C++ In Theory: The Singleton Pattern, Part I - The Meyers Singleton
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Luckily there is another solution that is very easy to implement and we call it the Meyers Singleton [Meyers]:
class Log {
public:
static Log& Instance() {
static Log theLog;
return theLog;
}
void Write(char const *logline);
bool SaveTo(char const *filename);
private:
Log(); // ctor is hidden
Log(Log const&); // copy ctor is hidden
Log& operator=(Log const&); // assign op is hidden
static std::list<std::string> m_data;
};
This construction relies on the fact that function-static objects are only initialized when the function is first being called upon; thus we maintain the benefit of dynamic initialization. (Function-static primitive variables like static int number=100; do get translated during compile time however!)
An additional safeguard is the returned reference instead of the pointer. The caller cannot be tempted to delete the singleton (as was the case when we were returning a pointer). The constructor is hidden, making it impossible for a user to instantiate a Log object; it would still be possible to assign the singleton Log to itself, however, and since this makes no sense we might just as well hide the assignment operator.
Log::Instance().Write(“Test line”); // works fine
Log &myLog = Log::Instance(); // works fine
Log another = myLog; // FAILS
myLog = myLog; // works until we hide the assign ops!
To wrap it all up, here is an abstract of the Meyers Singleton:
class Singleton {
public:
static Singleton& Instance() {
static Singleton theSingleton;
return theSingleton;
}
/* more (non-static) functions here */
private:
Singleton(); // ctor hidden
Singleton(Singleton const&); // copy ctor hidden
Singleton& operator=(Singleton const&); // assign op. hidden
~Singleton(); // dtor hidden
};
In the next article in this series, we will look at how to generalize this class, and at the construction/destruction sequencing problems.
REFERENCES
[Gamma] Design Patterns: E.Gamma, R.Helm, R.Johnson and J.Vlissides.
[Meyers] More Effective C++: S.Meyers.
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