How Do C 14 Generic Lambdas Achieve Type Flexibility Using the `auto` Keyword?

Understanding Generic Lambdas in C 14 Variant of Auto Keyword

In C 14, generic lambdas, a type of lambda expression with auto keyword as an argument type, provide enhanced flexibility. Contrary to C 11 lambdas with a non-template call operator, generic lambdas have a templated call operator within the closure type they define.

For example, the following code demonstrates a generic lambda:

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

In this case, the closure type of glambda will be defined as:

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

This means that glambda is an instance of a unique, unnamed functor with a templated call operator. Each occurrence of auto in the lambda's parameter declaration corresponds to an invented type template parameter, allowing the call operator to handle arguments of different types.

The C 14 standard (n3690) specifies that the call operator of the closure type for a generic lambda has a template-parameter-list with one invented type template-parameter for each auto in the lambda's parameter-declaration-clause. The return type and function parameters are derived from the lambda's trailing-return-type and parameter-declaration-clause with auto replaced by the name of the corresponding invented template-parameter.

In summary, generic lambdas in C 14 represent unique, unnamed functors with templated call operators. This differs from C template-based polymorphism, where the compiler generates new functions with replaced types for each argument type. It is more closely aligned with Java's generics, which involve type erasure during compilation.

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