c++ - declaring a const instance of a class

Question

Let's say I have a class defined as follows:

class foo{};

now, this is perfectly acceptable;

foo f;

how come this is a compiler error? (uninitialized const ‘f’)

const foo f;

Why do we have to do this?

const foo f = foo();

I know why we can't do this..

const foo f(); // though it compiles..

Interestingly, the following is valid:

const std::string f;

So what is missing from foo?

I realize that there are three questions there and it's bad form, but I'm hoping someone can clear this up for me in one answer.

EDIT: please feel free to close it if it's stupid...

Answer 1

Your class is a POD (essentially because it doesn’t provide a default constructor). POD variables are not initialized upon declaration. That is, this:

foo x;

does not initialize x to a meaningful value. This has to be done separately. Now, when you declare it as const, this may never happen because you cannot assign to or change x any more.

Consider the equivalence to int:

int x; // legal
const int y; // illegal

As you have noticed, using std::string instead of foo compiles. That’s because std::string is not a POD. A simple solution to your dilemma is to provide a default constructor for foo:

class foo {
public:
    foo() { }
};

Now your const foo x; code compiles.