Re: strings and NULL argument passing

James Kanze <>
Fri, 14 Nov 2008 01:33:23 -0800 (PST)
On Nov 14, 1:25 am, Rolf Magnus <> wrote:

James Kanze wrote:

It's undefined behavior. With a good implementation of
std::string, it won't compile.

Would it actually be allowed by the standard to have an
additional constructor in std::string?

That's a good question. If it doesn't affect the overload
resolution of legal calls to the constructor, I think so, if
only under the as if rule. Thus, if as a library implementor, I
do something like:

    namespace std {
    template< ... >
    class basic_string
        // ...
        struct _Hidden {} ;
        basic_string( int _Hidden::*,
                      Allocator const& = Allocator() ) ;
    } ;

can a legal program detect the presence of the additional

(One issue might be whether the following program is legal:

    #include <string>

    f( bool t )
        if ( t ) {
            std::string s(0) ;

        f( false ) ;


In practice, on thinking about it, I'm not sure that it's worth
the effort. I only catches the case where you initialize with a
null pointer constant (0, NULL or the like), which is, one would
hope, pretty rare. You still need a run-time check (generally
provided directly by the hardware) in case of a variable which
happens to contain a null ponter.

James Kanze (GABI Software)
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