c++ variable argument list

Question

I'm searching for answers but i can't find any relevant information on this. Let's take the example:

class MyClass
{
    //member functions and variables
};

void foo(int pivot,...)
{
    va_list arguments;  
    va_start(arguments,pivot);

    //va_arg(arguments,???)

    va_end(arguments);
}

void bar()
{
    MyClass a;
    MyClass * b = &a;
    const MyClass & c = a;
    foo(0,a,b,c);
}

How are the arguments a,b and c passed? By value , or by reference and how to ask for them using va_arg? What about the constructors/destructor for MyClass? Where in the c++ standard is this kind of behavior specified?

examples for va_arg: en.cppreference.com/w/cpp/utility/variadic/va_arg — Nate Kohl
– Nate Kohl, Commented Feb 22, 2013 at 14:50
variadic functions: en.cppreference.com/w/cpp/utility/variadic and variadic templates from c++11: cplusplus.com/articles/EhvU7k9E — CCJ
– CCJ, Commented Feb 22, 2013 at 14:53
@ForEveR: Actually it is just undefined behavior. C++03 5.2.2.7. However, g++ will give an error for it. — Vaughn Cato
– Vaughn Cato, Commented Feb 22, 2013 at 15:07

ForEveR · Accepted Answer · 2013-02-22 15:26:11Z

5

You should never use user-defined types in var-arg function. Use C++11 variadic templates.

If your class is not pod-type - it's unspecified by standard, thanks to Vaughn Cato for remark

n3337 5.2.2/7

Passing a potentially-evaluated argument of class type (Clause 9) having a nontrivial copy constructor, a non-trivial move constructor, or a non-trivial destructor, with no corresponding parameter, is conditionally-supported with implementation-defined semantics.

Else, you can and it will be correct, but you shouln't.

edited Feb 22, 2013 at 15:26

answered Feb 22, 2013 at 14:45

ForEveR

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2 Comments

Raxvan Over a year ago

Then it should be safe just to use MyClass* only if really needed. I tested this msvc and it seems to be legit.

James Kanze Over a year ago

And of course, he doesn't show enough for us to conclude that MyClass is not a POD. Although traditionally, POD classes are defined with the struct keyword, something like class MyClass { public : int a; int b; }; is perfectly legal, and is a POD.

James Kanze · Accepted Answer · 2013-02-22 15:11:31Z

3

By value. But beware, if MyClass is not a POD, the program has undefined behavior (C++03, §5.2.2/7), or if MyClass has a non-trivial copy constructor, move constructor or destructor, the operation is conditionally supported, with implementation defined semantics (C++11, §5.2.2/7).

In your example, passing a and passing c are exactly identical operations (except that c cannot be bound to a non-const reference, but that's not an issue here, since varargs are all pass by value). Thus, when calling foo, you pass 0, a copy of a, a copy of the pointer b, and a copy of a. In order to access them in foo, you need to declare the types in va_arg as int, MyClass, MyClass* and MyClass.

answered Feb 22, 2013 at 15:11

James Kanze

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