What is this crazy C++11 syntax ==> struct : bar {} foo {};?

Question

What could this possibly mean in C++11?

struct : bar {} foo {};

Answer 1

First, we'll take a bog-standard abstract UDT (User-Defined Type):

struct foo { virtual void f() = 0; }; // normal abstract type
foo obj;
// error: cannot declare variable 'obj' to be of abstract type 'foo'

Let's also recall that we can instantiate the UDT at the same time that we define it:

struct foo { foo() { cout << "!"; } };          // just a definition

struct foo { foo() { cout << "!"; } } instance; // so much more
// Output: "!"

Let's combine the examples, and recall that we can define a UDT that has no name:

struct { virtual void f() = 0; } instance; // unnamed abstract type
// error: cannot declare variable 'instance' to be of abstract type ''

We don't need the proof about the anonymous UDT any more, so we can lose the pure virtual function. Also renaming instance to foo, we're left with:

struct {} foo;

Getting close.

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Now, what if this anonymous UDT were to derive from some base?

struct bar {};       // base UDT
struct : bar {} foo; // anonymous derived UDT, and instance thereof

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Finally, C++11 introduces extended initialisers, such that we can do confusing things like this:

int x{0};

And this:

int x{};

And, finally, this:

struct : bar {} foo {};

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This is an unnamed struct deriving from bar, instantiated as foo with a blank initializer.