Template argument deduction

[kev82]

Given the following template:

template<class T>void luaK_pushiter(lua_State *l, T begin, T end,
 boost::function<void (lua_State *, boost::shared_ptr<T>)> wrapfunc) {

this class:

namespace DB {
class ProblemIterator
{

and this code:

void pushProblemIterator(lua_State *l, boost::shared_ptr<DB::ProblemIterator> pi) {
    lua_pushstring(l, (*pi)->name().c_str());
}

static int probIterator(lua_State *l) {
    luaK_pushiter(l, DB::ProblemIterator(""), DB::ProblemIterator(),
     pushProblemIterator);
    return 1;
}

Why can's T be deduced? i.e. why do I get this error?

localpdb_capi.cc: In function ‘int probIterator(lua_State*)’:
localpdb_capi.cc:74: error: no matching function for call to ‘luaK_pushiter(lua_State*&, DB::ProblemIterator, DB::ProblemIterator, void (&)(lua_State*, boost::shared_ptr<DB::ProblemIterator>))’

Whereas, if I specify the template, it works fine

static int probIterator(lua_State *l) {
    luaK_pushiter<DB::ProblemIterator>(l, DB::ProblemIterator(""), DB::ProblemIterator(),
     pushProblemIterator);
    return 1;        
}

I think it's got to do with the fact that pushProblemIterator isn't a boost::function<>, but I don't fully understand what's going on.

Thanks.

[Cubbi]

boost::function (and std::function) is a type-erasing class: there is no direct connection between the type of std::function and the type of the object you're constructing it from.

Normal practice is to template on the functor type

template<class T, class F>
void luaK_pushiter( lua_State *l, T begin, T end, F wrapFunc )
{

[kev82]

Templating on the Functor type as you suggest works fine, but I still don't really understand what's going on. Do you know of a simple-ish article on argument deduction?

[Cubbi]

To put it simple, implicit conversions are not considered during deduction. They are considered during overload resolution, but to get there, the compiler needs to generate the list of overloads first.