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of how a picture was formed The type of the picture object tells us how it was created: A String_Pic is created from character data that a user supplied to us; a Frame_Pic results from running frame on another Picture, and so on In addition to knowing how an object was created, we also need to store the object(s) from which it was created For a String_Pic, we'll need to remember the characters that the user gave us, which we can do in a vector<string> We create a Frame_Pic by framing another Picture, so we'll need to store the Picture that was framed Similarly, we create HCat_Pics and VCat_Pics by combining two other Pictures These classes will store the Pictures used in creating the resultant new object Before settling on a design that stores Pictures in the Pic_base derived classes, we should think through the implications of this design a bit more deeply Class Picture is an interface class intended for use by our users As such, it captures the interface to our problem domain but not the implementation Specifically, it does not have height, width, or display operations If we think a bit about how these functions might be implemented, we'll see that we'll need access to the corresponding operations on the Picture(s) stored in each of the derived types For example, to calculate height of a VCat_Pic, we need to add the heights of the two Pictures from which it was formed Similarly, we'll obtain the width by finding the maximum of the widths of the two component Pictures An implication of storing a Picture in each of the derived classes is that we'll have to give class Picture functions that duplicate the Pic_base operations Doing so obscures our initial design intent, which was that class Picture should be concerned with interface not implementation We can maintain our design by realizing that what we need in the derived classes is not an interface object but an implementation object This realization implies that instead of storing a Picture, we should store a Ptr<Pic_base> This design keeps a clean separation between interface and implementation, while still maintaining our intention to reference count our implementation objects to avoid unnecessary data duplication Although our design is clean, enough indirection is involved that a picture may help:
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Here we assume that we are generating three Pictures The first Picture represents a String_Pic object that holds the data that we got from the user The second one represents a Frame_Pic object that we constructed by calling frame on the initial Picture Finally, we construct a Picture that represents the output of vcat run on the two previous Pictures Each Picture has a single data member, which is a Ptr<Pic_base> That Ptr points to an object of the appropriate Pic_base derived type Each such object, in turn, contains either the vector that holds a copy of the data we got from the user, or one or two Ptrs that point to the Pic_base objects used to create the Picture Not shown in this diagram are the reference counts associated with the Ptr objects, because we assume that the Ptr class is doing its job, and we can ignore the details of that job
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What's different from what we did in 5 is that only String_Pic contains any characters The others hold one or two Ptrs Therefore, we won't copy any characters when we create f or v Instead, the Ptr will be yet another reference to the Ptrs in the Pictures that are used in creating a new Picture, and the Ptr class will take care of the reference counting for us So, when we call frame(pic), the effect is to create a new Frame_Pic object, and to point its Ptr at the same String_Pic that is stored in pic Similarly, the VCat_Pic contains two Ptrs pointing to the Frame_Pic and the String_Pic respectively We will not destroy any of these Pic_base objects; doing so is the responsibility of the Ptr class It will arrange to destroy each Pic_base object when the last Ptr that refers to each object has gone away At this point, we should capture these design decisions in code We know what data each object will contain, and we know what our operations will be:
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class Pic_base { // no public interface typedef std::vector<std::string>::size_type ht_sz; typedef std::string::size_type wd_sz; // this virtual virtual virtual }; class Frame_Pic: public Pic_base { // no public interface Ptr<Pic_base> p; Frame_Pic(const Ptr<Pic_base>& pic): p(pic) { } wd_sz width() const; ht_sz height() const; void display(std::ostream&, ht_sz, bool) const; }; class is an abstract base class wd_sz width() const = 0; ht_sz height() const = 0; void display(std::ostream&, ht_sz, bool) const = 0;
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Here we say that Frame_Pic inherits from Pic_base, and declare our intention to define class-specific versions of each of the three virtuals from that base class Thus, Frame_Pic will not be an abstract class, and we will be able to create Frame_Pic objects It is worth noting that we have declared these virtuals in the private section of the class Doing so lets the compiler enforce our design decision that only class Picture and operations on Pictures can access the Pic_base hierarchy Of course, because these virtuals are private, we may need to revisit the class definition to include friend declarations for class Picture, or the associated operations, as needed The Frame_Pic constructor needs only to copy the Ptr from the object that is being framed, which it does in the constructor initializer The constructor body is empty, because there is no other work to do Continuing with our other derived classes, the concatenation classes will operate similarly to Frame_Pic: Each class will need to remember its two constituent pictures How they were concatenated, vertically or horizontally, will be implicit in the type itself:
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class VCat_Pic: public Pic_base { Ptr<Pic_base> top, bottom; VCat_Pic(const Ptr<Pic_base>& t, const Ptr<Pic_base>& b): top(t), bottom(b) { } wd_sz width() const; ht_sz height() const; void display(std::ostream&, ht_sz, bool) const; }; class HCat_Pic: public Pic_base {
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This document is created with the unregistered version of CHM2PDF Pilot Ptr<Pic_base> left, right; HCat_Pic(const Ptr<Pic_base>& l, const Ptr<Pic_base>& r): left(l), right(r) { } wd_sz width() const; ht_sz height() const; void display(std::ostream&, ht_sz, bool) const; };
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The String_Pic class differs slightly from the others in that it stores a copy of the vector<char> that contains the picture's data:
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class String_Pic: public Pic_base { std::vector<std::string> data; String_Pic(const std::vector<std::string>& v): data(v) { } wd_sz width() const; ht_sz height() const; void display(std::ostream&, ht_sz, bool) const; };
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We still copy the underlying characters from our user's vector parameter v into our own member, which is called data This is the only place in our entire program that copies characters Everywhere else, we copy only Ptr<Pic_base> objects, which copies pointers and manipulates reference counts
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