More C++ Idioms/Construct On First Use

Construct On First Use Intent Ensure that an object is initialized before its first use. Specifically, ensure that non-local static object is initialized before its first use. Also Known As Lazy construction/evaluation Motivation Static objects that have non-trivial constructors must be initialized before they are used. It is possible to access an uninitialized non-local static object before its initialization if proper care is not exercised. struct Bar { Bar () { cout << "Bar::Bar()\n"; } void f () { cout << "Bar::f()\n"; } }; struct Foo { Foo () { bar_.f (); } static Bar bar_; }; Foo foo; Bar Foo::bar_; int main () {} In the above code, Bar::f() gets called before its constructor. It should be avoided. Solution and Sample Code There are 2 possible solutions, which depends upon whether the destructor of the object in consideration has non-trivial destruction semantics. Wrap the otherwise static object in a function so that function initializes it before it is used. Construct on first use using dynamic allocation struct Foo { Foo () { bar().f (); } Bar & bar () { static Bar *b = new Bar (); return *b; } }; If the object has a destructor with non-trivial semantics, local static object is used instead of dynamic allocation as given below. Construct on first use using local static struct Foo { Foo () { bar().f (); } Bar & bar () { static Bar b; return b; } }; Known Uses Singleton pattern implementations often use this idiom. ACE_TSS<T> class template in Adaptive Communication Environment (ACE) for creating and accessing objects in thread specific storage (TSS) uses this idiom. Related Idioms Nifty/Schwarz Counter References ja:More C++ Idioms/初回使用時生成(Construct On First Use)