invalid use of static data member in constructor parameter list

Question

I have a class something like this:

class Foo : public Bar {
double v1;
double v2;
...
public:
explicit Foo (double v1_ = 1.0, double v2_ = v1 > 0.0 ? 2.0 : 0.0)
    : v1(v1_), v2(v2_)
{
    // do something
}

// do other things

};

but I get the following compile error like so:

error: invalid use of non-static data member Foo::v1
note: declared here
double v1;
       ^

Any suggestions are appreciated to get around this error. Also, please point out the mistake in my code and explain a little so that I can understand better. Thanks in advance.

Answer 1

explicit Foo (double v1_ = 1.0, double v2_ = v1 > 0.0 ? 2.0 : 0.0)
                                             ^^

At the point where you use v1 it does not exist yet.

Unfortunately, you can't use v1_ at this point either. What you can do instead, is to split the constructor in two versions:

// for two arguments
Foo (double v1_, double v2_)
    : v1(v1_), v2(v2_)
{
    // do something
}

// for zero or one argument
explicit Foo (double v1_ = 1.0)
    : Foo(v1_, v1_ > 0.0 ? 2.0 : 0.0)
{
}

(here I used delegating constructors feature to avoid code duplication)

Answer 2

When you call the constructor the object doesn't exist yes. It's the constructor that creates and returns it. So you can't refer to v1 in the arguments of the constructor - there is still no object and no v1.