Empty Derivative Optimization

Question

Most C ++ programmers are aware of the empty base class optimizer, like idiom . What happens to empty child classes? for example

class EmptyBase {
    int i;
};

template<typename T>
class Derived : T { 
};

std::cout << sizeof(Derived<EmptyBase>); // Is there a standard verdic on this?

Similarly, EBO should be EDO, which states that since the derived class does not provide more members and does not introduce any virtual parameterizing type , it should not require more memory. Given various situations in which something like this might be apper (multiple inheritance, single inheritance ...):

• Is such an optimization standard / possible?
• If so, what are the mechanics of this optimization , are they similar to EBO ?

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Answer 1

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struct A {}; struct B : A { int x; };             // "equivalent" to { int }

struct X { int a; }; struct Y : X {};             // "equivalent" to { int }

B b; Y y;. , b, A - b (.. &static_cast<A&>(b)), b.x , , y, X - y, y.a - .

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struct S {}; struct T { S s; };                   // "equivalent" to { char }

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:

struct Foo { int x; char a; };                    // { int, char, char[3] }

struct Bar : Foo { short q; };                    // { int, char, char, short }

sizeof(int) == 4 sizeof(short) == 2. sizeof(Foo) == 8, sizeof(Bar) 8, .

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