Question

I’ve been trying to pass a std::vector to a function by reference with default value being an empty std::vector. The declaration of my function looks as follows:

void function( std::vector<double>& vec=std::vector<double>(0));

The definition of my function is:

void function( std::vector<double>& vec)
{
...
}

However my C++ compiler (gcc 4.6) is throwing an error here, saying:

error: default argument for parameter of type ‘std::vector&’ has type ‘std::vector’

I’ve seen this version of the code compile fine on a Microsoft VS 2010 compiler. And I’m wondering if this is an issue of different c++ standard interpretation between gcc and vs2010.

Answer 1

You can’t. You can’t bind a temporary to a ref-to-non-const, so you could only do this:

void function(const std::vector<double>& vec = std::vector<double>());

In your case, then, I suggest function overloading:

void function(std::vector<double>& vec)
{
    // ...
}

void function()
{
    std::vector<double> v;
    function(v);
}

If you don’t like the extra function call then you’re out of luck. 🙂

I don’t know whether C++11 rvalue refs might help here, if you have access to them.

I’ve seen this version of the code compile fine on a Microsoft VS 2010 compiler. And I’m wondering if this is an issue of different c++ standard interpretation between gcc and vs2010.

MSVS has a tendency to allow you to bind temporaries to refs-to-non-const, which is non-standard behaviour. GCC is correct.