Passing optional auto_ptr to function

Does anyone know if it is possible to pass an optional auto_ptr to a function? I want to pass in an auto_ptr to a file handle sometimes, but not others. My code looks like the following

Code:

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void generateTree(PhyloTree* tree, auto_ptr<ofstream>& file = NULL);

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Then later when the function is declared I want to check if it is NULL or not. But it is not working. Any suggestions??
Thanks
Mark

How is it not working? I don't see anything wrong with what you've posted so far.

You cannot assign NULL to a reference, even an auto_ptr reference. You can make it a const reference and assign a default constructed auto_ptr:

Code:

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void generateTree(PhyloTree* tree, const auto_ptr<ofstream>& file = auto_ptr<ofstream>());

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Although, there might be a better way. What do you do if it is null? If you write to cout, I would just take an ostream& and pass the ofstream or cout (or an ostringstream) as necessary.