How to set default parameter as class object in c++?

You have three obvious options here.

First, use overloads so the caller can choose to pass b or not.

int myfunc(int a) { ... }
int myfunc(int a, base& b) { ... }

This way you can pass b without having to use a pointer. Note that you should make b a reference or pointer type to avoid slicing the object.

Secondly, if you don't want 2 separate implementations, make b a pointer, which can be set to NULL.

int myfunc(int a, base* b = NULL) { ... }

Third, you could use something to encapsulate the concept of nullable, such as boost::optional.

int myfunc(int a, boost::optional<base&> b = boost::optional<base&>()) { ... }

Objects can't be NULL in C++.

To set the parameter to default, just use:

int myfunc(int a, base b = base())

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