How Can I Retrieve the Parameter and Return Types of a Lambda Expression in C ?

Retrieving Parameter and Return Types of Lambdas

In lambda expressions, determining parameter and return types can be essential for functions accepting lambdas as arguments. Let's explore the feasibility of this task and the approach to achieve it.

Lambda Traits

To obtain the parameter and return types of a lambda, it is proposed to define a lambda_traits template that can be used as follows:

auto lambda = [](int i) { return long(i*10); };

lambda_traits<decltype(lambda)>::param_type i; //i should be int
lambda_traits<decltype(lambda)>::return_type l; //l should be long

By leveraging lambda_traits, functions can introspect the parameter and return types of lambdas received as arguments, enabling advanced functionality like:

template<typename TLambda>
void f(TLambda lambda)
{
   typedef typename lambda_traits<TLambda>::param_type  P;
   typedef typename lambda_traits<TLambda>::return_type R;

   std::function<R(P)> fun = lambda; //I want to do this!
   //...
}

Using decltype to Introspect Parameter Types

Fortunately, it is possible to retrieve parameter types using the decltype of the lambda's operator(). This technique is elegantly demonstrated in the following function_traits implementation:

template <typename T>
struct function_traits
    : public function_traits<decltype(&amp;T::operator())>
{};

For generic lambdas, function_traits directly uses the signature of their operator(). For member function pointers, it specializes and provides arity, result_type, and argument types.

Example Usage

Consider the following example:

int main()
{
    auto lambda = [](int i) { return long(i*10); };

    typedef function_traits<decltype(lambda)> traits;

    static_assert(std::is_same<long, traits::result_type>::value, "err");
    static_assert(std::is_same<int, traits::arg<0>::type>::value, "err");

    return 0;
}

This example illustrates how to utilize function_traits to verify the expected parameter and return types of a lambda. Note that this approach may not work for generic lambdas like [](auto x) {}.

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