How Can I Determine the Parameter and Return Types of a C Lambda at Compile Time?

Determining the Parameter and Return Types of a Lambda

Given a lambda, it is possible to extract its parameter type and return type using compile-time techniques. This can be achieved through the use of lambda traits, which provide a way to access information about lambdas at compile time.

To define lambda traits, we can leverage the decltype operator to inspect the signature of the lambda's operator(). By specializing a template on this type, we can retrieve the parameter and return types.

For example, the following code implements lambda traits:

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

template<typename ReturnType, typename... Args>
struct lambda_traits<ReturnType(Args...)> {
    using param_type = Args...;
    using return_type = ReturnType;
};

With these traits, we can retrieve the parameter and return types of a lambda 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

This approach allows us to dynamically construct std::function objects from lambdas, as demonstrated in the following code:

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; // Construct the std::function object
}

Note that this approach fails for generic lambdas, such as [](auto x) {}, due to the inability to determine the exact types at compile time.

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