A reader posted this question on one of the C++ newsgroups. Assuming B is a base class for D, his program looks like this:
void g (B* p) ; void f (B*& p); // modifies p int main () { D* p = NULL ; g (p) ; // fine f (p) ; /* error: "cannot convert parameter 1 from 'class D *' to 'class B *& ' a reference that is not to 'const' cannot be bound to a non-lvalue" */ }Function f() works as expected, but g() causes a compilation error. The reader wondered what was wrong with the invocation of f() in main(). Can you see what is wrong with it? p is a D*, not a B*, and to convert it to a B*, the implementation creates a temporary pointer (recall that a reference must be bound to a valid object; in this case, a pointer). Now because temporaries are rvalues, and you can’t bind an rvalue to a non-const reference, the compiler complains. Declaring f() as follows:
void f (D* & p) { /* modifies p */ }solves this problem.
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