Re: Using SFINAE with member function types

Carl Barron <>
Sun, 22 Jul 2007 03:20:34 CST
In article <>,
Chris Fairles <> wrote:

Below is some code illustrating the wrong way to do what I'd like to
do. I have a class "A" with function "void f(int,int)" and class "B"
without that function. Is there a way to construct a traits-like
class, Q, that contains a constant expression (Q<T>::value) that
evaluates to true if some class T has the function "void f
(int,int)" ?

Pretend A and B cannot be changed. You only have Q to work with.

struct A {
  void f (int,int){}

struct B{};

template <class T,class F=void>
struct Q {

template<class T>
struct Q<T,void(T::*)(int,int)> {

int main() {
  if( Q<A>::value ) {cout << "A";}
  if( !Q<B>::value ){cout << "B";}

I'd like this to output "AB". I've tried playing around with
boost::enable_if and boost::function_traits but have not come up with
a solution.

The above code is a trivial case. The real case is, I'm trying to come
up with a "is_serializable" type trait (Q) so that an I/O "write"-like
function does something special for non-serializable classes (and just
does normal serialization for those that are). Every serializable
class has this function defined (as a mem func):
template <typename Archive>
void serialize(Archive &, const unsigned int);

If I could add a typedef to each serializable class, no problem. I
could even make a typelist of all serializable classes and use a
visitor/enable_if or something but I'd like to consider these last

See boost::mpl

template <class T>
struct is_serializable:boost::mpl::contains


this has a nested typedef typedef something type;
where something is either boost::mpl::true_ or boost::mpl::false_,
with obvious meanings. type also has an operator bool() const,
and a static const bool which return /is the boolean value true or
false accordingly.

so you can overload for boost::mpl::true_ and boost::mpl_false
or just add a boolean value that defaults to

template <class T>
void my_function(const T &a,bool which=is_serializable<T>::type::value)
   check the value of which at runtime.

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