Benefits of the [[nodiscard]] Attribute

The [[nodiscard]] attribute is a powerful feature in C++ that can help prevent subtle bugs and improve code clarity. Let's explore why it's beneficial, especially for getter functions in our Image class.

Purpose of [[nodiscard]]

The [[nodiscard]] attribute tells the compiler to issue a warning if the return value of a function is ignored. This is particularly useful for functions where discarding the return value is likely to be a mistake.

Why Use It for Getters?

Getter functions are designed to return information, and it's rarely intentional to call them without using their return value. By marking getters as [[nodiscard]], we can catch potential mistakes where a developer might accidentally ignore the returned value.

Let's look at an example using our Image class:

1class Image {
2 public:
3  [[nodiscard]]
4  int GetWidth() const {
5    return mImageSurface->w;
6  }
7
8  [[nodiscard]]
9  int GetHeight() const {
10    return mImageSurface->h;
11  }
12
13  [[nodiscard]]
14  ScalingMode GetScalingMode() const {
15    return mScalingMode;
16  }
17
18  void SetDimensions(int Width, int Height) {
19    // Implementation...
20  }
21};
22
23int main() {
24  Image MyImage{"example.png"};
25
26  MyImage.GetWidth();  
27
28  // This is fine
29  int Width = MyImage.GetWidth(); 
30
31  // Also fine
32  MyImage.SetDimensions(
33    MyImage.GetWidth(), MyImage.GetHeight()
34  );
35}

1warning: ignoring return value of function declared with 'nodiscard' attribute

In this example, the compiler will warn us about the line where we call GetWidth() without using its return value. This could indicate a logical error in our code.

Preventing Logical Errors

Consider a scenario where we want to check if an image's dimensions have changed:

1bool HasImageChanged(
2  const Image& OldImage, const Image& NewImage
3) {
4  OldImage.GetWidth();   
5  OldImage.GetHeight();  
6  NewImage.GetWidth();   
7  NewImage.GetHeight();  
8
9  return true;  // Always returns true!
10}

Without [[nodiscard]], this function would compile without warnings, even though it's not actually comparing anything. With [[nodiscard]], we get warnings that help us catch this mistake:

1warning: ignoring return value of function declared with 'nodiscard' attribute
2warning: ignoring return value of function declared with 'nodiscard' attribute
3warning: ignoring return value of function declared with 'nodiscard' attribute
4warning: ignoring return value of function declared with 'nodiscard' attribute

Improved Code Clarity

Using [[nodiscard]] also serves as a form of self-documentation. It clearly communicates to other developers (or your future self) that the return value of this function is important and should be used.

When Not to Use [[nodiscard]]

While [[nodiscard]] is great for getters, it's not appropriate for all functions. For example, a function like Render() in our Image class might not need [[nodiscard]] because it's normal to call it for its side effects without caring about a return value.

In conclusion, using [[nodiscard]] for getter functions in our Image class helps catch potential bugs, improves code clarity, and communicates intent to other developers. It's a simple addition that can significantly enhance the robustness of our code.