Re: exceptions

 James Kanze <>
Tue, 18 Sep 2007 01:27:34 -0700
On Sep 17, 5:47 pm, "Alf P. Steinbach" <> wrote:


If you have code like:

    c = 0; // initialize
    c = new C(...); // c of type C*
catch( E& e)

and an exception is thrown inside the constructor. Is it guaranteed
that c will be null?


Are you sure? It's what I would expect, and it might be the
intent of the standard, but I don't think that the standard
actually guarantees it anywhere. I can't find anything in the
standard which guarantees that the pointer will not be modified
until after the return from the constructor; formally, at least,
a compiler could break the statement with the new down into:

    c = operator new( sizeof( C ) ) ;
    c->C() ; // Call the constructor.

(Note that if the constructor exits via an exception, the memory
will be freed. which would mean that c would end up containing
an invalid pointer.)

If this is important, the obvious solution is:

    try {
        c = 0 ;
        C* tmp = new C(...) ;
        c = tmp ;
        // ...
    } catch ( E& e ) {
        // ...

If c is a smart pointer, the original code should also work,
since the assignment operator is actually a function call, and
thus introduces a sequence point.

James Kanze (GABI Software)
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