Re: Guarantee initialized legacy variables

Mathias Gaunard <>
Thu, 31 May 2007 13:15:18 CST
On May 31, 9:58 am, wrote:

I have a template: Initialised<T>, and a set of partial
specializations of Initialised<T>.

You may want to know that such a thing already exists in boost, called

T must be a PODT, or a sturct of PODTs, or array of PODTs. If T were
not a plain old data type, then it would have its own ctor, and
wouldn't need a templated wrapper to ensure its proper initialization.

An array of POD types is already a POD type.
And a structure of POD types is already a POD type too.

There is no rationale though for preventing usage of your template
with non-POD types.


P(1): I have a possible solution, but would like community feedback on
a technique for allowing an Initialised<T*> to be assigned NULL (I
certainly wish to allow this, its just a matter of what technique is

Pointers are PODs, there is no need to specialize them.

P(2): I am unable to discern how to partially specialize
Initialised<T> for T being an array of PODTs.

There is no need to specialize for arrays either.

P(3): I am unable to discern how to partially specialize
Initialised<T> for T being a struct of PODTs.

No need either.

Lets start with the basic definition of Initialised<T>:

template <typename T>
struct Initialised
        // default valued construction
        Initialised() : m_value(0) { }

replace m_value(0) by m_value() and it works as expected.

        // implicit valued construction (auto-conversion)
        template <typename U> Initialised(const U & rhs) : m_value(rhs) { }

        // assignment
        template <typename U> T & operator = (const U & rhs) { if
((void*)&m_value != (void*)&rhs) m_value = rhs; return *this; }

The test is not needed.
No need to optimize for a case that never happens.

Here is the partial specialization for raw pointers:

template <typename T>
struct Initialised<T*>
        // default valued construction
        Initialised() : m_value(NULL) { }

        // valued construction (auto-conversion)
        template <typename U> Initialised(const U * rhs) : m_value(rhs) { }

        // assignment
        template <typename U> T* & operator = (U * rhs) { if (m_value != rhs)
m_value = rhs; return *this; }
        template <typename U> T* & operator = (const U * rhs) { if (m_value !
= rhs) m_value = rhs; return *this; }

There is no need for the second assignment operator.
The first one works just as well, since U can be a const type too.

        // pointer semantics
        const T * operator -> () const { return m_value; }
        T * operator -> () { return m_value; }
        const T & operator * () const { return *m_value; }
        T & operator * () { return *m_value; }

Oh, so that's why you wanted to specialize it for pointers.
You might as well just overload operator-> in the general case
(operator* shouldn't be needed, thanks to the implicit conversion

Which brings me to question / problem P(2) - is there a syntax that
will allow me to do a partial specialization for an array of T?

There is, of course, but it's not needed at all.

I have tried:

template <typename T, size_t count>
struct Initialised<T(&)[count]>

T(&)[count] is a reference to an array, not an array.
Try with T[count].

Thanks for any feedback you may have to offer.

You just need this:

template<class T>
class Initialized
     Initialized() : x()

     template<typename U>
     Initialized(const U& u) : x(u)

     operator T&()
         return x;

     operator const T&() const
         return x;

     T& operator->()
         return x;

     const T& operator->() const
         return x;

     T x;

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