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Single Responsibility Principle

Single responsibility principle, or SRP is another of the SOLID guidelines for software designers. It’s especially useful in object-oriented design. The name suggests, that it will have something to do with decomposing the problem up to the point, where each entity in the system has one and only one responsibility. The principle alone states,

“There should never be more than one reason for class to change.”

Right, but where, the heck, is the responsibility we’re talking about? You see, a responsibility can be pretty hard to define and using the word directly would have definitely started a couple of fights. So the author defined it precisely as a ‘reason to change’. Let’s have a look at some example.

Here is a definition of my old MySQL class. It has interface for establishing and closing connection to a remote MySQL server and sending a query and receiving and processing the query result.

class MySQL
{
    public:
        bool connect();
        void disconnect();

        bool executeQuery(std::string queryString);
        MySQLResult* getQueryResult();
};

This class  has two reasons to change (i.e. responsibilities). It handles the initialization and closing of a connection to database server and also communication with the server (executing SQL queries). The two reasons to change are:

  • MySQL server will now accept only encrypted connections
  • The server implementation changes and it will respond differently to some queries

This violates the single responsibility principle. It would be a bad design to put together two things that change for different reasons. It might not seem that bad now, but the system will evolve and change. What now seem reasonable solution might kill you later on. The way I would now design things is this:

class MySQLConnection
{
    public:
        bool open(); /* former connect() */
        void close(); /* former disconnect() */
};

class MySQLQuery
{
    MySQLConnection* session;

    public:
        bool execute(std::string queryString);
        MySQLResult* getResult();
};

While SRP is fairly simple principle, it’s pretty hard to get it right. Putting responsibilities together is something that comes naturally to us and the separation (e.g. splitting the class into several smaller ones) might not seem as elegant at first. When I look back at some of my earlier designs, well, to be honest, I rarely stumble upon a class that conforms to this principle. When I look again, I can really see, how would the separation help reduce the complexity of the design and made my code easier to read and understand.

Following this principle religiously is definitely not a good idea, but it’s good to know it’s there and sometimes (especially, when you see a 500 lines in  my_class.h ) think, ‘Hey, would splitting my class to a couple more help?’. Usually it does :-P.

Sources