How to return a const ref to a struct object such that all its nested structures also are read only?

How do I prevent all the member variables in a nested struct to be non modifiable?

typedef struct A  
{  
    int a;  
    int b;  
}A1;  

typedef struct B  
{  
    A1* objA;  
}B1;  

class C  
{  
public:  
    const B& GetB() const { return objB; }  
    Po开发者_如何学CpulateB();  

private:  
    B objB;  
};  

int main()  
{  
    C objC;
    objC.PopulateB();  
    const B& objB2 = objC.GetB();  
    objB2.objA->a = 3; // compiler allows this  
}  

I just want a struct that is completely read only and I expected this to work. (In this case objB2)

---

The pointer is const. The data it points to is not. Thats just how pointers work The best solution is to not expose raw pointers. A non-const accessor function would not be allowed on a const reference, so you can avoid the tricky indirection.

As an aside, you don't have to typedef structs like that. It's a C-ism.

---

The pointer type in your struct B refers to a non-const value, and this is the definition that counts. If you wanted to control access, you should hide data members and provide accessors and mutators:

struct B
{
private:
    A1* objA;
public:
    A1 & a1() {return *objA;}
    A1 const & a1() const {return *objA;}
};

---

You can try:

typedef struct B  
{  
    const A1& objA;  
}B1;