Avoid calling constructor of member variable

Question

I have the following C++-class:

// Header-File
class A
{
    public:
    A();

    private:
    B m_B;
    C m_C;
};

// cpp-File
A::A()
: m_B(1)
{
    m_B.doSomething();
    m_B.doMore();
    m_C = C(m_B.getSomeValue());
}

I now would like to avoid the class A to call any constructor of C m_C. Because on the last line in A::A(), I'm anyways going to initialize m_C myself because I need to prepare m_B first. I could provide an empty default constructor for class B. But that's not the idea.

I have already tried to add m_C(NULL) to the init-list of A::A(). Sometimes it worked, sometimes it said there was no constructor taking NULL as an argument.

So how can I have m_C left uninitialized? I know that with pointers, the m_C(NULL)-way works. And I don't want to allocate it dynamically using new.

Any idea is appreciated.

Answer 1

What you ask is forbidden - and correctly so. This ensures that every member is correctly initialized. Do not try to work around it - try to structure your classes that they work with it.

Idea:

• C has a constructor that does nothing
• C has an initialization method that makes the class usable
• C tracks whether it has been initialized correctly or not and returns appropriate errors if used without initialization.

Answer 2

How about using technique described in this QA?

Prevent calls to default constructor for an array inside class

std::aligned_storage<sizeof(T[n]), alignof(T)>::type

---

Or, you also can consider using of union. AFAIK, unions will be initialized only with first named member's constructor.

For example,

union
{
   uint8_t _nothing = 0; 
   C c;
};

According to the standard mentioned in the QA, c will be zero-initialized, and its constructor will not be called.