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Full Discussion: Unclear pointer and array
Top Forums Programming Unclear pointer and array Post 302884282 by Corona688 on Sunday 19th of January 2014 06:22:32 PM
Old 01-19-2014
Quote:
The most difficult parts with C to me are: 1) dynamic memory allocation of pointers
I think you're overthinking this quite a whole lot. You've divided this into a lot of separate categories like "the correct way to use pointers when malloc is involved", "the correct way to use pointers when printf is involved", "the correct way to use pointers when sprintf is involved", "the correct way to use pointers when strtok is involved", etc -- as if these things had nothing to do with each other.

If you look at them though, their arguments help give you an idea how they work.

Code:
$ man malloc

void *malloc(size_t size);

$ man strtok

char *strtok(char *str, const char *delim);

$ man strcpy

char *strcpy(char *dest, const char *src);

$ man sprintf

int sprintf(char *str, const char *format, ...);

$

malloc returns a 'void *'. It's not const, so the memory it returns is memory you're allowed to write to. 'void' means its of no particular type, that you're expected to cast it into the type you need. So malloc returns memory which you're supposed to cast into some other type, memory which you're allowed to write to.

strtok takes a 'char *', which means you're giving it memory that strtok's allowed to alter, as well as a 'const char *', memory it's not allowed to change -- the list of tokens. It returns a 'char *', a pointer to memory you're allowed to write to. (As it turns out, a pointer to the same memory, even.)

strcpy's arguments show you which one is src and which one is dest, even without their names -- the 'const char *' has to be the source because that's memory strcpy can't write to.

and sprintf takes a pointer to memory it can write to(the output string), memory it can't write to(the command string), and a variable number of arguments just like printf.

As for the way printf and sscanf demand pointers and non-pointers, there's nothing really deep to learn about C there, it's just the way the printf and scanf calls were built. They could have made printf demand everything be pointers, just like scanf does, but didn't.
 

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LEARN ABOUT DEBIAN

graphics::primitive::component

Graphics::Primitive::Component(3pm)			User Contributed Perl Documentation		       Graphics::Primitive::Component(3pm)

NAME

       Graphics::Primitive::Component - Base graphical unit

DESCRIPTION

       A Component is an entity with a graphical representation.

SYNOPSIS

	 my $c = Graphics::Primitive::Component->new({
	   origin => Geometry::Primitive::Point->new({
	       x => $x, y => $y
	   }),
	   width => 500, height => 350
	 });

LIFECYCLE

       prepare
	   Most components do the majority of their setup in the prepare.  The goal of prepare is to establish it's minimum height and width so
	   that it can be properly positioned by a layout manager.

	     $driver->prepare($comp);

       layout
	   This is not a method of Component, but a phase introduced by the use of Layout::Manager.  If the component is a container then each of
	   it's child components (even the containers) will be positioned according to the minimum height and width determined during prepare.
	   Different layout manager implementations have different rules, so consult the documentation for each for details.  After this phase has
	   completed the origin, height and width should be set for all components.

	     $lm->do_layout($comp);

       finalize
	   This final phase provides and opportunity for the component to do any final changes to it's internals before being passed to a driver
	   for drawing.  An example might be a component that draws a fleuron at it's extremities.  Since the final height and width isn't known
	   until this phase, it was impossible for it to position these internal components until now.	It may even defer creation of this
	   components until now.

	   It is not ok to defer all action to the finalize phase.  If you do not establish a minimum hieght and width during prepare then the
	   layout manager may not provide you with enough space to draw.

	       $driver->finalize($comp);

       draw
	   Handled by Graphics::Primitive::Driver.

	      $driver->draw($comp);

METHODS

   Constructor
       new Creates a new Component.

   Instance Methods
       background_color
	   Set this component's background color.

       border
	   Set this component's border, which should be an instance of Border.

       callback
	   Optional callback that is fired at the beginning of the "finalize" phase.  This allows you to add some sort of custom code that can
	   modify the component just before it is rendered.  The only argument is the component itself.

	   Note that changing the position or the dimensions of the component will not re-layout the scene.  You may have weird results of you
	   manipulate the component's dimensions here.

       class
	   Set/Get this component's class, which is an abitrary string.  Graphics::Primitive has no internal use for this attribute but provides
	   it for outside use.

       color
	   Set this component's foreground color.

       fire_callback
	   Method to execute this component's "callback".

       get_tree
	   Get a tree for this component.  Since components are -- by definiton -- leaf nodes, this tree will only have the one member at it's
	   root.

       has_callback
	   Predicate that tells if this component has a "callback".

       height
	   Set this component's height.

       inside_bounding_box
	   Returns a Rectangle that defines the edges of the 'inside' box for this component.  This box is relative to the origin of the
	   component.

       inside_height
	   Get the height available in this container after taking away space for padding, margin and borders.

       inside_width
	   Get the width available in this container after taking away space for padding, margin and borders.

       margins
	   Set this component's margins, which should be an instance of Insets.  Margins are the space outside the component's bounding box, as in
	   CSS.  The margins should be outside the border.

       maximum_height
	   Set/Get this component's maximum height.  Used to inform a layout manager.

       maximum_width
	   Set/Get this component's maximum width.  Used to inform a layout manager.

       minimum_height
	   Set/Get this component's minimum height.  Used to inform a layout manager.

       minimum_inside_height
	   Get the minimum height available in this container after taking away space for padding, margin and borders.

       minimum_inside_width
	   Get the minimum width available in this container after taking away space for padding, margin and borders.

       minimum_width
	   Set/Get this component's minimum width.  Used to inform a layout manager.

       name
	   Set this component's name.  This is not required, but may inform consumers of a component.  Pay attention to that library's
	   documentation.

       origin
	   Set/Get the origin point for this component.

       outside_height
	   Get the height consumed by padding, margin and borders.

       outside_width
	   Get the width consumed by padding, margin and borders.

       finalize
	   Method provided to give component one last opportunity to put it's contents into the provided space.  Called after prepare.

       padding
	   Set this component's padding, which should be an instance of Insets.  Padding is the space inside the component's bounding box, as in
	   CSS.  This padding should be between the border and the component's content.

       page
	   If true then this component represents stand-alone page.  This informs the driver that this component (and any children) are to be
	   renderered on a single surface.  This only really makes sense in formats that have pages such as PDF of PostScript.

       prepare
	   Method to prepare this component for drawing.  This is an empty sub and is meant to be overridden by a specific implementation.

       preferred_height
	   Set/Get this component's preferred height.  Used to inform a layout manager.

       preferred_width
	   Set/Get this component's preferred width.  Used to inform a layout manager.

       to_string
	   Get a string representation of this component in the form of:

	     $name $x,$y ($widthx$height)

       visible
	   Set/Get this component's visible flag.

       width
	   Set/Get this component's width.

AUTHOR

       Cory Watson, "<gphat@cpan.org>"

BUGS

       Please report any bugs or feature requests to "bug-geometry-primitive at rt.cpan.org", or through the web interface at
       http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Geometry-Primitive <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Geometry-Primitive>.  I
       will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

COPYRIGHT &; LICENSE
       Copyright 2008-2009 by Cory G Watson.

       This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

perl v5.12.3							    2011-06-02				       Graphics::Primitive::Component(3pm)
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