Is using NULL references OK?

Question

I came across this code:

void f(const std::string &s);

And then a call:

f( *((std::string*)NULL) );

A

Answer 1

As others already said: A reference has to be valid. That's why it's a reference instead of a pointer.

If you want to make f() have a default behavior, you might want to use this:

static const std::string default_for_f;

void f(const std::string &s = default_for_f)
{
    if (&s == &default_for_f)
    {
        // make default processing
    }
    else
    ...
}
...
void bar()
{
    f();              // call with default behavior
    f(default_for_f); // call with default behavior
    f(std::string()); // call with other behavior
}

You can spare the default parameter for f(). (Some people hate default parameters.)