How Do C 14 Generic Lambdas Achieve Type-Generic Behavior?

Generic Lambdas in C 14

In C 14, generic lambdas allow defining lambdas with auto as an argument type, enabling type-generic behavior. Understanding how they work sheds light on their underlying mechanisms.

Templated Call Operator

Generic lambdas define a closure type with a templated call operator rather than a non-template one like C 11 lambdas. This means that when auto appears in the parameter list, the compiler creates a function template for the call operator.

Example:

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

The above lambda creates an instance of the following type:

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

Type Erasure vs. Template Polymorphism

Generic lambdas are closer to Java's generics, which employ type erasure. Unlike C templates, which generate multiple functions with different types, generic lambdas create a single function with a type-erasing call operator.

Type Parameter List

The generic lambda's call operator template includes a template parameter list for each occurrence of auto in the parameter declaration. The type of each template parameter corresponds to the type of the corresponding variable.

Conclusion:

Generic lambdas in C 14 enable type-generic programming by defining lambda expressions with a templated call operator. This provides a concise way to create type-erasing functions similar to Java's generics.

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