Object Oriented Software Design II
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- Beverley Peters
- 5 years ago
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1 Object Oriented Software Design II Real Application Design Christian Nastasi Scuola Superiore Sant Anna Pisa April 25, 2012 C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
2 Outline 1 Previously... 2 Non-local variable initialization 3 Application to Factories 4 Initialization Order Fiasco 5 Construct On First Use idiom 6 Back to the duck-lab C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
3 Outline 1 Previously... 2 Non-local variable initialization 3 Application to Factories 4 Initialization Order Fiasco 5 Construct On First Use idiom 6 Back to the duck-lab C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
4 Data AcQuisition System A basic DAQ system duck-lab: an open-source DAQ framework Project (temporary) home page: Mercurial revision control: (hg clone C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
5 duck-lab architecture I/O Library Core Library Storage Library Streamer Storage Vector... 0xFF 0xAF 0x47... Formatter Data Vector D1 D2 D3... Dn S1 S2 S3... Sm XML Configuration We have defined a hierarchy for Data We have defined the interaction with the Formatter, implemented by the FormatHandler class C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
6 duck-lab architecture Metadata #type: std::string #type_size: unsigned long #length: unsigned long +get_type(): std::string +get_type_size(): unsigned long +get_len(): unsigned long +get_tot_size(): unsigned long +~Metadata() #name: std::string Data +get_name(): std::string +format_data(buff:const unit8_t*,len:unsigned long): unsigned long data_vector <<bind>> <int8_t> T: DataTemplate +actual_data: T[] +DataTemplate() +~DataTemplate() <<bind>> <uint16_t> <<bind>> <float> +data_vector: Data[] FormatHandler DataInt8 DataUInt16 DataFloat32 -FormatHandler() +~FormatHandler() -format(in rawdata:uint8_t*,in len:unsigned long) C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
7 The FormatHandler class We have done Data creation/destruction We have done Data formatting class FormatHandler { public: FormatHandler(); ~FormatHandler(); void format(const uint8_t* rawdata, unsigned long len); private: std::vector<data*> data_vector; ; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
8 The FormatHandler class We have done Data creation/destruction We have done Data formatting class FormatHandler { public: FormatHandler(); ~FormatHandler(); void format(const uint8_t* rawdata, unsigned long len); private: std::vector<data*> data_vector; ; Factory Method Pattern with string-based look-up table (std::map) (almost good) Polymorphic destruction (good!) Polymorphic formatting with delegation to the Data classes (good!) C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
9 The FormatHandler class We have done Data creation/destruction We have done Data formatting class FormatHandler { public: FormatHandler(); ~FormatHandler(); void format(const uint8_t* rawdata, unsigned long len); private: std::vector<data*> data_vector; ; Factory Method Pattern with string-based look-up table (std::map) (almost good) Polymorphic destruction (good!) Polymorphic formatting with delegation to the Data classes (good!) So, given the following configuration (XML) <message_description> <data data_type="uint16" data_name="data_name_1" data_len="2"/> <data data_type="float32" data_name="data_name_2" data_len="1"/> <data data_type="int8" data_name="data_name_3" data_len="4"/> </message_description> C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
10 FormatHandler ctor and dtor DataExample5-to-exercise-solution/FormatHandler.cc FormatHandler::FormatHandler() { // We create the data vector according to XML config vector<xmldataelement> config = parse_xml_data(); vector<xmldataelement>::const_iterator it; for (it = config.begin(); it!= config.end(); ++it) { Data *d = DataFactory::create(it->type, it->name, it->len); if (!d) { cout << "Unable to create data type = " << it->type << "! Skipping" << endl; continue; data_vector.push_back(d); DataExample5-to-exercise-solution/FormatHandler.cc FormatHandler::~FormatHandler() { // We have to destroy the data vector properly vector<data*>::iterator it; for (it = data_vector.begin(); it!= data_vector.end(); ++it) delete *it; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
11 DataFactory Has anybody finished, if started, doing last exercise? C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
12 DataFactory Has anybody finished, if started, doing last exercise? DataExample5-to-exercise-solution/DataFactory.h class DataFactory { public: typedef Data* (*creator_type_t)(const std::string&, unsigned long); static Data* create(const std::string& id, const std::string& name, unsigned long len); private: static std::map<std::string, creator_type_t> lookup_table; ; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
13 DataFactory Has anybody finished, if started, doing last exercise? DataExample5-to-exercise-solution/DataFactory.h class DataFactory { public: typedef Data* (*creator_type_t)(const std::string&, unsigned long); static Data* create(const std::string& id, const std::string& name, unsigned long len); private: static std::map<std::string, creator_type_t> lookup_table; ; DataExample5-to-exercise-solution/DataFactory.cc std::map<string, DataFactory::creator_type_t> DataFactory::lookup_table; Data* DataFactory::create(const string& id, const string& n, unsigned long len) { if (lookup_table.empty()) { lookup_table["int8"] = DataInt8::create; lookup_table["uint16"] = DataUInt16::create; lookup_table["float32"] = DataFloat32::create; map<string, creator_type_t>::iterator elem = lookup_table.find(id); if (elem == lookup_table.end()) { cout << "Fatory Creation Error: Creator not registered" " for ID=" << id << endl; return NULL; return elem->second(n, len); C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
14 Outline 1 Previously... 2 Non-local variable initialization 3 Application to Factories 4 Initialization Order Fiasco 5 Construct On First Use idiom 6 Back to the duck-lab C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
15 Definitions Static storage duration Variables shall last for the entire duration of the program Variables with constructor/destructor having side effects cannot be eliminated C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
16 Definitions Static storage duration Variables shall last for the entire duration of the program Variables with constructor/destructor having side effects cannot be eliminated Non-local variables Typically known as global variables Either with static storage duration... or thread storage duration (defined in C++11) C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
17 Definitions Static storage duration Variables shall last for the entire duration of the program Variables with constructor/destructor having side effects cannot be eliminated Non-local variables Typically known as global variables Either with static storage duration... or thread storage duration (defined in C++11) Non-local variables with static storage duration are initialized as a consequence of program initiation. [C++11 standard: n3242, sec 3.6.2] C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
18 Variables with static storage duration Three possible cases: Non-local variables int var1; static float var2; int main(void) { return 0; Local variables void f(void) { static int var = 123; //... Static class data member (treated as non-local variable) class Test { //... static int var; ; int Test::var; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
19 Variables with static storage duration Three possible cases: Non-local variables int var1; static float var2; int main(void) { return 0; Static as linkage rule Local variables void f(void) { static int var = 123; //... Declaration of static durarion Static class data member (treated as non-local variable) class Test { //... static int var; ; int Test::var; Declare static class member Necessary to define (allocate) data member Notice the different usages of static C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
20 Initialization Type Two possible initializations C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
21 Initialization Type Two possible initializations static initialization zero initialization int var; int* ptr; int array[3]; float real; int main(void) {return 0; constant initialization int var = 123; int* ptr = &var1; int array[3] = {1, 2, 3; int main(void) {return 0; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
22 Initialization Type Two possible initializations static initialization zero initialization int var; int* ptr; int array[3]; float real; int main(void) {return 0; constant initialization int var = 123; int* ptr = &var1; int array[3] = {1, 2, 3; int main(void) {return 0; dynamic initialization: all other initializations inline int f() {return 123; int var = f(); float real = 3.14; // Dynamic Initilization! int main(void) {return 0; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
23 Initialization: Static vs Dynamic Considering non-local variable with static storage duration Static initialization Initialization value computed at compile time All static initializations occur before any dynamic initialization C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
24 Initialization: Static vs Dynamic Considering non-local variable with static storage duration Static initialization Initialization value computed at compile time All static initializations occur before any dynamic initialization Dynamic initialization Initialization value computed at runtime C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
25 Initialization: Static vs Dynamic Considering non-local variable with static storage duration Static initialization Initialization value computed at compile time All static initializations occur before any dynamic initialization Dynamic initialization Initialization value computed at runtime Under certain circumstances static initialization might be used in place of dynamic one C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
26 Initialization: Static vs Dynamic Considering non-local variable with static storage duration Static initialization Initialization value computed at compile time All static initializations occur before any dynamic initialization Dynamic initialization Initialization value computed at runtime Under certain circumstances static initialization might be used in place of dynamic one Initialization might occur before first statement of main, otherwise... it shall occur before the first odr-use of any function or variable defined in the same translation unit as the variable to be initialized. [C++11 standard: n3242, sec 3.6.2, par 4] A non-local variable with static storage duration having initialization with side-effects must be initialized even if it is not odr-used [C++11 standard: n3242, sec 3.6.2, note 34] C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
27 Initialization: Static vs Dynamic Considering non-local variable with static storage duration Static initialization Initialization value computed at compile time All static initializations occur before any dynamic initialization Dynamic initialization Initialization value computed at runtime Under certain circumstances static initialization might be used in place of dynamic one Initialization might occur before first statement of main, otherwise... it shall occur before the first odr-use of any function or variable defined in the same translation unit as the variable to be initialized. [C++11 standard: n3242, sec 3.6.2, par 4] A non-local variable with static storage duration having initialization with side-effects must be initialized even if it is not odr-used [C++11 standard: n3242, sec 3.6.2, note 34] Note: Variables might be static- (zero-) initialized first and dynamic-initialized later. User-defined types variable (classes) are subject to static and dynamic initialization too. C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
28 User-defined type: static initialization if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits [C++11 standard: n3242, sec 8.5, par 5] C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
29 User-defined type: static initialization if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits struct Simple { int var; int* ptr; char str[10]; ; Simple s; int main(void) { cout << "Simple has: " << s.var << " " << s.ptr << " " << s.str << " " << endl; return 0; [C++11 standard: n3242, sec 8.5, par 5] C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
30 User-defined type: static initialization if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits struct Simple { int var; int* ptr; char str[10]; ; Simple s; int main(void) { cout << "Simple has: " << s.var << " " << s.ptr << " " << s.str << " " << endl; return 0; [C++11 standard: n3242, sec 8.5, par 5] Simple has: 0 0 C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
31 User-defined type: dynamic initialization When zero-initializtion constuction is not sufficient... struct Simple { int var; int* ptr; char str[10]; Simple() : var(123), ptr(&var) { strcpy(str, "Hello"); ; Simple s; int main(void) { cout << "Simple has: " << s.var << " " << s.ptr << " " << s.str << " " << endl; return 0; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
32 User-defined type: dynamic initialization When zero-initializtion constuction is not sufficient... struct Simple { int var; int* ptr; char str[10]; Simple() : var(123), ptr(&var) { strcpy(str, "Hello"); ; Simple s; User-provided constructor Requesting a run-time call to the user-provided constructor int main(void) { cout << "Simple has: " << s.var << " " << s.ptr << " " << s.str << " " << endl; return 0; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
33 User-defined type: dynamic initialization When zero-initializtion constuction is not sufficient... struct Simple { int var; int* ptr; char str[10]; Simple() : var(123), ptr(&var) { strcpy(str, "Hello"); ; Simple s; User-provided constructor Requesting a run-time call to the user-provided constructor int main(void) { cout << "Simple has: " << s.var << " " << s.ptr << " " << s.str << " " << endl; return 0; Simple has: 123 0x6013a0 Hello C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
34 And now? The question is: why do we care about all this stuff? C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
35 And now? The question is: why do we care about all this stuff? It turns out that non-local variable initialization can be exploited to execute code at start-up. C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
36 And now? The question is: why do we care about all this stuff? It turns out that non-local variable initialization can be exploited to execute code at start-up. Suppose to have: Dynamic initialization might occur before first statement of main, otherwise... it shall occur before the first odr-use of any function or variable defined in the same translation unit as the variable to be initialized. A non-local variable with static storage duration having initialization with side-effects must be initialized even if it is not odr-used a non-local variable of user-defined type (class) which a user-provided constructor which has side-effects (produces changes in the execution environment) C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
37 Example struct MyClass { MyClass() { cout << "Make world..." << endl; //... //... ; MyClass c; int main(void) { cout << "Hello world!" << endl; return 0; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
38 Example struct MyClass { MyClass() { cout << "Make world..." << endl; //... //... ; MyClass c; int main(void) { cout << "Hello world!" << endl; return 0; Make world... Hello world! C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
39 Outline 1 Previously... 2 Non-local variable initialization 3 Application to Factories 4 Initialization Order Fiasco 5 Construct On First Use idiom 6 Back to the duck-lab C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
40 The Disney s cartoons problem The user wants to animate cartoons The user does not care about their creation process C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
41 C++ implementation FactoryExample4.h class DisneyProduct { public: virtual ~DisneyProduct() {; inline std::string get_name(void) const { return name; ; protected: DisneyProduct(const std::string& n) : name(n) {; private: DisneyProduct(const DisneyProduct&); DisneyProduct &operator=(const DisneyProduct&); std::string name; ; FactoryExample4.h class Goofy : public DisneyProduct { private: Goofy() : DisneyProduct("goofy") {; static DisneyProduct* create(void) { return new Goofy(); ; friend class DisneyFactory; ; class Pluto : public DisneyProduct { private: Pluto() : DisneyProduct("pluto") {; static DisneyProduct* create(void) { return new Pluto(); ; friend class DisneyFactory; ; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
42 C++ implementation FactoryExample4.h class DisneyUser { public: DisneyUser(); ~DisneyUser(); void animate(void); private: std::vector<disneyproduct*> toons; ; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
43 C++ implementation FactoryExample4.cc DisneyUser::DisneyUser() { const char* config[] = {"goofy", "pluto", "pluto", "invalid_id"; for (size_t i = 0; i < 4; i++) { DisneyProduct *p = DisneyFactory::create(config[i]); if (p) toons.push_back(p); DisneyUser::~DisneyUser() { vector<disneyproduct*>::iterator it; for (it = toons.begin(); it!= toons.end(); ++it) delete *it; void DisneyUser::animate(void) { vector<disneyproduct*>::const_iterator it; for (it = toons.begin(); it!= toons.end(); ++it) cout << "Hullo, my name is " << (*it)->get_name() << endl; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
44 C++ implementation FactoryExample4.h class DisneyFactory { public: static DisneyProduct* create(const std::string& id); private: typedef DisneyProduct* (*creator_type_t)(void); static std::map<std::string, creator_type_t> lookup_table; ; FactoryExample4.cc map<std::string, DisneyFactory::creator_type_t> DisneyFactory::lookup_table; DisneyProduct* DisneyFactory::create(const string& id) { if (lookup_table.empty()) { lookup_table["goofy"] = Goofy::create; lookup_table["pluto"] = Pluto::create; if (lookup_table.find(id) == lookup_table.end()) return NULL; return lookup_table[id](); C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
45 Consideration What about extensibility? Adding new DisneyProduct requires the following modifications: C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
46 Consideration What about extensibility? Adding new DisneyProduct requires the following modifications: 1 definition of the new DisneyProduct (e.g. DonalDuck) C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
47 Consideration What about extensibility? Adding new DisneyProduct requires the following modifications: 1 definition of the new DisneyProduct (e.g. DonalDuck) 2 adding a new entry in the look-up table... if (lookup_table.empty()) { lookup_table["goofy"] = Goofy::create; lookup_table["pluto"] = Pluto::create; lookup_table["donal duck"] = DonalDuck::create; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
48 Consideration What about extensibility? Adding new DisneyProduct requires the following modifications: 1 definition of the new DisneyProduct (e.g. DonalDuck) 2 adding a new entry in the look-up table... if (lookup_table.empty()) { lookup_table["goofy"] = Goofy::create; lookup_table["pluto"] = Pluto::create; lookup_table["donal duck"] = DonalDuck::create;... in the DisneyFactory class implementation C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
49 Consideration What about extensibility? Adding new DisneyProduct requires the following modifications: 1 definition of the new DisneyProduct (e.g. DonalDuck) 2 adding a new entry in the look-up table... if (lookup_table.empty()) { lookup_table["goofy"] = Goofy::create; lookup_table["pluto"] = Pluto::create; lookup_table["donal duck"] = DonalDuck::create;... in the DisneyFactory class implementation Using non-local variable initialization we can avoid step 2! C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
50 Factory with automatic registration The DisneyFactory shall contain the logic to register the creation method in the look-up table. This registration is performed at run-time and at start-up C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
51 Factory with automatic registration The DisneyFactory shall contain the logic to register the creation method in the look-up table. This registration is performed at run-time and at start-up But where do we write this code? C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
52 Factory with automatic registration The DisneyFactory shall contain the logic to register the creation method in the look-up table. This registration is performed at run-time and at start-up But where do we write this code? In the constructor, of course! FactoryExample5.h class DisneyFactory { public: typedef DisneyProduct* (*creator_type_t)(void); DisneyFactory(const std::string& id, creator_type_t func); static DisneyProduct* create(const std::string& id); private: static std::map<std::string, creator_type_t> lookup_table; ; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
53 Factory with automatic registration FactoryExample5.cc map<std::string, DisneyFactory::creator_type_t> DisneyFactory::lookup_table; DisneyFactory::DisneyFactory(const string& id, creator_type_t func) { cout << "Factory: registrating creator for id=" << id << endl; if (lookup_table.find(id)!= lookup_table.end()) { cout << "Error: ID already in look-up table" << endl; return; lookup_table[id] = func; DisneyProduct* DisneyFactory::create(const string& id) { if (lookup_table.find(id) == lookup_table.end()) return NULL; return lookup_table[id](); C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
54 Factory with automatic registration Each concrete DisneyProduct class shall automatically register itself... just defining a non-local variable with static storage duration of type DisneyFactory. class Goofy : public DisneyProduct { public: static DisneyProduct* create(void) { return new Goofy(); ; private: Goofy() : DisneyProduct("goofy") {; ; // In the implementation file... DisneyFactory register_goofy("goofy", Goofy::create); The full example is:./examples/10.real-application_moreoncreation-examples/fact C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
55 Outline 1 Previously... 2 Non-local variable initialization 3 Application to Factories 4 Initialization Order Fiasco 5 Construct On First Use idiom 6 Back to the duck-lab C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
56 Initialization order Variables with ordered initialization defined within a single translation unit shall be initialized in the order of their definitions in the translation unit. [C++11 standard: n3242, sec 3.6.2, par 2] In other words: global variables are initialized following the order of appearance in the source file; but the initialization order is not guaranteed between variables of different source files! C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
57 Initialization order fiasco Goofy.cc class Goofy : public DisneyProduct { public: static DisneyProduct* create(void) { return new Goofy(); ; private: Goofy() : DisneyProduct("goofy") {; ; DisneyFactory reg("goofy", Goofy::create); DisneyFactory.cc map<std::string, DisneyFactory::creator_type_t> DisneyFactory::lookup_table; DisneyFactory::DisneyFactory(const string& id, creator_type_t func) { if (lookup_table.find(id)!= lookup_table.end()) { cout << "Error: already in look-up table" << endl; return; lookup_table[id] = func; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
58 How to prevent the fiasco The problem of previous example: If DisneyFactory::lookup_table is not initialized before the non-local object reg... then the call to the constructor (registration mechanism) DisneyFactory::DisneyFactory(...) fails! C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
59 How to prevent the fiasco The problem of previous example: If DisneyFactory::lookup_table is not initialized before the non-local object reg... then the call to the constructor (registration mechanism) DisneyFactory::DisneyFactory(...) fails! Solution for the previous example? C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
60 How to prevent the fiasco The problem of previous example: If DisneyFactory::lookup_table is not initialized before the non-local object reg... then the call to the constructor (registration mechanism) DisneyFactory::DisneyFactory(...) fails! Solution for the previous example? Make DisneyFactory::lookup_table a pointer to map rather than a map object. As a pointer it shall be zero-initialized (static initialization) before any dynamic initialization. Delay the creation of the actual map until it is necessary: when it is used by DisneyFactory::DisneyFactory(...). C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
61 How to prevent the fiasco DisneyFactory.cc map<std::string, DisneyFactory::creator_type_t>* DisneyFactory::lookup_table; DisneyFactory::DisneyFactory(const string& id, creator_type_t func) { if (!lookup_table) lookup_table = new map<std::string, creator_type_t>(); if (lookup_table->find(id)!= lookup_table->end()) { cout << "Error: already in look-up table" << endl; return; (*lookup_table)[id] = func; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
62 How to prevent the fiasco DisneyFactory.cc map<std::string, DisneyFactory::creator_type_t>* DisneyFactory::lookup_table; DisneyFactory::DisneyFactory(const string& id, creator_type_t func) { if (!lookup_table) lookup_table = new map<std::string, creator_type_t>(); if (lookup_table->find(id)!= lookup_table->end()) { cout << "Error: already in look-up table" << endl; return; (*lookup_table)[id] = func; The generalization of this approach is known as Construct On First Use idiom. C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
63 Outline 1 Previously... 2 Non-local variable initialization 3 Application to Factories 4 Initialization Order Fiasco 5 Construct On First Use idiom 6 Back to the duck-lab C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
64 Construct On First Use Idiom Objective: Ensure that an object is initialized before its first use. Specifically, ensure that a non-local static object is initialized before its first use. Solution: Encapsulate the creation of the critical object in a function. Basically the object is now a local variable with static storage duration which is initialized only when the function is called! Also known as lazy initialization/evaluation (not at startup) MyClass& getmyobj() { static MyClass* p = new MyClass; return *p; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
65 Example [ Barney.cc x.cc #include "Fred.h" Fred x; y.cc #include "Barney.h" Barney y; #include "Barney.h" Barney::Barney() { //... x.gobowling(); //... C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
66 Example [ Barney.cc x.cc #include "Fred.h" Fred x; x.cc #include "Fred.h" y.cc #include "Barney.h" Barney y; #include "Barney.h" Barney::Barney() { //... x.gobowling(); //... Barney.cc #include "Barney.h" Fred& x() { static Fred* ans = new Fred(); return *ans; Barney::Barney() { //... x().gobowling(); //... C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
67 Alternative implementation... Dynamic allocation #include "Fred.h" Fred& x() { static Fred* ans = new Fred(); return *ans; Local static #include "Fred.h" Fred& x() { static Fred ans; return ans; The destructor for ans is not called, ever! Not good if destructor is non-trivial. The destructor for ans is called on program termination. Might generate destruction order fiasco! C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
68 Outline 1 Previously... 2 Non-local variable initialization 3 Application to Factories 4 Initialization Order Fiasco 5 Construct On First Use idiom 6 Back to the duck-lab C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
69 DataFactory with automatic registration The DataFactory class shall have a constructor to register the creator functions. The creator functions shall be defined by each concrete data-class: DataInt8 DataUInt16 DataFloat32... A utility macro might be used to generate the code for the creator function given a public constructor. The full example is:./examples/10.real-application_moreoncreation-examples/data C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
70 Consideration What about extensibility? Adding new Data requires the following modifications: C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
71 Consideration What about extensibility? Adding new Data requires the following modifications: 1 definition of the new concrete Data (e.g. DataFloat64, DataImageRGB) C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
72 Consideration What about extensibility? Adding new Data requires the following modifications: 1 definition of the new concrete Data (e.g. DataFloat64, DataImageRGB) 2 adding the automatic registration (one line if using the macro) C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
73 Consideration What about extensibility? Adding new Data requires the following modifications: 1 definition of the new concrete Data (e.g. DataFloat64, DataImageRGB) 2 adding the automatic registration (one line if using the macro) Notice: Both changes are to the implementation file of the new class! No more changes are required to other files! C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
74 Different creation parameters We have now an extensible solution with respect to new Data. C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
75 Different creation parameters We have now an extensible solution with respect to new Data. However, the creation function accepts always the same parameters! DataExample6/DataFactory.h class DataFactory { public: typedef Data* (*creator_type_t)(const std::string&, unsigned long); DataFactory(const std::string& id, creator_type_t func); Fine for basic data types... DataExample6/DataInt8.h class DataInt8 : public DataTemplate<int8_t> { public: DataInt8(const std::string& name, unsigned long len); ; C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
76 Different creation parameters Consider the DataImageRGB example... a new concrete data to manage images in RGB format class DataImageRGB : public Data { public: DataImageRGB(const std::string& name, unsigned long len, unsigned width, unsigned height) : Data("ImageRGB", name, width * height * 3, len) { // Allocate internal representation // for RGB image with resolution width x height //... ; ~DataImageRGB() { // Deallocations... ; ; unsigned long format_data(const uint8_t* buff, unsigned long len) { // Formatting... ; //... The constructor requires two more params: width and height C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
77 Different creation parameters The parameters width and height should be coded in the XML configuration file <message_description> <data data_type="imagergb" data_name="data_name_3" data_len="1"> <RGB_config width="160" height="120"> </data> </message_description> C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
78 Different creation parameters The parameters width and height should be coded in the XML configuration file <message_description> <data data_type="imagergb" data_name="data_name_3" data_len="1"> <RGB_config width="160" height="120"> </data> </message_description> Problem: is the configuration logic: DataExample5-to-exercise-solution/FormatHandler.cc FormatHandler::FormatHandler() { // We create the data vector according to XML config vector<xmldataelement> config = parse_xml_data(); vector<xmldataelement>::const_iterator it; for (it = config.begin(); it!= config.end(); ++it) { Data *d = DataFactory::create(it->type, it->name, it->len); if (!d) { cout << "Unable to create data type = " << it->type << "! Skipping" << endl; continue; data_vector.push_back(d); C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
79 Different creation parameters The parameters width and height should be coded in the XML configuration file <message_description> <data data_type="imagergb" data_name="data_name_3" data_len="1"> <RGB_config width="160" height="120"> </data> </message_description> Problem: is the configuration logic: DataExample5-to-exercise-solution/FormatHandler.cc FormatHandler::FormatHandler() { // We create the data vector according to XML config vector<xmldataelement> config = parse_xml_data(); vector<xmldataelement>::const_iterator it; for (it = config.begin(); it!= config.end(); ++it) { Data *d = DataFactory::create(it->type, it->name, it->len); if (!d) { cout << "Unable to create data type = " << it->type << "! Skipping" << endl; continue; data_vector.push_back(d); Solution: Delegate the XML parsing to the Factory... and to enforce extensibility, eventually delegate to the concrete data class! C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
80 duck-lab approach The solution adopted in the duck-lab uses a two-way factory : A Parser class is defined to access the XML A DataConfig class is defined to store the config information retrieved through the Parser The Data class is created from the DataConfig Motivation: separating the concrete Data class from its configuration. Parser DataConfig Data XML Configuration Factory 1 Factory 2 C. Nastasi (Scuola Superiore Sant Anna) C++ Intro April 25, / 44
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