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

Determining Parameter and Return Types of Lambda Expressions

Given a lambda expression, it can be challenging to ascertain its parameter and return types. This article explores how to extract this information using lambda traits.

In C 11, lambda expressions are introduced and can be used in various scenarios. One potential use case is to pass lambdas as arguments to function templates, but to successfully do this, knowing the lambda's parameter and return types is crucial.

Initially, attempts to use std::function to represent lambda expressions encountered errors. However, the introduction of lambda_traits provides a more robust solution. Using the decltype of the lambda's operator(), lambda_traits can determine the parameter types.

The following code snippet illustrates how lambda traits can be utilized to extract type information from a lambda expression:

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
}

In this example scenario, the f function template accepts a lambda as an argument and utilizes lambda traits to deduce its parameter and return types.

For generic lambdas that don't specify explicit types, like [](auto x) {}, this approach won't work. Nonetheless, lambda traits prove effective for lambdas with defined parameter and return types.

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