Is there any difference? (about unary_function<>)

[kyo]

class A
{
public:
int a;
};

struct testfun1 : public unary_function<const A&, bool>
{
bool operator()(const A &) const
{
return true;
}
};

struct testfun2 : public unary_function<A, bool>
{
bool operator()(const A &) const
{
return true;
}
};

struct testfun3 : public unary_function<A, bool>
{
bool operator()(const A *) const
{
return true;
}
};

struct testfun4 : public unary_function<const A*, bool>
{
bool operator()(const A *) const
{
return true;
}
};

When and Why that i should use testfun2 not testfun1, and testfun4 not testfun3.