Creating a template inner lambda function?

Istvan Orban Published at Dev

Istvan Orban

I been trying to solve the next functional problem in C++:

template <typename T>
auto createFunction(T& ClosuredData) {
    template <typename U>
    return [&](function<void(T&, U&)> Callback) {
        return [&](U& DataFromCallback) {
            Callback(ClosuredData, DataFromCallback);
        };
    };
};

// example use
struct Person {
   string name{"Foo"};
   int age = 20;
}
Person foo{};
auto mutate = createFunction(foo);
auto changeAge = mutate<int>([](Person& person, int& age) {
    person.age = age;
})

changeAge(20)

So basically a template Function creator with an inner template lambda. But of course as far as I know inner templates are not possible in C++. I tried to make the callback as an auto type, but of course that way I can't infer the DataFromCallback resulting in false type-safety.

Can I somehow achieve a proper type safety without introducing a class/struct?

Edit: One of the "solution" I seen is using an auto type instead of a functional:

template <typename T>
auto CreateFunction(T& ClosuredData) {
    return [&](auto Callback) {
        return [&, Callback](auto DataFromCallback) {
            Callback(ClosuredData, DataFromCallback);
        };
    };
};

This is really cool, but gives a false sense of typesafety, for example:

{
    Person person{};
    auto personMutator = CreateFunction(person);
    auto changeAge = personMutator([](Person& person, int age) {
        person.age = age;
    });

    changeAge(""); // Would not throw an error in this line, but at an upper stack

    changeAge(any); // same stuff
}

user7860670

It seems that you could use generic lambdas available since C++14 (which basically work as inner templates):

template <typename T>
auto CreateFunction(T& ClosuredData) {
    return [&](auto Callback) {
        return [&, Callback](auto DataFromCallback) {
            Callback(ClosuredData, DataFromCallback);
        };
    };
};

Collected from the Internet

Please contact [email protected] to delete if infringement.