Initializing member variables that are struts

[2b|!2b==?]

I have a class that contains C structs as member variables. By C
structs, I mean they cannot have ctors/dtors because they have C linkage
(extern "C").

For eg:

MyClass
{
//Impl
private:
MyStruct1 m_struct ;
};
//In C code, I could declare/define/initialize the m_struct variable
using 1 line:

MyStruct1 m_struct = {0} ;

In C++, I would have liked to resort to something similar (well
excluding the declaration part), using an initialization list like so:

MyClass()
:m_struct({0})
{}

Of course, this dosent work. The only way I found to initilalize a C
struct was to do the ff:

MyClass()
{
MyStruct m_struct = {0} ;
}

which compiles ok - but I can't help wondering if this is correct. It
looks like a declaration - which if it is, means that my member variable
did not get initialized. What does the C++ spec say about this ?

[Ian Collins]

2b|!2b==? wrote:

I have a class that contains C structs as member variables. By C
structs, I mean they cannot have ctors/dtors because they have C linkage
(extern "C").

The linkage isn't the issue, use by C code is.

For eg:

MyClass
{
//Impl
private:
MyStruct1 m_struct ;
};
//In C code, I could declare/define/initialize the m_struct variable
using 1 line:

MyStruct1 m_struct = {0} ;

In C++, I would have liked to resort to something similar (well
excluding the declaration part), using an initialization list like so:

MyClass()
:m_struct({0})
{}

Of course, this dosent work. The only way I found to initilalize a C
struct was to do the ff:

MyClass()
{
MyStruct m_struct = {0} ;
}

which compiles ok - but I can't help wondering if this is correct. It
looks like a declaration - which if it is, means that my member variable
did not get initialized. What does the C++ spec say about this ?

Does it? I'd expect at least one warning that the function scope
variable shadows the class member.

One tidy solution is to derive form the C sourced struct and provide a
default constructor in the derived struct:

struct UsedByC { int n; };

struct UsedByCpp : UsedByC {
UsedByCpp() { n = 0; }
};

class MyClass {
UsedByCpp m_struct;
};

--
Ian Collins.

[Rolf Magnus]

2b|!2b==? wrote:

I have a class that contains C structs as member variables. By C
structs, I mean they cannot have ctors/dtors because they have C linkage
(extern "C").

For eg:

MyClass
{
//Impl
private:
MyStruct1 m_struct ;
};
//In C code, I could declare/define/initialize the m_struct variable
using 1 line:

MyStruct1 m_struct = {0} ;

In C++, I would have liked to resort to something similar (well
excluding the declaration part), using an initialization list like so:

MyClass()
:m_struct({0})
{}

Of course, this dosent work. The only way I found to initilalize a C
struct was to do the ff:

MyClass()
{
MyStruct m_struct = {0} ;
}

which compiles ok - but I can't help wondering if this is correct.

Well, it's correct, but it doesn't seem to be what you want. This creates a
new local struct within the constructor and has nothing to do with the
member variable.

It looks like a declaration - which if it is, means that my member
variable did not get initialized.

That's right.

What does the C++ spec say about this ?

MyClass()
:m_struct()
{}

[ajk]

On Tue, 01 May 2007 10:42:54 +0100, "2b|!2b==?" <ro**@your.box.com>
wrote:

>In C++, I would have liked to resort to something similar (well
excluding the declaration part), using an initialization list like so:

MyClass()
:m_struct( MyStruct() )
{ }

[Fei Liu]

2b|!2b==? wrote:

I have a class that contains C structs as member variables. By C
structs, I mean they cannot have ctors/dtors because they have C linkage
(extern "C").

For eg:

MyClass
{
//Impl
private:
MyStruct1 m_struct ;
};
//In C code, I could declare/define/initialize the m_struct variable
using 1 line:

MyStruct1 m_struct = {0} ;

In C++, I would have liked to resort to something similar (well
excluding the declaration part), using an initialization list like so:

MyClass()
:m_struct({0})
{}

Of course, this dosent work. The only way I found to initilalize a C
struct was to do the ff:

MyClass()
{
MyStruct m_struct = {0} ;
}

which compiles ok - but I can't help wondering if this is correct. It
looks like a declaration - which if it is, means that my member variable
did not get initialized. What does the C++ spec say about this ?

As it has been explained, as long as you only use the struct in C++
code, there is no problem defining constructors etc in your struct. It's
no different from a class except the default access is public.

[James Kanze]

On May 1, 11:55 am, Rolf Magnus <[e-mail]:

2b|!2b==? wrote:

I have a class that contains C structs as member variables. By C
structs, I mean they cannot have ctors/dtors because they have C linkage
(extern "C").

For eg:

MyClass
{
//Impl
private:
MyStruct1 m_struct ;
};

//In C code, I could declare/define/initialize the m_struct variable
using 1 line:

MyStruct1 m_struct = {0} ;

In C++, I would have liked to resort to something similar (well
excluding the declaration part), using an initialization list like so:

MyClass()
:m_struct({0})
{}

Of course, this dosent work. The only way I found to initilalize a C
struct was to do the ff:

MyClass()
{
MyStruct m_struct = {0} ;
}

which compiles ok - but I can't help wondering if this is correct.

Well, it's correct, but it doesn't seem to be what you want. This createsa
new local struct within the constructor and has nothing to do with the
member variable.

It looks like a declaration - which if it is, means that my member
variable did not get initialized.

That's right.

What does the C++ spec say about this ?

MyClass()
:m_struct()
{}

That only works if he wants zero initialization (which
corresponds to the example he actually gave, of course). A more
general solution would be:

namespace {
MyStruct1 myStructInit = { ... } ;
}

MyClass()
: m_struct( myStructInit )
{
}

--
James Kanze (GABI Software) email:ja*********@gmail.com
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