Steven Watanabe wrote:

Boost.Function doesn't provide such a utility. I can't see a way to get this information out of boost::bind using the public interface, either. visit_each skips placeholders. IMHO it should be a feature of boost::function.

If there is a way you would need something like this

#include #include

struct bar {};

class tSlot { public: template<class T> tSlot(const T& t) : uses_placeholder1(false), uses_placeholder2(false), f(t) { // do some magic to find which arguments are used. } But what is the magic here? An approach could be the introduction of a class function_invoker leaving boost::function untouched:

---- struct function_invoker { template<class T> function_invoker(function<T>& f); // throws, if f.empty(); // true, if a placeholder is used by f (should the index i // be zero-based?): bool uses_placeholder(int i) const; // binds arg to a placeholder template<class T> void bind_placeholder(int i, T& arg); template<class T> void bind_placeholder(int i, const T& arg); // throws, if a placeholder used by f is not bound to an argument: fn_result call(); }; ---- I'm aware that the implementation won't be that easy, esp. the static computing of the type of fn_result could become difficult. However it would solve my problem without bloating boost::function: void foo(const tSlot& s) { function_invoker i(s); s.bind_placeholder(1, 1); bar b; if (i.uses_placeholder(2)) // index is one-based) { expensive_calculation(b); s.bind_placeholder(2, b); } s.call(); } I'm wondering if Douglas Gregor is lurking and what his opinion is. Best regards Olaf Krzikalla