Re: vector swap time complexity

James Kanze <>
Wed, 16 Sep 2009 04:40:44 -0700 (PDT)
On Sep 15, 4:29 pm, "Francesco S. Carta" <> wrote:

On 15 Set, 16:20, Victor Bazarov <> wrote:

Is the following code "good" to check an implementation
for the above behavior?


I disagree, if we understand that the goal is to investigate
what a particular implementation actually does, and not
something to be used in production code.

#include <iostream>
#include <vector>

using namespace std;

void dump(vector<char>* pv) {
  size_t* pc = reinterpret_cast<size_t*>(pv);

Huh??? 8-O

The only question I have to pose here is the use of size_t. In
practice, it's probably an adequate choice for almost any
implementation, but formally, anything but a character type
results in undefined behavior.

I have the following in my library, expressedly for such cases:

    // Dump:
    // =====
    //! This is a simple class which can be used to generate a hex
    //! dump of any object. (It generates the hex dump all on a
    //! single line, as a sequence of two byte hex values,
    //! by spaces.) In almost all cases, the actual instance of
    //! class will be a temporary, generated as the return value
    //! the function template dump(), so that template type
    //! can be used.
    //! \warning
    //! This class does <strong>not</strong> save a copy of
    //! object, but only a reference to it. This means that
    //! will contain a dangling reference if the object ceases
    //! exist before the instance of the class. On the other
    //! hand, it also means that it is possible to dump the
    //! of objects which cannot be copied, a signaling NaN,
    //! example. It is, however strong recommended that only
    //! temporary instances of this class be used (and that no
    //! instance be bound to a reference, other than as a
    //! argument); the rules concerning the lifetime of
    //! make this always safe.
    template< typename T >
    class Dump : Gabi::Util::IOStreamOperators< Dump< T > >
        explicit Dump( T const& obj ) ;
        void print( std::ostream& dest ) const ;

        //!@cond implementation
        unsigned char const*myObj ;
    } ;

    //! Allows the use of template type deduction, i.e.:
    //! \code
    //! std::cout << dump( someObject ) ;
    //! \endcode
    //! rather than
    //! \code
    //! std::cout << Dump< ObjectType >( someObject ) ;
    //! \endcode
    //! \param obj
    //! The object for which an instance of Dump<T> is
    //! \return
    //! The instance of Dump<T>.
    template< typename T >
    inline Dump< T >
        T const& obj )
        return Dump< T >( obj ) ;

    template< typename T >
    Dump< T >::Dump(
        T const& obj )
        : myObj( reinterpret_cast< unsigned char const* >( &obj ) )

    template< typename T >
    Dump< T >::print(
        std::ostream& dest ) const
        IOSave saver( dest ) ;
        dest.fill( '0' ) ;
        dest.setf( std::ios::hex, std::ios::basefield ) ;
        char const* baseStr = "" ;
        if ( (dest.flags() & std::ios::showbase) != 0 ) {
            baseStr = "0x" ;
            dest.unsetf( std::ios::showbase ) ;
        unsigned char const* const
                            end = myObj + sizeof( T ) ;
        for ( unsigned char const* p = myObj ; p != end ; ++ p ) {
            if ( p != myObj ) {
                dest << ' ' ;
            dest << baseStr << std::setw( 2 ) << (unsigned int)
( *p ) ;

(Note that the class template IOStreamOperators generates the <<
and >> operators automatically, and IOSave, of course, saves the
various formatting information, and restores it in its

Again, I don't think I've ever used it in production code, but
it's often useful for checking assumptions about the
implementation (big endian or little, are doubles IEEE, etc.).


BTW, should I have preferred a C-style cast such as
"(size_t*)pv" instead of reinterpret_cast, in such a cheat?

Absolutely not. The purpose of reinterpret_cast is to allow
such cheats. And to make them visible as such.

James Kanze

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