Can unnamed structures inherit?

c++, inheritance, struct, unnamed-class

Solution

Unnamed classes can inherit. This is useful, for example, in situations when you must inherit in order to override a virtual function, but you never need more than one instance of the class, and you do not need to reference the derived type, because a reference to the base type is sufficient.

Here is an example:

#include <iostream>
using namespace std;

struct Base {virtual int process(int a, int b) = 0;};
static struct : Base {
    int process(int a, int b) { return a+b;}    
} add;
static struct : Base {
    int process(int a, int b) { return a-b;}    
} subtract;
static struct : Base {
    int process(int a, int b) { return a*b;}    
} multiply;
static struct : Base {
    int process(int a, int b) { return a/b;}    
} divide;

void perform(Base& op, int a, int b) {
    cout << "input: " << a << ", " << b << "; output: " << op.process(a, b) << endl;
}

int main() {
    perform(add, 2, 3);
    perform(subtract, 6, 1);
    perform(multiply, 6, 7);
    perform(divide, 72, 8);
    return 0;
}

This code creates four anonymous derivations of `Base` - one for each operation. When the instances of these derivations are passed to the `perform` function, the proper override is called. Note that `perform` does not need to know about any of the specific types - the base type with its virtual function is enough to complete the process.

Here is the output of running the above code:

input: 2, 3; output: 5
input: 6, 1; output: 5
input: 6, 7; output: 42
input: 72, 8; output: 9

Demo on ideone.

Problem

The following looks like a compilation error : ``` struct : Base { }; ``` Yet when used [1] it seems to work : ``` #include <iostream> using namespace std; template<bool B> struct A { struct : std::integral_constant<bool, B> { } members; }; int main() { A<true> a; cout << a.members.value << endl; return 0; } ``` In c++ is it valid for unnamed structures to inherit? Are there any examples where this is userful? [1] Disclaimer: I'm not pretending the provided example is useful. I rarely use unnamed structs, and when I do they're usually bundling together some built-in member variables, in order to provide a cleaner interface for a class. The question came up from the observation that memberspaces need not be nammed structures

Original source