C ++在构造函数的初始化列表中初始化嵌套结构？

Question

  struct B
  {

    int hh;
    int ii;
  };          

  struct A
  {
    B b_memberVar;
    void *p_data;
  };

  struct C
  {
    A array[13];
    int bb;
    int cc;
    int dd;
    int ee;
    int ff;
    int gg;
  };

  struct D
  {
    C c_memberVar;
    int aa;
  };

  class XYZ
  {
    XYZ();
    D m_DMemberVar;
    int zz;
  }

  XYZ::XYZ():
    m_DMemberVar(
    ({{{
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr},
            {{0,0},nullptr}
            },0,0,0,0,0,0},0}),
            zz(0)
    ){}

How do I initialize a nested struct that's a class member variable on the constructor's initialization list? I have tried using a lot of nested {} as shown as an example above but that produced syntax errors. The errors give "expected an expression" and "expected a ")" "

Answer 1

Well, you have a ton of errors with your class definitions:

• Wrong order and no forward declarations
• Missing semicolon terminators
• Missing constructor declaration
• Missing constructor definition body
• Sample/placeholder initialiser invalid

But, once those are fixed, it really is just a case of matching your braces.

Here we go!

struct D
{
 int D_MemberVar;
};

struct C
{
 int* c_ptrMemberVar;
};

struct B
{
 C c_memberVar;
 D* d_ptrMemberVar;
};

struct A
{
 B b_memberVar;
 int int_memberVar;
};

class XYZ
{
 A m_memberVar;
 int z;

public:
 XYZ();
};

XYZ::XYZ()
    : m_memberVar{
        {    // A::b_memberVar
            {   // B::c_memberVar
                nullptr    // B::c_ptrMemberVar
            },
            nullptr  // B::d_ptrMemberVar
        },
        3    // A::int_memberVar
    }
    , z(0)
{}

int main()
{
    XYZ xyz;
}

( live demo )

I suspect the key is that you were trying to use A 's constructor, but there is no non-default constructor in that class. So you needed {} at the top level too.