How can I alloc member value with "Get" function?

Question

Here's a simple code.

class Sub
{
    ...
public:
    Sub()
    {
        ...
    }
}

class Main
{
private:
    Sub*    m_pSub

public:

    Main()
    {
        // I don't want construct "Sub" here
        m_pSub = nullptr;
    }    

    Sub*    GetSub()
    {
        return m_pSub;
    }
}


/////////////////////
// in source
Main*   pMain;

pMain->GetSub() = new Sub()

Of course, pMain->GetSub() = new Sub() does not work because the left value of '=' in the above code must be a correctable value.

Therefore, please teach me various ways to implement similarly (which can be used as short as possible).

Thank you !

Answer 1

The simplest way to make your code work is to have GetSub() return a reference, eg:

class Main
{
private:
    Sub* m_pSub = nullptr;

public:

    Main() = default;

    Sub*& GetSub()
    {
        return m_pSub;
    }
};

However, this isn't very good class design.

Another option is to have GetSub() create the object on its first call, eg:

class Main
{
private:
    Sub* m_pSub = nullptr;

public:

    Main() = default; 

    Sub* GetSub()
    {
        if (!m_pSub) m_pSub = new Sub;
        return m_pSub;
    }
};

Otherwise, use an explicit setter method, eg:

class Main
{
private:
    Sub* m_pSub = nullptr;

public:

    Main() = default;    

    Sub* GetSub()
    {
        return m_pSub;
    }

    void SetSub(Sub* newSub)
    {
        m_pSub = new Sub;
    }
};

...
pMain->SetSub(new Sub);

Either way, you really should be using smart pointers, either std::unique_ptr or std::shared_ptr, to make it clear who owns the Sub object and is responsible for destroying it. A raw pointer does not convey that information.