Chain of Responsibility em C++
O Chain of Responsibility é um padrão de projeto comportamental que permite passar a solicitação ao longo da cadeia de handlers em potencial até que um deles lide com a solicitação.
O padrão permite que vários objetos tratem a solicitação sem acoplar a classe remetente às classes concretas dos destinatários. A cadeia pode ser composta dinamicamente em tempo de execução com qualquer handler que siga uma interface de handler padrão.
Complexidade:
Popularidade:
Exemplos de uso: O padrão Chain of Responsibility não é um padrão frequente em um programa C++, pois é relevante apenas quando o código opera com cadeias de objetos.
Identificação: O padrão é reconhecível pelos métodos comportamentais de um grupo de objetos que indiretamente chamam os mesmos métodos em outros objetos, enquanto todos os objetos seguem a interface comum.
Exemplo conceitual
Este exemplo ilustra a estrutura do padrão de projeto Chain of Responsibility . Ele se concentra em responder a estas perguntas:
De quais classes ele consiste?
Quais papéis essas classes desempenham?
De que maneira os elementos do padrão estão relacionados?
main.cc: Exemplo conceitual
/**
* The Handler interface declares a method for building the chain of handlers.
* It also declares a method for executing a request.
*/
class Handler {
public:
virtual Handler *SetNext(Handler *handler) = 0;
virtual std::string Handle(std::string request) = 0;
};
/**
* The default chaining behavior can be implemented inside a base handler class.
*/
class AbstractHandler : public Handler {
/**
* @var Handler
*/
private:
Handler *next_handler_;
public:
AbstractHandler() : next_handler_(nullptr) {
}
Handler *SetNext(Handler *handler) override {
this->next_handler_ = handler;
// Returning a handler from here will let us link handlers in a convenient
// way like this:
// $monkey->setNext($squirrel)->setNext($dog);
return handler;
}
std::string Handle(std::string request) override {
if (this->next_handler_) {
return this->next_handler_->Handle(request);
}
return {};
}
};
/**
* All Concrete Handlers either handle a request or pass it to the next handler
* in the chain.
*/
class MonkeyHandler : public AbstractHandler {
public:
std::string Handle(std::string request) override {
if (request == "Banana") {
return "Monkey: I'll eat the " + request + ".\n";
} else {
return AbstractHandler::Handle(request);
}
}
};
class SquirrelHandler : public AbstractHandler {
public:
std::string Handle(std::string request) override {
if (request == "Nut") {
return "Squirrel: I'll eat the " + request + ".\n";
} else {
return AbstractHandler::Handle(request);
}
}
};
class DogHandler : public AbstractHandler {
public:
std::string Handle(std::string request) override {
if (request == "MeatBall") {
return "Dog: I'll eat the " + request + ".\n";
} else {
return AbstractHandler::Handle(request);
}
}
};
/**
* The client code is usually suited to work with a single handler. In most
* cases, it is not even aware that the handler is part of a chain.
*/
void ClientCode(Handler &handler) {
std::vector<std::string> food = {"Nut", "Banana", "Cup of coffee"};
for (const std::string &f : food) {
std::cout << "Client: Who wants a " << f << "?\n";
const std::string result = handler.Handle(f);
if (!result.empty()) {
std::cout << " " << result;
} else {
std::cout << " " << f << " was left untouched.\n";
}
}
}
/**
* The other part of the client code constructs the actual chain.
*/
int main() {
MonkeyHandler *monkey = new MonkeyHandler;
SquirrelHandler *squirrel = new SquirrelHandler;
DogHandler *dog = new DogHandler;
monkey->SetNext(squirrel)->SetNext(dog);
/**
* The client should be able to send a request to any handler, not just the
* first one in the chain.
*/
std::cout << "Chain: Monkey > Squirrel > Dog\n\n";
ClientCode(*monkey);
std::cout << "\n";
std::cout << "Subchain: Squirrel > Dog\n\n";
ClientCode(*squirrel);
delete monkey;
delete squirrel;
delete dog;
return 0;
}
Output.txt: Resultados da execução
Chain: Monkey > Squirrel > Dog
Client: Who wants a Nut?
Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
Monkey: I'll eat the Banana.
Client: Who wants a Cup of coffee?
Cup of coffee was left untouched.
Subchain: Squirrel > Dog
Client: Who wants a Nut?
Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
Banana was left untouched.
Client: Who wants a Cup of coffee?
Cup of coffee was left untouched.
Chain of Responsibility em outras linguagens