Unraveling the Mystery: How does the ODR interact with “unique” types that use a non-type template defaulted to a lambda expression?

As C++ developers, we’ve all been there – staring at a complex piece of code, scratching our heads, and wondering how on earth the ODR (One Definition Rule) interacts with those mysterious “unique” types that seem to defy the rules of traditional templating. Well, wonder no more! In this article, we’ll delve into the depths of C++ and explore the intricacies of how the ODR handles these special types.

What are “unique” types, anyway?

Before we dive into the ODR’s interactions, let’s take a step back and understand what makes these types so, well, unique. In C++, a “unique” type is a type that uses a non-type template defaulted to a lambda expression. This might sound like a mouthful, but it’s actually quite simple once you break it down.

A non-type template parameter is a template parameter that’s not a type, but rather a value or an expression. For example:

template <int N>
struct my_struct {
    static int value = N;
};

In this example, `N` is a non-type template parameter of type `int`. Now, let’s add a lambda expression to the mix:

template <auto LAMBDA>
struct my_struct {
    static auto value = LAMBDA();
};

Here, `LAMBDA` is a non-type template parameter that’s defaulted to a lambda expression. This is where things get interesting. When we use this template, we can pass in a lambda expression as an argument:

auto my_instance = my_struct<[]() { return 42; }>{};

This creates an instance of `my_struct` with `value` set to the result of calling the lambda expression, which is 42.

How does the ODR interact with these “unique” types?

Now that we have our “unique” type, let’s see how the ODR interacts with it. The ODR is a fundamental rule in C++ that states that each program shall contain exactly one definition of every non-inline function or object of static storage duration.

When we use our “unique” type, the ODR comes into play in a few ways. Let’s explore each scenario:

Scenario 1: Multiple definitions with the same lambda expression

What happens when we define multiple instances of our “unique” type with the same lambda expression?

auto instance1 = my_struct<[]() { return 42; }>{};
auto instance2 = my_struct<[]() { return 42; }>{};

In this case, the ODR ensures that there’s only one definition of `my_struct` with the given lambda expression. The compiler treats both instances as the same type, and the program is valid.

Scenario 2: Multiple definitions with different lambda expressions

What about when we define multiple instances with different lambda expressions?

auto instance1 = my_struct<[]() { return 42; }>{};
auto instance2 = my_struct<[]() { return 21; }>{};

In this scenario, the ODR allows multiple definitions of `my_struct` with different lambda expressions. Each instance is treated as a separate type, and the program is still valid.

Scenario 3: Mixing lambda expressions and other non-type template parameters

What happens when we mix lambda expressions with other non-type template parameters?

template <int N, auto LAMBDA>
struct my_struct {
    static auto value = LAMBDA();
};

auto instance1 = my_struct<42, []() { return 42; }>{};
auto instance2 = my_struct<21, []() { return 21; }>{};

In this case, the ODR ensures that each combination of non-type template parameters (including lambda expressions) results in a unique type. The program is valid, and each instance is treated as a separate type.

Key takeaways

Here are the key points to remember when working with “unique” types that use non-type templates defaulted to lambda expressions:

• The ODR ensures that each program shall contain exactly one definition of every non-inline function or object of static storage duration.
• Multiple definitions of the same “unique” type with the same lambda expression are treated as a single definition.
• Multiple definitions of the same “unique” type with different lambda expressions are treated as separate types.
• Mixing lambda expressions with other non-type template parameters results in unique types for each combination.

Conclusion

In conclusion, the ODR’s interaction with “unique” types that use non-type templates defaulted to lambda expressions can seem complex at first, but it’s actually quite straightforward once you understand the rules. By following the scenarios outlined in this article, you’ll be well-equipped to handle even the most complex templating situations. Remember to keep it simple, keep it unique, and always keep the ODR in mind!

Scenario	ODR Behavior
Multiple definitions with the same lambda expression	Treated as a single definition
Multiple definitions with different lambda expressions	Treated as separate types
Mixing lambda expressions with other non-type template parameters	Unique types for each combination

Happy coding, and may the ODR be with you!

Frequently Asked Question

Curious about how the ODR (ODR stands for “Opaque Debug Representation”) interacts with “unique” types that use a non-type template defaulted to a lambda expression? Let’s dive into the details!