Help - New to Managed C++ Question

[Steve Marsden]

Hi

I am an experienced C programmer and we have a large app written in C which
I have just recompiled with /clr to start to add in managed code to use .NET
framework and windows forms bit by bit.

I am new to managed C ++(and C++ for that matter).

I am struggling with the fact that it seems you cannot have global managed
variables although I think I understand why you can't.

i.e. if I declare a managed type or class or object as I would a normal C
global variable, I get a complier error:

error C3145 cannot declare a global or static managed type object or an _gc
pointer.

My big problem is I initially want to write some managed C++
functions/classes in a new C++ file which I then want to call from my
existing C code. However these new managed C++ functions need to preserve
data between themselves and between calls from the C code so it seemed
global variables where the ideal solution.

So the scenario is something like

C code

call managed C++ function 1 to initialize data

loop
do some processing
call managed C++ function 2 to work with initialized data
end loop

C++ code

function1
creates some managed data (but where does it preserve this if I cant have
global managed variables)

funtion2
works on previously created managed data

Can anyone advise on how best to tackle this problem.

Thanks

Steve

[Steve Marsden]

Below is a very very simple example of my problem (in reality things are
more complicated obviously but the principle is the same)

I want to call functions test to initialize the data and test 2 to retrieve
it later from my C code which obviously requires that dog is preserved
between calls

In my C++ file I have:

__gc class animal
{
public:
int legs;
};

extern "C" void test()
{
animal* dog;
dog=new animal;
dog->legs=4;
}

extern "C" int test2()
{
return dog->legs;
}

Firstly this obvioulsy doesn't compile as test2 doesnt know about dog. A
global; variable would solve it but if I declare dog as variable I get the
C3145 compiler error.

Secondly, won't dog be garbage collected once test has been called by by C
code so that when I later call test2 dog doesnt exist anyway.

It seems to me a global variable would solve all this but managed C++ won't
allow it.

Steve

[Steve McLellan]

Hi,

You could make a class that initialises and holds 'dog' and makes it
statically available. MC++ has static constructors, so you can use that to
ensure you never access dog before it's been initialised.

Steve

"Steve Marsden" <st***@conquest .ltd.uk> wrote in message
news:Of******** ******@TK2MSFTN GP09.phx.gbl...

Below is a very very simple example of my problem (in reality things are
more complicated obviously but the principle is the same)

I want to call functions test to initialize the data and test 2 to
retrieve it later from my C code which obviously requires that dog is
preserved between calls

In my C++ file I have:

__gc class animal
{
public:
int legs;
};

extern "C" void test()
{
animal* dog;
dog=new animal;
dog->legs=4;
}

extern "C" int test2()
{
return dog->legs;
}

Firstly this obvioulsy doesn't compile as test2 doesnt know about dog. A
global; variable would solve it but if I declare dog as variable I get the
C3145 compiler error.

Secondly, won't dog be garbage collected once test has been called by by C
code so that when I later call test2 dog doesnt exist anyway.

It seems to me a global variable would solve all this but managed C++
won't allow it.

Steve

[Steve Marsden]

Steve

Thanks for the reply. Could you expand on this a bit please. I'm not sure
exaclty what you mean.

Thanks

Steve

"Steve McLellan" <sjm AT fixerlabs DOT com> wrote in message
news:OB******** ******@tk2msftn gp13.phx.gbl...

Hi,

You could make a class that initialises and holds 'dog' and makes it
statically available. MC++ has static constructors, so you can use that to
ensure you never access dog before it's been initialised.

Steve

"Steve Marsden" <st***@conquest .ltd.uk> wrote in message
news:Of******** ******@TK2MSFTN GP09.phx.gbl...

Below is a very very simple example of my problem (in reality things are
more complicated obviously but the principle is the same)

I want to call functions test to initialize the data and test 2 to
retrieve it later from my C code which obviously requires that dog is
preserved between calls

In my C++ file I have:

__gc class animal
{
public:
int legs;
};

extern "C" void test()
{
animal* dog;
dog=new animal;
dog->legs=4;
}

extern "C" int test2()
{
return dog->legs;
}

Firstly this obvioulsy doesn't compile as test2 doesnt know about dog. A
global; variable would solve it but if I declare dog as variable I get
the C3145 compiler error.

Secondly, won't dog be garbage collected once test has been called by by
C code so that when I later call test2 dog doesnt exist anyway.

It seems to me a global variable would solve all this but managed C++
won't allow it.

Steve