Question

I’m learning C++ (coming from Java) and this is bugging the hell out of me, say I have…

class Foo {

public:

    Bar &x; // <- This is a ref, Bar is not instantiated (this is just a placeholder)
    Bar *y; // <- This is a pointer, Bar is not instantiated (this is just another placeholder)
    Bar z;  // <- What is this???

};

class Bar {

public:

    Bar(int bunchOfCrap, int thatNeedsToBeHere, double toMakeABar);

};

In this example Bar has one constructor that needs a bunch of fields specified in order to create a “Bar.” Neither x, nor y creates a Bar, I understand that x creates a reference that can be pointed to a Bar and that y creates a pointer that can also represent a Bar.

What I don’t understand is what exactly z is.

• Is it a Bar? And if so, how can it be given Bar’s only constructor?

• Is it a Bar sized chunk of memory belonging to instances of Foo that can be initialized into a Bar?

• Or is it something else?

Thanks!

Answer 1

For pedagogical reasons, let’s try to compile this code:

class Bar {
public:
    Bar(int bunchOfCrap, int thatNeedsToBeHere, double toMakeABar);
};

class Foo {
public:
    Bar &x; // <- This is a ref, Bar is not instantiated (this is just a placeholder)
    Bar *y; // <- This is a pointer, Bar is not instantiated (this is just a *safer* placeholder)
    Bar z;  // <- What is this???
};

int main() {
    Foo foo;
}

Output:

c++     uuu.cpp   -o uuu
uuu.cpp:10:7: error: implicit default constructor for 'Foo' must explicitly initialize the reference member 'x'
class Foo {
      ^
uuu.cpp:14:10: note: declared here
    Bar &x; // <- This is a ref, Bar is not instantiated (this is just a placeholder)
         ^
uuu.cpp:10:7: error: implicit default constructor for 'Foo' must explicitly initialize the member 'z' which does not
      have a default constructor
class Foo {
      ^
uuu.cpp:16:9: note: member is declared here
    Bar z;  // <- What is this???
        ^
uuu.cpp:2:7: note: 'Bar' declared here
class Bar {
      ^
uuu.cpp:21:9: note: implicit default constructor for 'Foo' first required here
    Foo foo;
        ^
2 errors generated.
make: *** [uuu] Error 1

As it says, you must initialize both the member reference Bar &x and the member variable Bar z;, because Bar doesn’t have a default constructor. y doesn’t have to be initialized, and it will default to NULL.

Both x and y indirectly refer to the object. You can’t change what x refers to (so it must be initialized when Foo is instantiated). You can change what y refers to. z is a Bar-sized chunk of memory which lives inside Foo; for you to legally declare a member variable like this, you have to put the complete definition of Bar before Foo so that the compiler knows how big Bar is.