Why the Correct Syntax Works

Oct 13, 2025 by windailys.com 23 views

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 throw an exception or terminate the program. However, when it comes to friend functions, declaring them with the [[noreturn]] attribute can be tricky. In this article, we will explore the correct syntax for declaring a friend function with the [[noreturn]] attribute.

Friend Functions and the [[noreturn]] Attribute

A friend function is a non-member function that has access to the private and protected members of a class. To declare a friend function, we use the friend keyword followed by the function declaration. However, when we try to declare a friend function with the [[noreturn]] attribute, we encounter a problem.

Incorrect Syntax

Let's take a look at the incorrect syntax:

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

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

As we can see, the [[noreturn]] attribute is applied to the friend function declaration inside the class definition. However, this is not the correct way to declare a friend function with the [[noreturn]] attribute.

Correct Syntax

The correct syntax for declaring a friend function with the [[noreturn]] attribute is to apply the attribute to the function definition outside the class definition. Here's the corrected code:

struct a
{
    friend int b();
};

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

In this corrected code, we declare the friend function b() inside the class definition without the [[noreturn]] attribute. Then, we define the function b() outside the class definition with the [[noreturn]] attribute.

Why the Correct Syntax Works

The correct syntax works because the [[noreturn]] attribute is a function attribute, not a class attribute. When we apply the [[noreturn]] attribute to a function definition, it indicates that the function does not return. However, when we apply it to a friend function declaration inside a class definition, it is not clear whether the attribute applies to the function definition or the friend function declaration.

By applying the [[noreturn]] attribute to the function definition outside the class definition, we ensure that the attribute is applied to the correct entity, which is the function definition.

Example Use Case

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

struct a
{
    friend int b();
};

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

int main()
{
    try
    {
        b();
    }
    catch (int e)
    {
        std::cerr << "Caught exception: " << e << std::endl;
    }
    return 0;
}

In this example, we declare a friend function b() inside the class definition a. Then, we define the function b() outside the class definition with the [[noreturn]] attribute. In the main() function, we call the function b() and catch the exception that is thrown.

Conclusion

In conclusion, declaring a friend function with the [[noreturn]] attribute requires careful syntax. By applying the attribute to the function definition outside the class definition, we ensure that the attribute is applied to the correct entity, which is the function definition. This corrected syntax is essential for using the [[noreturn]] attribute with friend functions in C++17.

Best Practices

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

Always apply the [[noreturn]] attribute to the function definition outside the class definition.
Use the friend keyword to declare the friend function inside the class definition.
Define the friend function outside the class definition with the [[noreturn]] attribute.
Use the try-catch block to catch exceptions thrown by the friend function.

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

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

Q: Can I apply the [[noreturn]] attribute to a friend function declaration inside a class definition?

A: No, you cannot apply the [[noreturn]] attribute to a friend function declaration inside a class definition. The correct syntax is to apply the attribute to the function definition outside the class definition.

Q: Why can't I apply the [[noreturn]] attribute to a friend function declaration inside a class definition?

A: The [[noreturn]] attribute is a function attribute, not a class attribute. When you apply it to a friend function declaration inside a class definition, it is not clear whether the attribute applies to the function definition or the friend function declaration.

Q: What is the correct syntax for declaring a friend function with the [[noreturn]] attribute?

A: The correct syntax is to declare the friend function inside the class definition without the [[noreturn]] attribute, and then define the function outside the class definition with the [[noreturn]] attribute.

Q: Can I use the [[noreturn]] attribute with other function attributes?

A: Yes, you can use the [[noreturn]] attribute with other function attributes. For example, you can use it with the [[noreturn]] attribute to indicate that a function does not return and also has a specific calling convention.

Q: How do I use the [[noreturn]] attribute with a friend function that throws an exception?

A: You can use the [[noreturn]] attribute with a friend function that throws an exception by applying the attribute to the function definition outside the class definition. For example:

struct a
{
    friend int b();
};

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

In this example, the friend function b() throws an exception and is declared with the [[noreturn]] attribute.

Q: Can I use the [[noreturn]] attribute with a friend function that terminates the program?

A: Yes, you can use the [[noreturn]] attribute with a friend function that terminates the program. For example:

struct a
{
    friend int b();
};

[[noreturn]] int b(){ std::exit(1); }

In this example, the friend function b() terminates the program using the std::exit() function and is declared with the [[noreturn]] attribute.

Q: What are some best practices for using the [[noreturn]] attribute with friend functions?

A: Some best practices for using the [[noreturn]] attribute with friend functions include:

Always apply the [[noreturn]] attribute to the function definition outside the class definition.
Use the friend keyword to declare the friend function inside the class definition.
Define the friend function outside the class definition with the [[noreturn]] attribute.
Use the try-catch block to catch exceptions thrown by the friend function.

By following these best practices, you can ensure that your friend functions with the [[noreturn]] attribute are declared and defined correctly.