[[noreturn]] Attribute On Friend Functios

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Introduction

In C++17, the [[noreturn]] attribute was introduced to indicate that a function does not return. This attribute is useful for functions that never return, such as those that terminate the program or enter an infinite loop. However, when it comes to friend functions, the syntax can be a bit tricky. In this article, we will explore how to declare a friend function with the [[noreturn]] attribute and provide a solution to the common issue of failing to compile.

Friend Functions in C++

Before we dive into the [[noreturn]] attribute, let's quickly review what friend functions are. In C++, a friend function is a non-member function that has access to the private and protected members of a class. Friend functions are declared using the friend keyword followed by the function declaration.

Declaring Friend Functions with [[noreturn]]

To declare a friend function with the [[noreturn]] attribute, we need to use the following syntax:

struct a {
    [[noreturn]] friend int b();
};

However, as you've encountered, this syntax is not enough to make the friend function [[noreturn]]. The reason is that the [[noreturn]] attribute is applied to the function declaration, not the function definition.

The Correct Syntax

To fix this issue, we need to apply the [[noreturn]] attribute to the function definition, not the declaration. Here's the corrected code:

struct a {
    friend int b();
};

[[noreturn]] int a::b() { throw 42; }

In this code, we first declare the friend function b() without the [[noreturn]] attribute. Then, we define the function b() with the [[noreturn]] attribute applied to it. Note that we use the scope resolution operator :: to specify that the function b() is a member of the class a.

Why the Syntax Matters

So, why does the syntax matter? The reason is that the [[noreturn]] attribute is a function attribute, not a class attribute. It's applied to the function definition, not the function declaration. By applying the attribute to the function definition, we ensure that the compiler knows that the function does not return.

Example Use Case

Here's an example use case for the [[noreturn]] attribute on a friend function:

struct a {
    friend int b();
};

[[noreturn]] int a::b() {
    // Simulate an infinite loop
    while (true) {}
}

In this example, the function b() is declared as a friend function of the class a. The [[noreturn]] attribute is applied to the function definition, indicating that the function does not return. This is useful for functions that never return, such as those that enter an infinite loop.

Conclusion

In conclusion, declaring a friend function with the [[noreturn]] attribute requires a specific syntax. By applying the attribute to the function definition, we ensure that the compiler knows that the function does not return. This is useful for functions that never return, such as those that terminate the program or enter an infinite loop. By following the correct syntax, we can write more robust and efficient code.

Best Practices

Here are some best practices to keep in mind when using the [[noreturn]] attribute on friend functions:

  • Always apply the [[noreturn]] attribute to the function definition, not the declaration.
  • Use the scope resolution operator :: to specify that the function is a member of the class.
  • Make sure to declare the friend function without the [[noreturn]] attribute before defining it with the attribute.
  • Use the [[noreturn]] attribute to indicate that a function does not return, such as those that terminate the program or enter an infinite loop.

Introduction

In our previous article, we explored how to declare a friend function with the [[noreturn]] attribute in C++17. However, we know that there are still many questions and concerns about this topic. In this article, we will address some of the most frequently asked questions about the [[noreturn]] attribute on friend functions.

Q: What is the purpose of the [[noreturn]] attribute?

A: The [[noreturn]] attribute is used to indicate that a function does not return. This is useful for functions that never return, such as those that terminate the program or enter an infinite loop.

Q: How do I declare a friend function with the [[noreturn]] attribute?

A: To declare a friend function with the [[noreturn]] attribute, you need to use the following syntax:

struct a {
    friend int b();
};

[[noreturn]] int a::b() { throw 42; }

Note that you need to apply the [[noreturn]] attribute to the function definition, not the declaration.

Q: Why do I need to use the scope resolution operator (::) when defining a friend function with the [[noreturn]] attribute?

A: The scope resolution operator :: is used to specify that the function is a member of the class. This is necessary because the [[noreturn]] attribute is applied to the function definition, not the declaration.

Q: Can I use the [[noreturn]] attribute on a non-friend function?

A: Yes, you can use the [[noreturn]] attribute on a non-friend function. The [[noreturn]] attribute is a function attribute, not a class attribute, so it can be applied to any function.

Q: What happens if I forget to apply the [[noreturn]] attribute to a friend function?

A: If you forget to apply the [[noreturn]] attribute to a friend function, the compiler will not generate any warnings or errors. However, the function will still be compiled and linked as usual.

Q: Can I use the [[noreturn]] attribute on a function that returns a value?

A: No, you cannot use the [[noreturn]] attribute on a function that returns a value. The [[noreturn]] attribute is used to indicate that a function does not return, so it cannot be applied to a function that returns a value.

Q: Are there any other attributes that I can use on friend functions?

A: Yes, there are several other attributes that you can use on friend functions, including:

  • [[noexcept]]: This attribute is used to indicate that a function does not throw any exceptions.
  • [[nodiscard]]: This attribute is used to indicate that a function's return value should not be discarded.
  • [[const]]: This attribute is used to indicate that a function does not modify the object it is called on.

Q: Can I use the [[noreturn]] attribute on a function that is marked as [[noexcept]]?

A: Yes, you can use the [[noreturn]] attribute on a function that is marked as [[noexcept]]. The [[noreturn]] attribute is used to indicate that a function does not return, while the [[noexcept]] attribute is used to indicate that a function does not throw any exceptions.

Conclusion

In conclusion, the [[noreturn]] attribute on friend functions is a powerful tool that can be used to indicate that a function does not return. By following the correct syntax and best practices, you can use this attribute to write more robust and efficient code. We hope that this Q&A article has helped to address some of the most frequently asked questions about the [[noreturn]] attribute on friend functions.