Using std::function
, we can get the type of an argument using the argument_type
, second_argument_type
etc. typedefs, but I can't see a way to do the same thing with lambdas. Is it possible? (I'm using VS2010)
Say I want something like the following in my deserialization system used to read an object and pass it to a setter function:
template<typename F> static void forward(F f) { // Create an object of the type of the first // parameter to the function object F typedef typename F::argument_type T; T t; //...do something with 't' here (deserialize in my case) // Forward the object to the function f(t); }
It can be used like this and everything works fine:
std::function<void(int)> f = [](int i) -> void { setValue(i); }; forward(f);
But it will not work directly with lambdas:
forward([](int i) -> void { setValue(i); }); //error C2039: 'argument_type' : is not a //member of '`anonymous-namespace'::<lambda1>'
Is there a way to access the parameter types in a way that will work for both lambdas and std::function
objects? Maybe a way to get the std::function
type of a lambda first, and then the argument_type
from that?
Following on from the answer below, a version that works with lambdas and std::function
is:
template<typename T, typename F> static void forward(F f) { T t; //...do something with 't' here (deserialize in my case) f(t); } forward<int>([](int i) -> void { setValue(i); });
Since int
is repeated here I was hoping to get rid of it - not so bad for int
but more annoying for long-named types in a couple of namespaces. C'est la vie!
The return type of a lambda expression is automatically deduced. You don't have to use the auto keyword unless you specify a trailing-return-type. The trailing-return-type resembles the return-type part of an ordinary function or member function.
A lambda function is a stateless instantatiation of a function interface because the lambda expression syntax does not allow for the declaration of instance variables (fields). Therefore, there is no statically-defined data and no other methods to be accessed, negating the need for a self-reference.
A lambda expression can refer to identifiers declared outside the lambda expression. If the identifier is a local variable or a reference with automatic storage duration, it is an up-level reference and must be "captured" by the lambda expression.
Calls of the lambda are translated to direct calls to its operator() and can therefore be inlined.
It's not desirable in the general case. (Note that it's quite easy for std::function<T(A)>
to specify what e.g. argument_type
is: it's just A
! It's available in the type definition.)
It would be possible to require each and every function object type to specify its argument types, and in turn mandate that the closure types generated from lambda expression do so. In fact, pre-C++0x features like adaptable functors would only work for such types.
However, we're moving from that with C++0x and for good reasons. The simplest of which is simply overloading: a functor type with a templated operator()
(a.k.a a polymorphic functor) simply takes all kind of arguments; so what should argument_type
be? Another reason is that generic code (usually) attempts to specify the least constraints on the types and objects it operates on in order to more easily be (re)used.
In other words, generic code is not really interested that given Functor f
, typename Functor::argument
be int
. It's much more interesting to know that f(0)
is an acceptable expression. For this C++0x gives tools such as decltype
and std::declval
(conveniently packaging the two inside std::result_of
).
The way I see it you have two choices: require that all functors passed to your template use a C++03-style convention of specifying an argument_type
and the like; use the technique below; or redesign. I'd recommend the last option but it's your call since I don't know what your codebase looks like or what your requirements are.
For a monomorphic functor type (i.e. no overloading), it is possible to inspect the operator()
member. This works for the closure types of lambda expressions.
So we declare these helpers
template<typename F, typename Ret, typename A, typename... Rest> A helper(Ret (F::*)(A, Rest...)); template<typename F, typename Ret, typename A, typename... Rest> A helper(Ret (F::*)(A, Rest...) const); // volatile or lvalue/rvalue *this not required for lambdas (phew)
that accept a pointer to member function taking at least one argument. And now:
template<typename F> struct first_argument { typedef decltype( helper(&F::operator()) ) type; };
[ an elaborate trait could successively query the lvalue-rvalue/const/volatile overloads and expose the first argument if it's the same for all overloads, or use std::common_type
.]
@Luc's answer is great but I just came across a case where I also needed to deal with function pointers:
template<typename Ret, typename Arg, typename... Rest> Arg first_argument_helper(Ret(*) (Arg, Rest...)); template<typename Ret, typename F, typename Arg, typename... Rest> Arg first_argument_helper(Ret(F::*) (Arg, Rest...)); template<typename Ret, typename F, typename Arg, typename... Rest> Arg first_argument_helper(Ret(F::*) (Arg, Rest...) const); template <typename F> decltype(first_argument_helper(&F::operator())) first_argument_helper(F); template <typename T> using first_argument = decltype(first_argument_helper(std::declval<T>()));
This can be used on both functors and function pointers:
void function(float); struct functor { void operator() (int); }; int main() { std::cout << std::is_same<first_argument<functor>, int>::value << ", " << std::is_same<first_argument<decltype(&function)>, int>::value << std::endl; return 0; }
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