在初始化程序中调用的已删除构造函数

Question

Yes, I know destructors are important but this is a situation where both Foo and Bar are going to be singletons. And for debugging purposes I want to make sure the singleton object is never deleted (I accidentally made that happen once).

class Bar {
public:
    Bar();
    ~Bar() = delete;
}
class Foo {
public:
    Foo();
    Bar b;
}

All constructor and destructor bodies are empty. So the cpp just looks like

Foo::Foo() {
}

This code causes an error that I do not expect. I would understand it to be frustrated about not having a destructor in the destructor method of Foo but instead I get an error on the constructor. For some reason the compiler says it is referencing a deleted function. Why does it care about this in the constructor?

Answer 1

Having Bar b; as class member implies that there must be an accessible destructor for b .

The rationale for this is so that it can be destroyed if an exception is thrown some point during construction of a Foo , after b was constructed but before construction of the Foo was complete.

Unfortunately, even if your code is such that there is no possible exception, this rule still stands. You will have to redesign your class layout so that singletons are not included directly as data members of other classes, even if they are singletons too.

References:

C++17 [class.base.init]/12:

In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked. [Note: This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown (18.2). —end note ]

Also [class.dtor]/12.4:

A program is ill-formed if a destructor that is potentially invoked is deleted or not accessible from the context of the invocation.