How to initialize the reference member variable of a class?

Question

Consider the following code C++:

    #include<iostream>

using namespace std;

class Test {
    int &t;
public:
    Test (int &x) { t = x; }
    int getT() { return t; }
};

int main()
{
    int x = 20;
    Test t1(x);
    cout << t1.getT() << " ";
    x = 30;
    cout << t1.getT() << endl;
    return 0;
}

It is showing the following error while using gcc compiler

    est.cpp: In constructor ‘Test::Test(int&)’:
    est.cpp:8:5: error: uninitialized reference member ‘Test::t’ [-fpermissive]

Why doesn't the compiler directly call the Constructor?

Answer 1

That is because references can only be initialized in the initializer list. Use

Test (int &x) : t(x) {}

To explain: The reference can only be set once, the place where this happens is the initializer list. After that is done, you can not set the reference, but only assign values to the referenced instance. Your code means, you tried to assign something to a referenced instance but the reference was never initialized, hence it's not referencing any instance of int and you get the error.

Answer 2

My compiler emits this error:

error C2758: 'Test::t' : must be initialized in constructor base/member initializer list

And that's exactly what you must do. References must be initialized in the initializer list:

#include<iostream>

using namespace std;

class Test {
    int &t;
public:
    Test (int &x) : t(x) {  } // <-- initializer list used, empty body now
    int getT() { return t; }
};

int main()
{
    int x = 20;
    Test t1(x);
    cout << t1.getT() << " ";
    x = 30;
    cout << t1.getT() << endl;
    return 0;
}

Explanation:

If the reference is not in the initiliazer list, it's next to impossible for the compiler to detect if the reference is initialized. References must be initialized. Imagine this scenario:

Test (int &x, bool b) 
{
   if( b ) t = x;
}

Now it would be up to the caller of the constructor to decide if valid code was generated. That cannot be. The compiler must make sure the reference is initialized at compile time.