How I set a reference after constructor? - c++

Question

I want to set a reference after constructor
Example:
class OtherClass
{
public:
OtherClass() : m_class(Class()){}
inline void SetData(int data1, int data2)
{
//calculate data3
// I tried:
m_class = Class(data3);
//but it doesn't worked
}
protected:
private:
Class& m_class;
};
Edit:
The Exception is: vector subscript out of range
because I have glm vectors in the Class.
I need also call functions in my Class.
Edit 2:
Why I need this?
Because I have an other class [ExClass] which extends and which have to calulate in constructor:
ExClass::ExClass(float d1, float d2, ...) {
//calculate data from given values
SetData(data);
}

Answer 1

The proper way to do that is to use a pointer and not a reference, as opposed to references - pointers can be set after object creation. Note also that referring (or pointing) to a local variable whose lifetime will end, while still in use, is a bad idea.
Your code may be changed to use a pointer and dynamic allocation or alternatively, std::unique_ptr. There are of course other options, these are just examples.
Option 1 - a pointer and dynamic allocation
class OtherClass
{
public:
OtherClass() : m_class(nullptr){}
~OtherClass() {
delete m_class;
}
// block copy and assignment (or implement them)
OtherClass(const OtherClass&) = delete;
OtherClass& operator=(const OtherClass&) = delete;
void setData(int data1, int data2)
{
// ... calculate data3 ...
m_class = new Class(data3);
}
bool hasInnerObj() const {
return m_class; // or: return m_class != nullptr;
}
/** call this function only if hasInnerObj() returned true */
Class& getInnerObj() {
return *m_class;
}
private:
Class* m_class;
};
Option 2 - std::unique_ptr
class OtherClass
{
public:
void setData(int data1, int data2)
{
// ... calculate data3 ...
m_class = std::make_unique<Class>(data3);
}
bool hasInnerObj() const {
return m_class; // or: return m_class != nullptr;
}
/** call this function only if hasInnerObj() returned true */
Class& getInnerObj() {
return *m_class;
}
private:
std::unique_ptr<Class> m_class;
};

Answer 2

You have two problems:
Reference class members (i.e. m_class) need to be initialized when object is created.
However, both your Class instances (one in the constructor, and one in SetData) are put on the stack and popped right away, making the reference invalid.
What you need to do is make sure that your class object actually lives through the function call. One way of achieving that is allocating it prior to passing it to the OtherClass constructor or SetData function:
class Class {};
class OtherClass
{
public:
OtherClass(Class& c) : m_class(c){}
inline void SetData(Class& c)
{
m_class = c;
}
protected:
private:
Class& m_class;
};
int main()
{
Class a;
OtherClass c(a);
Class b;
c.SetData(b); // changes m_class
return 0;
}
Live example here.