Avoid Hardcoding Values for Better Libraries

Naming constants is good practice, but they are still hardcoded values that can make library code less reusable and less predictable.

Bad

constexpr int kThumbnailSizes[] = {480, 576, 720};

// Returns thumbnails of various sizes for the given image.
std::vector<Image> GetThumbnails(const Image& image) {
  std::vector<Image> thumbnails;
  for (const int size : kThumbnailSizes) {
    thumbnails.push_back(ResizeImage(image, size));
  }
  return thumbnails;
}

Instead, let the caller decide.

Good

// Returns thumbnails of various sizes for the given image.
std::vector<Image> GetThumbnails(const Image& image,
                                 absl::Span<const int> sizes) {
  std::vector<Image> thumbnails;
  for (const int size : sizes) {
    thumbnails.push_back(ResizeImage(image, size));
  }
  return thumbnails;
}

// Declared in the public header.
inline constexpr int kDefaultThumbnailSizes[] = {480, 576, 720};

// Default argument allows the function to be used without specifying a size.
std::vector<Image> GetThumbnails(
    const Image& image, absl::Span<const int> sizes = kDefaultThumbnailSizes);