How Can We Determine the Parameter and Return Types of a C Lambda Function?

Discovering Parameter and Return Types of Lambdas

Question

Can we determine the parameter and return types of a lambda function in C ? If so, how?

This inquiry is driven by the need to know the types within a function template that takes a lambda as an argument.

Answer

The solution lies in employing function_traits, as demonstrated by this code:

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

template <typename ClassType, typename ReturnType, typename... Args>
struct function_traits<ReturnType(ClassType::*)(Args...) const>
{
    enum { arity = sizeof...(Args) };
    typedef ReturnType result_type;

    template <size_t i>
    struct arg
    {
        typedef typename std::tuple_element<i, std::tuple<Args...>>::type type;
    };
};

// Example usage
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 implementation leverages the decltype of the lambda's operator() to derive its parameter types.

Notable Limitation

It's important to note that this technique is ineffective with generic lambdas like [](auto x) {}.

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