Why does C++ disallow anonymous structs?

by

As others have pointed out anonymous unions are permitted in standard C++, but anonymous structs are not.

The reason for this is that C supports anonymous unions but not anonymous structs*, so C++ supports the former for compatibility but not the latter because it’s not needed for compatibility.

Furthermore, there’s not much use to anonymous structs in C++. The use you demonstrate, to have a struct containing three floats which can be referred to either by .v[i], or .x, .y, and .z, I believe results in undefined behavior in C++. C++ does not allow you to write to one member of a union, say .v[1], and then read from another member, say .y. Although code that does this is not uncommon it is not actually well defined.

C++’s facilities for user-defined types provide alternative solutions. For example:

struct vector3 {
  float v[3];
  float &operator[] (int i) { return v[i]; }
  float &x() { return v[0]; }
  float &y() { return v[1]; }
  float &z() { return v[2]; }
};

* C11 apparently adds anonymous structs, so a future revision to C++ may add them.

More Related Contents:

  1. How to compile C code with anonymous structs / unions?
  2. union ‘punning’ structs w/ “common initial sequence”: Why does C (99+), but not C++, stipulate a ‘visible declaration of the union type’?
  3. How do I compare chars (or strings) using void functions, also comparing chars that were taken from a struct array
  4. what does this error mean in general? and how I fix it in this case? [closed]
  5. Why isn’t sizeof for a struct equal to the sum of sizeof of each member?
  6. Accessing inactive union member and undefined behavior?
  7. Difference between ‘struct’ and ‘typedef struct’ in C++?
  8. When should you use a class vs a struct in C++?
  9. Unions and type-punning
  10. Purpose of Unions in C and C++
  11. Difference between a Structure and a Union
  12. How to use a C++ string in a structure when malloc()-ing the same structure?
  13. sizeof a union in C/C++
  14. memset() or value initialization to zero out a struct?
  15. Structure of a C++ Object in Memory Vs a Struct
  16. using struct keyword in variable declaration in C++
  17. Struct initialization of the C/C++ programming language?
  18. How to read / write a struct in Binary Files?
  19. What does “request for member ‘*******’ in something not a structure or union” mean?
  20. What is the difference between using a struct with two fields and a pair?
  21. What happened to the “aggregate or union type that includes one of the aforementioned types” strict aliasing rule?
  22. Force C++ structure to pack tightly
  23. Mixing class and struct
  24. Access struct members as if they are a single array?
  25. Is it safe to return a struct in C or C++?
  26. how to assign multiple values into a struct at once?
  27. Initializing an object to all zeroes
  28. Purpose of struct, typedef struct, in C++
  29. Variable Sized Struct C++
  30. memcpy/memmove to a union member, does this set the ‘active’ member?

Leave a Comment