Cannot use Macro in a C++ constructor? - c++

Question

I have a complex class that I was trying to simplify with a Macro. The Macro works fine with some compilers, but not with others. I was thinking that a macro was just text replacement, am I wrong?
struct FooManager
{
FooManager(){}
void Add( Foo* i_pFoo ){ m_FooObjects.Add( i_pFoo ); }
private:
DynamicArray<Foo*> m_FooObjects;
};
struct Foo
{
Foo( FooManager& mgr, int param1, int param2 = 0 )
: m_Param1( param1 )
, m_Param2( param2 )
{
mgr.Add( this );
}
private:
const int m_Param1, m_Param2;
}
class Bar
{
FooManager m_Manager;
Foo m_Foo1, m_Foo2;
public:
Bar();
};
Then in the .cpp file...
#define Macro( f, a, ... ) f( m_Manager, a, __VA_ARGS__ )
Bar::Bar()
: m_Manager()
, Macro( m_Foo1, 1 )
, Macro( m_Foo2, 2, 3 )
{}
I get an "error 29: expected an expression" when using a gcc compiler.
All I really want is my Bar to have a Manager that knows about all of the Foo's in it. I can unroll the Macro, but was hoping I didn't have to because it makes things look a lot cleaner, and it can provide other shared parameters based on usage.
I am inclined to believe that there is a rule I am breaking by doing this, and that the compilers that it does work with are for some reason ignoring this rule.

Answer 1

In situations when __VA_ARGS__ is empty (e.g. when expanding Macro( m_Foo1, 1 )) your macro will generate the following entry in the constructor initializer list
m_Manager(<some argument>, )
This is obviously invalid. The idea of __VA_ARGS__ is that you should supply at least one argument for the __VA_ARGS__ part.
In order to make __VA_ARGS__ easier to use in such contexts, compilers like MSVC are known to implement non-standard behavior in this case: they quietly remove the excessive comma, making your original code to compile as intended.
GCC also implements a non-standard extension that serves the same purpose, but you have to activate it by using a non-standard trick with ##
#define Macro( f, a, ... ) f( m_Manager, a, ##__VA_ARGS__ )
This will automatically remove the trailing comma in GCC when __VA_ARGS__ is empty.