Reference to Member Variable in Function declaration in C++?

Question

I'm trying to do something like the following:

class FOO {
 void bar(int& var = m_var) {
   // ....
 }
 int m_var;
};

Why doesn't this compile? Why didn't they program this into the language? Is there any way to mimic this behavior?

Answer 1

This is not allowed because m_var is a member variable and needs to be accessed through the object.
It would compile successfully if m_var was a static member of the class.

A simple workaround is calling an overloaded function with same name or another member function through bar() (which is a member function & has access to m_var ) and pass m_var as an parameter by reference.It will have the same effect you want to achieve.

Answer 2

I agree, this is a limitation of the language. It can be implemented into compilers (in my humble opinion) with no difficulty.

If you want this behaviour, you have to write:

class FOO 
{
    void bar(int& var) { ... }
    void bar() { this->bar(m_var); }

    int m_var;
};

and the extra function call will be inlined by any half-decent compiler, in case you worry about it.

Answer 3

One solution is to to have static-variables, that would act as placeholders:

 class FOO 
    {
    private:
        static int _ph_m_var;
        void bar(int& var = _ph_var)
        {
              if(&var == &_ph_var) { // Default }
        }
};

Answer 4

Declare m_var to be a static const. It will compile and run.

static const int m_var should do the trick