Unnamed struct is allowed (type has no name)

void foo()
{
    struct /* No name */ {
        float x;
        float y;
    } point;
    
    point.x = 42;
}

or

struct Circle
{
    struct /* No name */ {
        float x;
        float y;
    } center; // but a member name
    float radius;
};

and later

Circle circle;
circle.center.x = 42.f;

but NOT anonymous struct (unnamed type and unnamed object)

struct InvalidCircle
{
    struct /* No name */ {
        float centerX;
        float centerY;
    }; // No member either.
    float radius;
};

Note: Some compilers allow anonymous struct as extension.

• lamdba can be seen as a special unnamed struct.

• decltype allows to retrieve the type of unnamed struct:

  decltype(circle.point) otherPoint;
  

• unnamed struct instance can be parameter of template method:

  void print_square_coordinates()
  {
      const struct {float x; float y;} points[] = {
          {-1, -1}, {-1, 1}, {1, -1}, {1, 1}
      };
  
      // for range relies on `template <class T, std::size_t N> std::begin(T (&)[N])`
      for (const auto& point : points) { 
          std::cout << "{" << point.x << ", " << point.y << "}\\n";
      }
  
      decltype(points[0]) topRightCorner{1, 1};
      auto it = std::find(points, points + 4, topRightCorner);
      std::cout << "top right corner is the "
                << 1 + std::distance(points, it) << "th\\n";
  }