Is it possible to use anonymous classes in C++?

Question

I have seen anonymous classes in C++ code on Quora. It's successfully compiled and run.

Code here:

#include <iostream>  auto func() {     class // no name     {       public:              int val;     } a;      a.val = 5;      return a; }  int main() {     std::cout << func().val << std::endl;     return 0; } 

So, Is it valid in C++?

Also, I am curious to know, Is it possible to use anonymous classes in C++?

Answer 1

Not only that, you can create more instances of the class by using decltype.

#include <iostream>  class  {    public:       int val; } a;   int main() {    decltype(a) b;    a.val = 10;    b.val = 20;     std::cout << a.val << std::endl;    std::cout << b.val << std::endl;    return 0; } 

Output:

10 20 

Answer 2

In C++, an anonymous union is a union of this form:

 union { ... } ; 

It defines an unnamed object of an unnamed type. Its members are injected in the surrounding scope, so one can refer to them without using an <object>. prefix that otherwise would be necessary.

In this sense, no anonymous classes (that are not unions --- in C++ unions are classes) exist.

On the other hand, unnamed classes (including structs and unions) are nothing unusual.

union { ... } x; class { ... } y; typedef struct { ... } z; 

x and y are named object of unnamed types. z is a typedef-name that is an alias for an unnamed struct. They are not called anonymous because this term is reserved for the above form of a union.

[](){} 

Lambdas are unnamed objects of unnamed class types, but they are not called anonymous either.