我们可以通过初始化列表中的智能指针来初始化结构成员吗？

Question

#include <boost/scoped_ptr.hpp>

class classA
{
protected:
    struct StructB
    {
        int iAge;
        double dPrice;
    };

    boost::scoped_ptr<StructB> m_scpStructB;

public:
    classA(int age, double price)
        : m_scpStructB(new StructB)
    {
        m_scpStructB->iAge = age;
        m_scpStructB->dPrice = price;
    }

    /* DO NOT COMPILE  <= second block
    classA(int age, double price)
        : m_scpStructB(new StructB),
          m_scpStructB->iAge(age),
          m_scpStructB->dPrice(price)
    {}
    */

};

Question1> I have found that I cannot use the second block of code to initialize structure members pointed by a smart pointer. Is it a general c++ rule that we just cannot do it.

Please discard this question if the answer to the first question is "You cannot do it".

Question2> As far as I know, the order of assignment on initialization list is based on the order of member variables definition. Assume that you can initialize the member variables through smart pointer. How can you guarantee the order so that the smart point is initialized always first?

Answer 1

If you don't need StructB to be an aggregate/POD type, then just give it a constructor too:

#include <boost/scoped_ptr.hpp>

class classA
{
protected:
    struct StructB
    {
        StructB(int age, double price) : iAge(age), dPrice(price) { }

        int iAge;
        double dPrice;
    };

    boost::scoped_ptr<StructB> m_scpStructB;

public:
    classA(int age, double price) : m_scpStructB(new StructB(age, price)) { }
};

Otherwise you can just use a factory function, so that it remains a POD type:

#include <boost/scoped_ptr.hpp>

class classA
{
protected:
    struct StructB
    {
        int iAge;
        double dPrice;

        static StructB* make(int age, double price)
        {
            StructB* ret = new StructB;
            ret->iAge = age;
            ret->dPrice = price;
            return ret;
        }
    };

    boost::scoped_ptr<StructB> m_scpStructB;

public:
    classA(int age, double price) : m_scpStructB(StructB::make(age, price)) { }
};