How Do C 14 Generic Lambdas Achieve Type Deduction Without Traditional Templates?

Generic Lambdas in C 14: A Deeper Dive

The advent of generic lambdas in C 14 introduced a novel feature to lambda expressions, allowing for functions with automatically deduced argument types. However, the working mechanism of these generics has been subject to speculation.

Contrary to initial assumptions, generic lambdas do not rely on C templates for their implementation. Instead, they follow a unique approach involving a templated call operator for the closure type of the lambda.

For example, the generic lambda expression:

auto glambda = [](auto a) { return a; };

produces a closure type with the following templated call operator:

class /* unnamed */
{
public:
    template <typename T>
    T operator () (T a) const { return a; }
};

The call operator is templated based on an invented type template-parameter for each instance of auto in the lambda's parameter declaration. These parameters enable the deduced type to vary for each invocation of the lambda.

This templated approach differs from C templates, which generate new functions for each unique argument type. Instead, it operates more like Java's generics, where the underlying type erasure allows for efficient implementation without requiring specialized function generations.

In summary, generic lambdas in C 14 leverage a templated call operator rather than traditional C templates. This design choice strikes a balance between expressive power and optimization, providing a convenient means of working with functions with automatically deduced argument types.

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