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From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2000-02-03 05:28:23

I have in mind a small (pretty??) class, which seems to me to be a yet
another candidate to our boost utility library. I've seen it to be
reinvented at least 2 times after I'd invented it myself , so I will be not
much surprised if someone say 'I have this one too'..
And even if you didn't, but you had ever written a code which assign some
temporary value to a variable at some place in a block (usually at the
beginning), and restore the old value at the end of it, I am sure you would
appreciate this class :). It does exactly such thing - assign a new value to
a variable, storing the old one within itself, and restore the original
value in its destructor.
So the class itself:

namespace boost {
template<class T>
class temp_value
    typedef T value_type;
    temp_value( T* object_ptr, const T& new_value )
      : object_ptr_( object_ptr )
      , old_value_( *object_ptr ) { *object_ptr_ = new_value; }

    ~temp_value() { *object_ptr_ = old_value_; }

    T old_value_;
    T* object_ptr_;

(Dave, I know, it must be renamed to 'temporary_value' =), and I agree with
you, unless we choose a totally different name.)
And a typical usage (and test) may look like this:

void temp_value_test() {
  bool flag = true;
  int count = 15;
  if ( flag ) {
    boost::temp_value<bool> tmp( &flag, false );
    assert( !flag );
    if ( !flag ) {
      boost::temp_value<int> tmp( &count, 0 );
      assert( !count );
    assert( count == 15 );
  assert( flag );

Any thoughts about it?


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