easily enough done in this case.

But I'm still curious as to whether or not it's possible to initialize an
array of classes if the class doesn't have a default constructor.

[David Wilkinson]

Re: Initialize an array of classes?

Bob Altman wrote:

Hi all,
>
I have a class that contains a member variable that is an array of class
instances:
>
class MyClass {
private:
SomeClass m_someClass;
SomeClass m_arrayOfClasse s[2];
};
>
Now, the SomeClass class doesn't have a default (paramterless) constructor,
so I need to call its constructor in my initialization list of the MyClass
constructor, kind of like this:
>
// Constructor
MyClass::MyClas s(void) :
// This compiles ok
m_someClass( constructor_arg uments ),
>
// This doesn't compile
m_arrayOfClasse s[0]( constructor_arg uments ),
m_arrayOfClasse s[1]( constructor_arg uments )
{}
>
How do I initialize the array of class instances?

Bob:

If SomeClass is copyable you can do

MyClass::MyClas s():
m_someClass( constructor_arg uments )
{
m_arrayOfClasse s[0] = SomeClass( constructor_arg uments0 );
m_arrayOfClasse s[1] = SomeClass( constructor_arg uments1 );
}

--
David Wilkinson
Visual C++ MVP

[Bo Persson]

Re: Initialize an array of classes?

Bob Altman wrote:

Well, I found a way to get kind of what I'm after. Instead of
trying to have an array of class instances, I can create an array
of pointers to class instances. That way I can use the "new"
operator in the body of the constructor to initialize the array. Of
course, this means that I need to go through my code and change
all of the relevant "." operators to "->" operators, but that's
easily enough done in this case.
But I'm still curious as to whether or not it's possible to
initialize an array of classes if the class doesn't have a default
constructor.

No, there is no way of initializing member array in the current
language.

In the upcoming revision, C++09, there is a generalized "initialize r
list" feature that will possibly help in cases like this.


Bo Persson

[Charles Wang [MSFT]]

Re: Initialize an array of classes?

Hi Bob,
Currently you cannot use member initialization list for a nonstatic array of a class if it does not have a default constructor. When you try to initialize the array, you will encounter the compiler
error C2536. You may refer to:
Compiler Error C2536

http://msdn.microsoft.com/en-us/library/9f53ks1w.aspx

If the class has a default constructor, you can explicitly initialize it like the following code:
class CInner {
public:
// Constructor
CInner(int x) : Value(x) {}
CInner(){}

// Member variable
int Value;
};

class COuter {
public:
CInner InnerValue;
CInner m_Inners[2];

// Constructor
COuter() :
InnerValue(2),m _Inners()
{}
};

The code can be compiled, however you may encounter the warning C4351 and I do not think that it is useful to use such initialization for an array.
You can refer to this article for further inforamtion:
Compiler Warning (level 1) C4351

http://msdn.microsoft.com/en-us/library/1ywe7hcy.aspx

Best regards,
Charles Wang
Microsoft Online Community Support
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[Ben Voigt [C++ MVP]]

Re: Initialize an array of classes?

David Wilkinson wrote:

Bob Altman wrote:

>Hi all,
>>
>I have a class that contains a member variable that is an array of
>class instances:
>>
>class MyClass {
>private:
> SomeClass m_someClass;
> SomeClass m_arrayOfClasse s[2];
>};
>>
>Now, the SomeClass class doesn't have a default (paramterless)
>constructor, so I need to call its constructor in my initialization
>list of the MyClass constructor, kind of like this:
>>
>// Constructor
>MyClass::MyCla ss(void) :
> // This compiles ok
> m_someClass( constructor_arg uments ),
>>
> // This doesn't compile
> m_arrayOfClasse s[0]( constructor_arg uments ),
> m_arrayOfClasse s[1]( constructor_arg uments )
>{}
>>
>How do I initialize the array of class instances?

>
Bob:
>
If SomeClass is copyable you can do
>
MyClass::MyClas s():
m_someClass( constructor_arg uments )
{
m_arrayOfClasse s[0] = SomeClass( constructor_arg uments0 );
m_arrayOfClasse s[1] = SomeClass( constructor_arg uments1 );
}

Don't think so... all members have to be fully constructed before the body
of the user-defined constructor begins to execute. With no parameter-less
constructor, and no way to pass parameters to the array element
constructors, it's not possible.

OTOH, you should be able to make a fixed-length std::vector alike that does
this, using an embedded char array, sizeof, and placement new. Be careful
about destroying the right set of already-fully-constructed elements if any
of the element constructors throw, be careful to destroy the elements in the
array's destructor, and block copy construction and assignment operators,
and you should be golden. This will, of course, be a lot more general in
C++0x with variadic templates and closures, but C++03 is perfectly capable
of making a type-specific initializable array class.

[David Wilkinson]

Re: Initialize an array of classes?

Ben Voigt [C++ MVP] wrote:

David Wilkinson wrote:

>Bob Altman wrote:

>>Hi all,
>>>
>>I have a class that contains a member variable that is an array of
>>class instances:
>>>
>>class MyClass {
>>private:
>> SomeClass m_someClass;
>> SomeClass m_arrayOfClasse s[2];
>>};
>>>
>>Now, the SomeClass class doesn't have a default (paramterless)
>>constructor , so I need to call its constructor in my initialization
>>list of the MyClass constructor, kind of like this:
>>>
>>// Constructor
>>MyClass::MyCl ass(void) :
>> // This compiles ok
>> m_someClass( constructor_arg uments ),
>>>
>> // This doesn't compile
>> m_arrayOfClasse s[0]( constructor_arg uments ),
>> m_arrayOfClasse s[1]( constructor_arg uments )
>>{}
>>>
>>How do I initialize the array of class instances?

>Bob:
>>
>If SomeClass is copyable you can do
>>
>MyClass::MyCla ss():
>m_someClass( constructor_arg uments )
>{
> m_arrayOfClasse s[0] = SomeClass( constructor_arg uments0 );
> m_arrayOfClasse s[1] = SomeClass( constructor_arg uments1 );
>}

>
Don't think so... all members have to be fully constructed before the body
of the user-defined constructor begins to execute. With no parameter-less
constructor, and no way to pass parameters to the array element
constructors, it's not possible.
>
OTOH, you should be able to make a fixed-length std::vector alike that does
this, using an embedded char array, sizeof, and placement new. Be careful
about destroying the right set of already-fully-constructed elements if any
of the element constructors throw, be careful to destroy the elements in the
array's destructor, and block copy construction and assignment operators,
and you should be golden. This will, of course, be a lot more general in
C++0x with variadic templates and closures, but C++03 is perfectly capable
of making a type-specific initializable array class.

Ben:

Mmmm, I guess you're right. Oh well.

Of course, this could be fixed by creating a default constructor for SomeClass...

--
David Wilkinson
Visual C++ MVP

[=?Utf-8?B?UGF1bCBQYXZsaWNrbw==?=]

Re: Initialize an array of classes?

Bob - I am trying to do something very similar by having an array of a class
inside the other class but have the array created dynamically through the
main class constructor.

can I see your code on how you did this with the pointers?


"Bob Altman" wrote:

Well, I found a way to get kind of what I'm after. Instead of trying to
have an array of class instances, I can create an array of pointers to class
instances. That way I can use the "new" operator in the body of the
constructor to initialize the array. Of course, this means that I need to
go through my code and change all of the relevant "." operators to "->"
operators, but that's easily enough done in this case.
>
But I'm still curious as to whether or not it's possible to initialize an
array of classes if the class doesn't have a default constructor.
>
>
>

[Ben Voigt [C++ MVP]]

Re: Initialize an array of classes?

Paul Pavlicko wrote:

Bob - I am trying to do something very similar by having an array of
a class inside the other class but have the array created dynamically
through the main class constructor.
>
can I see your code on how you did this with the pointers?

Just use std::vector<Tfo r this, if a native class, or clr::array<Tif a
ref class.

>
>
"Bob Altman" wrote:
>

>Well, I found a way to get kind of what I'm after. Instead of
>trying to have an array of class instances, I can create an array of
>pointers to class instances. That way I can use the "new" operator
>in the body of the constructor to initialize the array. Of course,
>this means that I need to go through my code and change all of the
>relevant "." operators to "->" operators, but that's easily enough
>done in this case.
>>
>But I'm still curious as to whether or not it's possible to
>initialize an array of classes if the class doesn't have a default
>constructor.

[Bob Altman]

Re: Initialize an array of classes?

Thanks all. That answers my question.

[Bob Altman]

Re: Initialize an array of classes?

Paul, I just hacked up my original post. No guarantees that I didn't mangle
something...

#include "stdafx.h"
#include <iostream>
using namespace std;

class CInner {
public:
// Constructor
CInner(int x) : Value(x) {}

// Member variable
int Value;
};

class COuter {
public:
CInner InnerValue[5]; // Won't compile
CInner* InnerValue[5] // Array of pointers

// Constructor
COuter() :
// InnerValue(2) // Remove initializer
{
// Initialize the array in the body of the constructor
for (int i = 0; i < 5; i++) {
InnerValue[i] = new CInner(i);
}
}
};

int _tmain(int argc, _TCHAR* argv[])
{
COuter x;
cout << x.InnerValue.Va lue << endl;
return 0;
}