I'm having a problem with optional function parameter in C++ What I'm trying to do is to write function with optional parameter which is passed by reference, so that I can use it in two ways (1) and (2), but on (2) I don't really care what is the value of mFoobar. I've tried such a code: void foo(double &bar, double &foobar = NULL) { bar = 100; foobar = 150; } int main() { double mBar(0),mFoobar(0); foo(mBar,mFoobar); // (1) cout << mBar << mFoobar; mBar = 0; mFoobar = 0; foo(mBar); // (2) cout << mBar << mFoobar; return 0; } but it crashes at void foo(double &bar, double &foobar = NULL) with message : error: default argument for 'double& foobar' has type 'int' Is it possible to solve it without function overloading? Thanks in advance for any suggestions. Pawel