My specific question is that when implementing a singleton class in C++, is there any substantial differences between the two below codes regarding performance, side issues or something:
class singleton
{
// ...
static singleton& getInstance()
{
// allocating on heap
static singleton* pInstance = new singleton();
return *pInstance;
}
// ...
};
and this:
class singleton
{
// ...
static singleton& getInstance()
{
// using static variable
static singleton instance;
return instance;
}
// ...
};
(Note that dereferencing in the heap-based implementation should not affect performance, as AFAIK there is no extra machine-code generated for dereferencing. It's seems only a matter of syntax to distinguish from pointers.)
UPDATE:
I've got interesting answers and comments which I try to summarize them here. (Reading detailed answers is recommended for those interested.):
- In the singleton using static local variable, the class destructor is automatically invoked at process termination, whereas in the dynamic allocation case, you have to manage object destruction someway at sometime, e.g. by using smart pointers:
static singleton& getInstance() {
static std::auto_ptr<singleton> instance (new singleton());
return *instance.get();
}
The singleton using dynamic allocation is "lazier" than the static singleton variable, as in the later case, the required memory for the singleton object is (always?) reserved at process start-up (as part of the whole memory required for loading program) and only calling of the singleton constructor is deferred to
getInstance()call-time. This may matter whensizeof(singleton)is large.Both are thread-safe in C++11. But with earlier versions of C++, it's implementation-specific.
The dynamic allocation case uses one level of indirection to access the singleton object, whereas in the static singleton object case, direct address of the object is determined and hard-coded at compile-time.
P.S.: I have corrected the terminology I'd used in the original posting according to the @TonyD's answer.
