How to initialize a member struct

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Bob Altman - 29 Nov 2007 01:01 GMT

Hi all,

If I have a class that includes an instance of a struct as a member, how do
I initialize that struct? I can't find a syntax for the constructor
"initializer list" that works.

For example, suppose MyStruct has 3 int members. I've tried something like
this:

class Test {
private:
MyStruct X
}

// Constructor
Test::Test(void) : X(1,2,3) // Try initializing all 3 struct members...

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Jeffrey Tan[MSFT] - 29 Nov 2007 02:55 GMT

Hi Bob,

Using initializer list for another structure member means calling that
structure's constructor. So that structure must has a constructor matching
the signature with the initializer list calling, like this:
struct MyStruct
{
    int m_a;
    int m_b;
    int m_c;

    MyStruct(int a, int b, int c)
    {
        m_a = a;
        m_b = b;
        m_c = c;
    }
};

class Test {
    Test::Test(void) : X(1,2,3)
    {
    }
private:
    MyStruct X;
};
Hope it helps.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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Reply to this Message

Bob Altman - 29 Nov 2007 16:54 GMT

Hi Jeffrey,

I can't (easily) give this struct a constructor. Here's the longer story
(which I didn't want to get into because it's a little bit complicated):

Optimally, I'd like to just give this struct a parameterless constructor so
that the struct initializes itself. But when I do that, my code breaks.
Or, more specifically, the code in another DLL breaks.

I pass this struct type (call it "MyStruct") as an argument to a templated
class constructor (call it "TemplatedClass") in another DLL library.
Because it is a templated class, some of source code for the class lives in
the public header file and some of the code is compiled into the resultant
DLL. For some reason that I haven't tried to figure out, when I add a
parameterless constructor to MyStruct then the compiler issues an error in
the templated source code. (Sorry, I'm not at my work computer, so I don't
have the error number handy.) The error tells me that the DLL that contains
TemplatedClass needs to expose a fully constructed template for
TemplatedClass<MyStruct>.

Now, I've dealt with this error in other DLL projects. Under some (not well
understood) circumstances, if a DLL exposes a templated class, you need to
include some magic code in the public interface to expose fully constructed
definitions of the template for some of the types that callers use when
constructing instances of the templated class.

I don't want to monkey with the code in the DLL that contains
TemplatedClass. That is a general purpose library that shouldn't have code
in it that knows about my desire to use it with MyStruct as a template
parameter. So... That gets me back to trying to figure out a way to
initialize MyStruct without giving it a constructor.

If I create a MyStruct in "normal" code, I can just write:

MyStruct x = {1, 2, 3};

But I apparently can't include the initializer list (the "{1, 2, 3}") in a
class definition header file where I declare a member variable of type
MyStruct. This leaves me with trying to come up with a way to initialize
the struct in the class constructor. The ugly way to do this is to
explicitly initialize each member of the structure in the constructor code.
But I'm looking for a way to use an initializer list to initialize the
entire struct all at once.

Sigh... I told you it was complicated...

> Hi Bob,
>
[quoted text clipped - 46 lines]
> This posting is provided "AS IS" with no warranties, and confers no
> rights.

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Ben Voigt [C++ MVP] - 29 Nov 2007 18:24 GMT

> If I create a MyStruct in "normal" code, I can just write:
>
[quoted text clipped - 7 lines]
> code. But I'm looking for a way to use an initializer list to initialize
> the entire struct all at once.

With a helper function:

static MyStruct init_my_struct(int a, int b, int c)
{
const MyStruct x = { a, b, c };
return x;
}

MyConstructor() : member(init_my_struct(1, 2, 3)) {}

> Sigh... I told you it was complicated...

>> Hi Bob,
>>
[quoted text clipped - 49 lines]
>> This posting is provided "AS IS" with no warranties, and confers no
>> rights.

Reply to this Message

Bob Altman - 29 Nov 2007 18:58 GMT

Thanks Ben. I guess that's about as clean a solution as I'm likely to get.

- Bob

>> If I create a MyStruct in "normal" code, I can just write:
>>
[quoted text clipped - 77 lines]
>>> This posting is provided "AS IS" with no warranties, and confers no
>>> rights.

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How to initialize a member struct

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