One line solution for unused out parameter reference

Question

I use a function which use 2 reference parameters to return values, only one of this value interess me. Is there a proper way to do it in one line.

I'll explain myself with the code

Function code

void Limits (double& min, double &max)
{
    min = MIN;
    max = MAX;
}

using the function

double min;
double unused;

Limits(min, unused);

// using myDouble but not unused

What i would like to write is something like

double min;

Limits(min, double());

I can't find a way without previously properly declare the object. Is there an elegant solution?

I can't modify the function.

Answer 1

I think, with C++11 and its rvalue references, you can implement the requested one-liner. It reads as follows:

template<class T> T& make_dummy_out(T&& t) { return t; }

Then you can call your Limit function as follows:

double min;
Limits(min, make_dummy_out(double()));

This is safe, because the lifetime of the ad-hoc-created double() will be until the end of the statement, which is after the Limits() call has been completed.

Note, that make_dummy_out() is basically the opposite of std::move(): While std::move() turns an lvalue reference explicitely into an rvalue reference, make_dummy_out() converts an rvalue reference explicitely into an lvalue reference. Of course, you need to declare the template function only once, and then can use it to create dummy output parameters whereever you need it.

Answer 2

I'm not sure why having to declare an additional variable is a problem. Regardless, if you can't modify the function, make a wrapper:

void MyLimits(double& min) {
  double unused;
  Limits(min, unused);
}

Or you could do something funky like this:

double min;

Limits(min, *std::unique_ptr<double>(new double));

Answer 3

The most obvious solution would be to use pointers:

void
Limits( double* min, double* max )
{
    if ( min != nullptr ) {
        *min = myMin;
    }
    if ( max != nullptr ) {
        *max = myMax;
    }
}

//  ...
double min;
Limits( &min, nullptr );

Alternatively, you could just define separate functions:

void Limits( double& min, double& max );
void UpperLimit( double& max );
void LowerLimit( double& min );

EDIT:

Since the orginal poster has since indicated that he cannot modify the function, he should wrap it with one which uses pointers:

void myLimits( double* min, double* max )
{
    double ignore;
    Limits( min == nullptr ? ignore : *min,
            max == nullptr ? ignore : *max );
}