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Full Discussion: Static variables in C++
Top Forums Programming Static variables in C++ Post 302741095 by achenle on Friday 7th of December 2012 11:38:56 AM
Old 12-07-2012
Quote:
Originally Posted by Don Cragun
If the application is assuming that based on the location in memory of dl (&dl) it "knows" the address of df or du and makes use of that knowledge to get the value of df or to store a value into du that application will produce undefined results. This is true even though there is a good chance that with a little testing the user can figure out how a given compiler will arrange the locations of the integers declared by:
Code:
static int  df, dl, du, count;

or by:
Code:
static int  df;
static int  dl;
static int  du;
static int  count;

There is probably a much better than 50% chance that the two above ways of declaring these variables will result in them being allocated in the same positions relative to each other, but there is a non-zero percent chance that some compilers would allocate them in reverse order (or alphabetic order by name or some other order) in memory.

Any application that makes an assumption like this is depending on undefined behavior. As Corona688 said:

Is it possible that an assumption like this could change the behavior of a section of code? Absolutely.
Is it likely that an assumption like this would change the behavior of a section of code? Who knows?
Saying code like that "depend[s] on undefined behavior" is too kind. Anyone who writes code that assumes the location of a variable based on the location of another variable is bat-!*()% crazystupid.

FWIW, multiple definitions on one line are a bad idea for other reasons, too.

What's this mean:
Code:
char* ptrA, ptrB;

 

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Graph(3pm)						User Contributed Perl Documentation						Graph(3pm)

NAME
SVG::Graph - Visualize your data in Scalable Vector Graphics (SVG) format. SYNOPSIS
use SVG::Graph; use SVG::Graph::Data; use SVG::Graph::Data::Datum; #create a new SVG document to plot in... my $graph = SVG::Graph->new(width=>600,height=>600,margin=>30); #and create a frame to hold the data/glyphs my $frame = $graph->add_frame; #let's plot y = x^2 my @data = map {SVG::Graph::Data::Datum->new(x=>$_,y=>$_^2)} (1,2,3,4,5); my $data = SVG::Graph::Data->new(data => @data); #put the xy data into the frame $frame->add_data($data); #add some glyphs to apply to the data in the frame $frame->add_glyph('axis', #add an axis glyph 'x_absolute_ticks' => 1, #with ticks every one #unit on the x axis 'y_absolute_ticks' => 1, #and ticks every one #unit on the y axis 'stroke' => 'black', #draw the axis black 'stroke-width' => 2, #and 2px thick ); $frame->add_glyph('scatter', #add a scatterplot glyph 'stroke' => 'red', #the dots will be outlined #in red, 'fill' => 'red', #filled red, 'fill-opacity' => 0.5, #and 50% opaque ); #print the graphic print $graph->draw; DESCRIPTION
SVG::Graph is a suite of perl modules for plotting data. SVG::Graph currently supports plots of one-, two- and three-dimensional data, as well as N-ary rooted trees. Data may be represented as: Glyph Name Dimensionality supported 1d 2d 3d tree -------------------------------------------------------- Axis x Bar Graph x Bubble Plot x Heatmap Graph x Line Graph x Pie Graph x Scatter Plot x Spline Graph x Tree x SVG::Graph 0.02 is a pre-alpha release. Keep in mind that many of the glyphs are not very robust. PLOTTING
You need to create a SVG::Graph::Frame instance from the parent SVG::Graph instance for each set of data to be plotted. Datasets can be hierarchical, and to represent this, SVG::Graph::Frame instances can themselves contain subframes. SVG::Graph::Frame can contain: - multiple subframes as instances of SVG::Graph::Frame - a single SVG::Graph::Data instance - multiple SVG::Graph::Glyph instances with which to render the attached SVG::Graph::Data instance, and all SVG::Graph::Data instances attached to SVG::Graph::Frame subinstances See SVG::Graph::Frame and SVG::Graph::Glyph for details. ONE DATA SET 1. create an SVG::Graph instance 2. create an SVG::Graph::Frame instance by calling SVG::Graph::add_frame(); 3. create an SVG::Graph::Data instance, containing an SVG::Graph::Data::Datum instance for each data point. 4. Attach the SVG::Graph::Data instance to your SVG::Graph::Frame using SVG::Graph::Frame::add_data(); 5. Attach glyphs to the SVG::Graph::Frame instance using SVG::Graph::Frame::add_glyph(); 6. Call SVG::Graph::draw(); MULTIPLE DATA SETS 1. create an SVG::Graph instance 2. create an SVG::Graph::Frame instance by calling SVG::Graph::add_frame(); 3. create an SVG::Graph::Data instance, containing an SVG::Graph::Data::Datum instance for each data point. 4. Attach the SVG::Graph::Data instance to your SVG::Graph::Frame using SVG::Graph::Frame::add_data(); 5. Attach glyphs to the SVG::Graph::Frame instance using SVG::Graph::Frame::add_glyph(); 6. repeat [2-5] for each additional data set to be added. add_frame() can be called on SVG::Graph to add top-level data sets, or SVG::Graph::Frame to add hierarchical data sets. 7. Call SVG::Graph::draw(); FEEDBACK
Send an email to the svg-graph-developers list. For more info, visit the project page at http://www.sf.net/projects/svg-graph AUTHORS
Allen Day, <allenday@ucla.edu> Chris To, <crsto@ucla.edu> CONTRIBUTORS
James Chen, <chenj@seas.ucla.edu> Brian O'Connor, <boconnor@ucla.edu> SEE ALSO
SVG METHODS
new Title : new Usage : my $graph = SVG::Graph->new(width=>600, height=>600, margin=>20); Function: creates a new SVG::Graph object Returns : a SVG::Graph object Args : width => the width of the SVG height => the height of the SVG margin => margin for the root frame init Title : init Usage : Function: Example : Returns : Args : width Title : width Usage : $obj->width($newval) Function: Example : Returns : value of width (a scalar) Args : on set, new value (a scalar or undef, optional) height Title : height Usage : $obj->height($newval) Function: Example : Returns : value of height (a scalar) Args : on set, new value (a scalar or undef, optional) margin Title : margin Usage : $obj->margin($newval) Function: Example : Returns : value of margin (a scalar) Args : on set, new value (a scalar or undef, optional) svg Title : svg Usage : $obj->svg($newval) Function: Example : Returns : value of svg (a scalar) Args : on set, new value (a scalar or undef, optional) add_frame Title : add_frame Usage : my $frame = $graph->add_frame Function: adds a Frame to the current Graph Returns : a SVG::Graph::Frame object Args : a hash. usable keys: frame_transform (optional) 'top' default orientation 'bottom' rotates graph 180 deg (about the center) 'right' points top position towards right 'left' points top position towards left frames Title : frames Usage : get/set Function: Example : Returns : Args : xoffset Title : xoffset Usage : $obj->xoffset($newval) Function: Example : Returns : value of xoffset (a scalar) Args : on set, new value (a scalar or undef, optional) yoffset Title : yoffset Usage : $obj->yoffset($newval) Function: Example : Returns : value of yoffset (a scalar) Args : on set, new value (a scalar or undef, optional) draw Title : draw Usage : $graph=>draw Function: depends on child glyph implementations Returns : xmlifyied SVG object Args : none perl v5.10.0 2009-01-07 Graph(3pm)
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