C++ Default argument for vector<int>& ?

Question

I have a function,

void test( vector<int>& vec );

How can I set the default argument for vec ? I have tried

void test( vector<int>& vec = vector<int>() );

But there's a warning "nonstandard extension used : 'default argument' : conversion from 'std::vector<_Ty>' to 'std::vector<_Ty> &'"

Is there a better way to do this ? Instead of

void test() {
    vector<int> dummy;
    test( dummy );
}

Regards, Voteforpedro

Answer 1

Have you tried:

void test(const vector<int>& vec = vector<int>());

C++ does not allow temporaries to be bound to non-const references.

If you really to need to have a vector<int>& (not a const one), you can declare a static instance and use it as a default (thus non-temporary) value.

static vector<int> DEFAULT_VECTOR;

void test(vector<int>& vec = DEFAULT_VECTOR);

But beware, because DEFAULT_VECTOR will (can) be modified and won't reset on each call ! Not sure that this is what you really want.

---

Thanks to stinky472, here is a thread-safe alternative:

Instead of providing a default value, you might as well overload test() with a zero-parameter version which calls the other version:

void test()
{
  vector<int> vec;
  test(vec);
}

Answer 2

I find it questionable for a non-const reference argument to have a default value. What is this supposed to mean?

Commonly, a function taking a non-const reference means "I might change the argument". If the argument is optional why no pass a (non-const) pointer? That way, when callers don't want to pass an arguments, they can pass NULL.
(See here for more information on how to pass function arguments.)

Edit: Well, and of course, there's also overloading. Just add another overload which doesn't take this argument.

Answer 3

We cannot initialize mutable references to temporaries. Only const references allow this. What you are after is most likely this:

void test( const vector<int>& vec = vector<int>() );

Aside from avoiding undefined behavior, this makes the most sense logically and from a const-correctness perspective. If you wanted 'test' to modify the original vector being passed to it, you would not have been able to sensibly provide a default value. Thus it's obvious you are using 'vec' here for read-only purposes and should therefore make it a const reference.