Mastering Function Declarations in C++: Returning Pointers to Arrays

Introduction

In C++, returning an array from a function is impossible because arrays cannot be copied. However, it’s possible to return a pointer or a reference to an array. The syntax for doing so can be quite tricky and intimidating. Luckily, C++ provides several ways to simplify these declarations, making it easier to work with pointers to arrays.

Let's dive into how to declare functions that return pointers to arrays and how modern C++ features like typedef, using, trailing return types, and decltype can simplify this process.

1. Using Type Aliases to Simplify Declarations

One of the simplest ways to declare a function that returns a pointer to an array is by using type aliases. These aliases allow you to create more readable and manageable code, especially when working with complex types like arrays.

Example with typedef and using:

typedef int arrT[10];   // arrT is a synonym for an array of ten ints

using arrT = int[10];   // equivalent declaration of arrT

arrT* func(int i);      // func returns a pointer to an array of ten ints        

Here, arrT is a synonym for an array of ten integers. Since we cannot return an array directly, we define the return type as a pointer to that array (arrT*). The function func takes an integer as an argument and returns a pointer to an array of 10 integers.

2. Declaring a Function That Returns a Pointer to an Array Without Aliases

If you prefer not to use aliases, you must carefully place the array’s dimension after the function name and parameter list, as array dimensions in C++ follow the name being defined.

Understanding the Syntax:

int arr[10];            // arr is an array of ten ints

int *p1[10];            // p1 is an array of ten pointers to ints

int (*p2)[10] = &arr;   // p2 is a pointer to an array of ten ints        

When declaring a function that returns a pointer to an array, the function’s return type and parameter list must be enclosed in parentheses to ensure the correct type is returned.

Example Without Type Aliases:

int (*func(int i))[10];  // func returns a pointer to an array of ten ints        

Breaking Down the Declaration:

• func(int) indicates that func accepts an int as a parameter.
• (*func(int)) indicates that the result of func(int) is dereferenceable (i.e., a pointer).
• (*func(int))[10] tells us that dereferencing this result yields an array of 10 integers.
• int (*func(int))[10] specifies that the array's element type is int.

3. Using a Trailing Return Type

C++11 introduced trailing return types to simplify complex function declarations. This feature is especially useful for functions that return pointers to arrays, where the return type can be difficult to understand if placed before the function’s name.

Example Using Trailing Return Type:

auto func(int i) -> int(*)[10];  // func takes an int and returns a pointer to an array of ten ints        

Here’s how it works:

• The auto keyword indicates that the return type will be defined after the parameter list.
• The trailing return type (-> int(*)[10]) shows that func returns a pointer to an array of 10 integers.

This approach clarifies the function signature, making it easier to read and maintain.

4. Using decltype to Infer Return Types

Another modern approach is to use decltype to declare the return type. decltype can be particularly useful when a function returns a pointer to a known array, such as odd and even arrays in the example below.

Example Using decltype:

int odd[] = {1, 3, 5, 7, 9};

int even[] = {0, 2, 4, 6, 8};

decltype(odd) *arrPtr(int i) 
{
    return (i % 2) ? &odd : &even;  // returns a pointer to an array of five ints
}        

In this example, decltype(odd) infers the return type from the odd array. Since odd is an array, the function arrPtr returns a pointer to that array. However, it's crucial to remember that decltype doesn’t automatically convert an array to a pointer type. Therefore, we must explicitly indicate that arrPtr returns a pointer by adding * to decltype(odd).

Open Question:

What are the benefits of using this function declaration int (*func(int x))[10]; over this simpler declaration int *func(int x); when we want to return a pointer to an array?

This is an important question to consider because both declarations involve returning a pointer.

Conclusion

Returning a pointer to an array may initially seem complex due to the complex syntax. However, using tools like type aliases, trailing return types, and decltype, C++ developers can make their code more readable and manageable.

By mastering these techniques, you'll not only write cleaner and more efficient code but also gain a deeper understanding of how function declarations work in C++. Whether you're working with fixed-size arrays, dynamic memory, or complex data structures, these features are invaluable in simplifying the return types of your functions.

Key Takeaways:

- Use type aliases (typedef or using) to simplify complex return types.

- Trailing return types (auto + ->) can make function declarations much clearer.

- decltype can automatically infer the return type, reducing manual type specification.

Understanding these techniques will enhance your ability to write more maintainable C++ code, especially when dealing with pointers to arrays.

References:

• C++ Primer, 5th Edition, Section 6.3.3, page 228