gt5595c@prism.gatech.EDU (BARTZ) (05/22/91)
I have a question about derived class constructors.
Below is the skeleton of the problem.
class A
{
A(int f1,int f2,double first ...); // Constructor of base class
...
};
class B : public A
{
B(int g1,double first ...) : Matrix(g1,g1,first) {};
// Constructor for derived class
...
};
The constructor for the base class works OK with the ellipse
but the derived class construction does not work. The base class
constructor is not called with the list of doubles.
The cfront output for this declaration
B X(1,3,0.1,0.3,0.2);
looks something like this:
struct B __1X ;
( (__ct__6ClassA_FiT1d3 ( ((struct A *)(& __1X )), 1 , 3 , 0.1 ) ), (((& __1X )))) ;
instead of
__ct__6ClassA_FiT1d3 ( ((struct A *)(& __1X )), 1 , 3 , 0.1 , 0.3, 0.2);
Is it not legal to pass variable length arguments through to
base class constructors?
-----------------------------------------------------------------------
Michael Bartz
Coding and Information Theory Laboratory
Georgia Institute of Technology
School of Electrical Engineering, Atlanta, GA 30332-0250
E-MAIL: gt5595c@prism.gatech.edu
PHONE: (404) 853-9362mittle@blinn.watson.ibm.com (Josh Mittleman) (05/22/91)
In article <29534@hydra.gatech.EDU>, gt5595c@prism.gatech.EDU (BARTZ) writes: > class A > { > A(int f1,int f2,double first ...); // Constructor of base class > ... > }; > > class B : public A > { > B(int g1,double first ...) : Matrix(g1,g1,first) {}; > // Constructor for derived class > ... > }; I assume that instead of "Matrix" you meant "A". Ellipsis is not an argument, it is a compiler directive. It means that the compler may match a call with any number of trailing arguments with this function declaration. In particular, it is *not* a list of values. When you write A(g1, g1, first), all you are passing is the single value, first. The only way to access the additional values which may follow first is to use the va_list macros, or some similar method. These only work within the function which has the ellipsis signature. I have been able to pass a pointer to the list of values to second function by initializing a va_list in the calling function, and then passing a reference to it. In the recipient function, va_arg works just fine with this reference. =========================================================================== Josh Mittleman (mittle@watson.ibm.com or joshua@paul.rutgers.edu) J2-C28 T.J. Watson Research Center, PO Box 704, Yorktown Heights, NY 10598
steve@taumet.com (Stephen Clamage) (05/22/91)
gt5595c@prism.gatech.EDU (BARTZ) writes: >class A >{ > A(int f1,int f2,double first ...); // Constructor of base class > ... >}; >class B : public A >{ > B(int g1,double first ...) : Matrix(g1,g1,first) {}; > // Constructor for derived class > ... >}; >The constructor for the base class works OK with the ellipse >but the derived class construction does not work. The base class >constructor is not called with the list of doubles. I assume you meant B(int g1, double first ...) : A(g1, g1, first) {}; There is no syntax to support passing along a variable parameter list this way. Two solutions come immediately to mind: 1. Don't use variable parameter lists for the B constructor. Use an array of doubles and a count, for example, and add an A constructor which does the same: B::B(int g1, double *aray, int count) : A(g1, g1, aray, count) {} 2. Make A a member of B instead of deriving B from A. Then A can have an additional constructor taking a va_list parameter, and the B constructor can set up the va_list and invoke the A constructor. -- Steve Clamage, TauMetric Corp, steve@taumet.com