nested structures using pointers - initialization and use - c++

Question

So I have a header file:
struct inner1 {
int w;
int x;
};
struct inner2 {
int y;
int z;
};
struct outer{
inner1 *a;
inner2 *b;
};
Now in main, I want to initialize these structures and use them. How do I initialize all three structures so that outer recognizes inner1 and inner2 as the specific structs it points to. Then lets say I want to access the variable w. Is it outer.a->w? I am unfamiliar with nested structures dealing with pointers. Any help is appreciated.

Answer 1

struct inner1 {
int w;
int x;
};
struct inner2 {
int y;
int z;
};
struct outer{
inner1 *a;
inner2 *b;
};
int _tmain(int argc, _TCHAR* argv[])
{
outer o;
o.a = new inner1();
o.b = new inner2();
o.a->w = 1;
o.a->x = 2;
o.b->y = 3;
o.b->z = 4;
//do stuff
delete o.a;
delete o.b;
return 0;
}
This will permit you to declare and initialize the structures.

Answer 2

Here is a demonstrative program
#include <iostream>
int main()
{
struct inner1 {
int w;
int x;
};
struct inner2 {
int y;
int z;
};
struct outer{
inner1 *a;
inner2 *b;
};
outer o = { new inner1 { 10, 20 }, new inner2 { 30, 40 } };
std::cout << "o.a->w = " << o.a->w << ", o.a->x = " << o.a->x
<< ", o.b->y = " << o.b->y << ", o.b->z = " << o.b->z
<< std::endl;
delete o.a;
delete o.b;
}
The program output is
o.a->w = 10, o.a->x = 20, o.b->y = 30, o.b->z = 40
Or you could do the following way
#include <iostream>
int main()
{
struct inner1 {
int w;
int x;
};
struct inner2 {
int y;
int z;
};
struct outer{
inner1 *a;
inner2 *b;
};
inner1 i1 = { 10, 20 };
inner2 i2 = { 30, 40 };
outer o = { &i1, &i2 };
std::cout << "o.a->w = " << o.a->w << ", o.a->x = " << o.a->x
<< ", o.b->y = " << o.b->y << ", o.b->z = " << o.b->z
<< std::endl;
}
The approach depends on the task and using of the objects.

Answer 3

You are correct in both your assumptions.
As long as these three structures are defined in this order no further declarations have to be made.

Answer 4

Welcome to c++.
The biggest favour you will do yourself is to learn about the smart pointers early on.
It's almost always a mistake to write structures in terms of raw pointers.
#include <memory>
#include <iostream>
struct inner1 {
inner1(int a, int b) : w(a), x(b) {}
int w;
int x;
};
struct inner2 {
inner2(int a, int b) : y(a), z(b) {}
int y;
int z;
};
struct outer{
outer(int w, int x, int y, int z)
: a(std::make_unique<inner1>(w,x))
, b(std::make_unique<inner2>(y,z))
{}
std::unique_ptr<inner1> a;
std::unique_ptr<inner2> b;
};
int main()
{
auto p_outer = std::make_unique<outer>(1, 2, 3, 4);
return 0;
// p_outer.a and .b are destroyed automatically here
// and all memory is reclaimed.
}