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--- enum_class_rather_than_typedef_enum [2017/09/01 01:55]
shane
+++ enum_class_rather_than_typedef_enum [2017/09/01 14:01] (current)
shane [Conclusion and analysis]
@@ Line 1: / Line 1: @@
-====== C++ enum declaration vs. C "typedef enum" ======
+====== C++ "enum class" vs. C "typedef enum" ======
+===== The problems =====
 In the 1980's, it was common to use the preprocessor's ''#define'' capability to define //symbolic constants//, as a way of documenting the specific semantic interpretation intended for particular numeric values. For example, one could write:
 <code c>
@@ Line 23: / Line 24: @@
 Animal animal;
 </code>
-This //looks// very nice, and it's tempting to think that we have succeeded in defining a truly new data-type, but in reality this is all just [[wp>Syntactic_sugar|syntactic sugar]]. The C compiler treats ''enum'' variables just like integers (the number of bits used is compiler-specific). The user-specified symbolic names are assigned actual integer values by the compiler in an ordered sequence, usually starting with 0, and are also treated just like integers. That is, the declarations above are entirely equivalent to the older ''#define''-based code above.
+This //looks// very nice, and it's tempting to think that we have succeeded in defining a truly new data-type, but in reality this is nothing but [[wp>Syntactic_sugar|syntactic sugar]]. The C compiler treats ''enum'' variables just like integers (the number of bits used is compiler-specific). The user-specified symbolic names are assigned actual integer values by the compiler in an ordered sequence, usually starting with 0, and are also treated just like integers. That is, the declarations above are entirely equivalent to the older ''#define''-based code above.
+The following assignments would therefore be quite legitimate, and we quickly see that our hopes of defining a truly new data-type are just empty hopes after all:
-The following assignments would therefore be quite legitimate, and we quickly see that our hopes of defining a truly new data-type are nothing but empty hopes after all:
 <code c>
 int x = kDog;        /* assign a symbolic Animal value to an int                   */
@@ Line 32: / Line 32: @@
 </code>
 This is why ''enum'' constant-names are commonly prefixed with a lowercase "k" just like other ''#define''d constants---because that's basically what they are.
+In programming for [[wp>Embedded_system|embedded systems]] (especially [[wp>Microcontroller|microcontrollers]]), ''enum'' declarations are often used as a short-form alternative to groups of ''#define''s, in cases where we would like to use symbolic names for values, but retain the power to specify their exact binary representation. This is fine, and can even be done in C++, but for higher-level programming we would like something a bit cleaner and, well, //higher-level//.
-In programming for [[wp>Embedded_system|embedded systems]] (especially [[wp>Microcontroller|microcontrollers]]), ''enum'' declarations are often used as a short-form alternative to groups of ''#define''s, in cases where we would like to use symbolic names for values, but retain the power to specify their exact binary representation. This is entirely valid, even when carried over to C++.
 The original C++ specification introduced a slightly different ''enum'' declaration syntax for defining ''enum'' types without having to use the ''typedef'' keyword:
@@ Line 39: / Line 38: @@
 enum Animal { kCat, kDog, kCow, kHorse };
 </code>
-Unfortunately, this is semantically equivalent to the C ''typedef'' declaration above, so now we have two different syntaxes defining an enumerated type, //neither of which offers any type-safety//. This has been rectified in the [[wp>C++11|C++11]] standard, with the introduction of //enum classes//. In C++11 and later, we can write
+Unfortunately, this is semantically equivalent to the C ''typedef'' declaration above, so now we have two different syntaxes defining an enumerated type, //neither of which offers any type-safety//. This has finally been rectified in the [[wp>C++11|C++11]] standard, with the introduction of //enum classes//. In C++11 and later, we can write
 <code cpp>
 enum class Animal { cat, dog, cow, horse };
 </code>
 This defines a new data-type ''Animal'', and a group of permissible symbolic-constant values, which work in a type-safe fashion. The "k" prefix is no longer needed, because the ''class'' declaration puts these symbols into their own [[wp>Namespace|namespace]], which we have to use as a prefix, e.g. ''Animal::cat'', which has the nice result that we could use the name ''cat'' as a constant in any number of ''enum class'' types, with no chance of getting confused among them. The C++ compiler will enforce type-safety rules, making it illegal to try to assign, say, a plain integer value to a variable of ''Animal'' type.
+===== Applying enum class in VanillaJuce: case 1 =====
 In an early version of VanillaJuce, the file ''SynthEnvelopeGenerator.h'' included the declaration
@@ Line 69: / Line 70: @@
 and all uses of the symbolic names ''idle'', ''attack'', etc. are now changed to e.g. ''EG_Segment::idle''.
 Moreover, I realized that the ''EG_Segment'' data type and values are not even used anywhere outside the //SynthEnvelopeGenerator// class, so I was able to move the entire ''enum class'' declaration inside the //SynthEnvelopeGenerator// class declaration---into the ''private'' part, in fact.
+This was a nice simple case, because an ''EG_Segment'' is a true //categorical variable//---an element drawn from a finite set of value-items which have no special relationship other than their shared set membership.
+===== Applying enum class in VanillaJuce: case 2 =====
 I had also used a ''typedef enum'' declaration in ''SynthParameters.h'', so I could use the declared type in a few different ''.cpp'' files:
@@ Line 99: / Line 104: @@
 </code>
 I had also used constructions like ''String(WFname[osc1Waveform])'' to convert the value of a ''SynthOscillatorWaveform'' variable (in this case ''osc1Waveform'') to a //juce::String//.
-I cleaned this up by adding a couple of simple functions in ''SynthParameters.cpp'', for serializing and deserializing ''SynthOscillatorWaveform'' values:
+My first thought was that maybe I could add //Serialize// and //Deserialize// member functions to the new ''SynthOscillatorWaveform'' class, but alas, fancy new standards notwithstanding, C++ is still up to its old tricks of reusing keywords---in this case, ''class''---in new ways that don't quite mean what we might expect based on experience. It turns out that C++11's "enum classes" are not //classes// at all, and hence we can't augment them with member functions.
+I ended up just adding a couple of free-standing functions in ''SynthParameters.cpp'', for serializing and deserializing ''SynthOscillatorWaveform'' values:
 <code cpp>
 String Serialize_SynthOscillatorWaveform(SynthOscillatorWaveform wf)
@@ Line 125: / Line 133: @@
 }
 </code>
-In ''GuiOscTab.cpp'', I needed to create similar code to convert between ''SynthOscillatorWaveform'' values and integer indices for the //juce::ComboBox// controls used to select oscillator waveforms, but this was straightforward.
+Then I found I had to write about the same amount of new code, toconvert between ''SynthOscillatorWaveform'' values and integer indices for the //juce::ComboBox// controls used to select oscillator waveforms. This code was starting to smell, and the more I thought about it, the more uncomfortable I was with the fact that I had defined the human-readable string names for waveform types in //four places:// once in each of my new serialize/deserialize functions, and once for each of the //juce::ComboBox// controls in ''GuiOscTab.cpp'':
+<code cpp>
+waveformCB1->addItem (TRANS("Sine"), 1);
+waveformCB1->addItem (TRANS("Triangle"), 2);
+waveformCB1->addItem (TRANS("Square"), 3);
+waveformCB1->addItem (TRANS("Sawtooth"), 4);
+...
+waveformCB2->addItem (TRANS("Sine"), 1);
+waveformCB2->addItem (TRANS("Triangle"), 2);
+waveformCB2->addItem (TRANS("Square"), 3);
+waveformCB2->addItem (TRANS("Sawtooth"), 4);
+</code>
+(The above code was automatically generated by the Projucer.)
+Clearly, I needed to re-think the whole notion of waveform selection.
+===== Re-thinking waveform selection in VanillaJuce =====
+The notion of //waveform// has more to it than just "an element of a finite set" (which is what a C++11 "enum class" models):
+  * The set of waveforms might not always be finite. What if, down the road, I were to add some facility for users to define their own?
+  * For humans, waveforms are identified by //string constants//, i.e., their names, typically through GUI widgets like //juce::ComboBox// which have an inherent linear //order//. These names are also useful when serializing preset-parameters to XML, because the names remain valid even if the inherent order of the chosen binary representation should change.
+  * For program code, a more compact, unambiguous representation is needed, e.g. for my //SynthOscillator::getSample()// function, which is called //very often// and needs a quick way to look up the right chunk of code for the selected waveform.
+The set of available waveforms in a synthesizer is essentially an //ordered collection// of objects which have at least three attributes: a unique integer //index// (basis of the ordering), a human-readable //name//, and some associated //sample-generating code//. I realized that my original instinct to use an integer-indexed representation, with a static array of human-readable names, was correct; I just hadn't implemented it very cleanly.
+I decided to create a new class //SynthWaveform// to encapsulate the notion of an integer waveform //index// and the relationship between indices and human-readable //names//, and allow only the //SynthOscillator// class to make direct use of the index type (for efficient selection of sample-generating code). Here is  //SynthWaveform//:
+<code cpp>
+class SynthWaveform
+{
+private:
+    enum WaveformTypeIndex {
+        kSine, kTriangle, kSquare, kSawtooth,
+        kNumberOfWaveformTypes
+    } index;
+    friend class SynthOscillator;
+public:
+    // default constructor
+    SynthWaveform() : index(kSine) {}
+    // set to default state after construction
+    void setToDefault() { index = kSine; }
+    // serialize: get human-readable name of this waveform
+    String name();
+    // deserialize: set index based on given name
+    void setFromName(String wfName);
+    // convenience funtions to allow selecting SynthWaveform from a juce::comboBox
+    static void setupComboBox(ComboBox& cb);
+    void fromComboBox(ComboBox& cb);
+    void toComboBox(ComboBox& cb);
+private:
+    // waveform names: ordered list of string literals
+    static const char* const wfNames[];
+};
+const char* const SynthWaveform::wfNames[] = {
+    "Sine", "Triangle", "Square", "Sawtooth"
+};
+void SynthWaveform::setFromName(String wfName)
+{
+    for (int i = 0; i < kNumberOfWaveformTypes; i++)
+    {
+        if (wfName == wfNames[i])
+        {
+            index = (WaveformTypeIndex)i;
+            return;
+        }
+    }
+    // Were we given an invalid waveform name?
+    jassertfalse;
+}
+String SynthWaveform::name()
+{
+    return wfNames[index];
+}
+void SynthWaveform::setupComboBox(ComboBox& cb)
+{
+    for (int i = 0; i < kNumberOfWaveformTypes; i++)
+        cb.addItem(wfNames[i], i + 1);
+}
+void SynthWaveform::fromComboBox(ComboBox& cb)
+{
+    index = (WaveformTypeIndex)(cb.getSelectedItemIndex());
+}
+void SynthWaveform::toComboBox(ComboBox& cb)
+{
+    cb.setSelectedItemIndex((int)index);
+}
+</code>
+I have used a traditional C++ ''enum'' type declaration (without the archaic ''typedef''), with full awareness of its near-equivalence to ''int'' and consequent lack of type-safety. I have mitigated the risk considerably, however, by making the //SynthWaveform// type ''private'', accessible only within this class and to the ''friend'' class //SynthOscillator//, which has a genuine pragmatic reason to use it (for efficient selection of code at runtime):
+<code cpp>
+float SynthOscillator::getSample()
+{
+    float sample = 0.0f;
+    switch (waveform.index)
+    {
+    case SynthWaveform::kSine:
+        sample = (float)(std::sin(phase * 2.0 * double_Pi));
+        break;
+    case SynthWaveform::kSquare:
+        sample = (phase <= 0.5) ? 1.0f : -1.0f;
+        break;
+    case SynthWaveform::kTriangle:
+        sample = (float)(2.0 * (0.5 - std::fabs(phase - 0.5)) - 1.0);
+        break;
+    case SynthWaveform::kSawtooth:
+        sample = (float)(2.0 * phase - 1.0);
+        break;
+    }
+    phase += phaseDelta;
+    while (phase > 1.0) phase -= 1.0;
+    return sample;
+}
+</code>
+===== Conclusion and analysis =====
+A C++11 "enum class" is an excellent, type-safe replacement for "typedef enum" when dealing with what amounts to //categorical variables//, which are just symbolic names drawn from a finite, unordered set.
+For waveform selection in **VanillaJuce**, I realized that I was dealing with something considerably more complicated. My first attempt, to blindly apply the substitution ''typedef enum'' -> ''enum class'' resulted in //increased// line-count, and served to highlight what was already messy code. By re-thinking the representation deeply, I was able to achieve reduced line-count //and// cleaner code.

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