Implicit instantiation (C++ only)

Unless a template specialization has been explicitly instantiated or explicitly specialized, the compiler will generate a specialization for the template only when it needs the definition. This is called implicit instantiation.

If the compiler must instantiate a class template specialization and the template is declared, you must also define the template.

For example, if you declare a pointer to a class, the definition of that class is not needed and the class will not be implicitly instantiated. The following example demonstrates when the compiler instantiates a template class:

template<class T> class X {
  public:
    X* p;
    void f();
    void g();
};

X<int>* q;
X<int> r;
X<float>* s;
r.f();
s->g();

The compiler requires the instantiation of the following classes and functions:

Therefore, the functions X<T>::f() and X<T>::g() must be defined in order for the above example to compile. (The compiler will use the default constructor of class X when it creates object r.) The compiler does not require the instantiation of the following definitions:

The compiler will implicitly instantiate a class template specialization if it is involved in pointer conversion or pointer to member conversion. The following example demonstrates this:

template<class T> class B { };
template<class T> class D : public B<T> { };

void g(D<double>* p, D<int>* q)
{
  B<double>* r = p;
  delete q;
}

The assignment B<double>* r = p converts p of type D<double>* to a type of B<double>*; the compiler must instantiate D<double>. The compiler must instantiate D<int> when it tries to delete q.

If the compiler implicitly instantiates a class template that contains static members, those static members are not implicitly instantiated. The compiler will instantiate a static member only when the compiler needs the static member's definition. Every instantiated class template specialization has its own copy of static members. The following example demonstrates this:

template<class T> class X {
public:
   static T v;
};

template<class T> T X<T>::v = 0;

X<char*> a;
X<float> b;
X<float> c;

Object a has a static member variable v of type char*. Object b has a static variable v of type float. Objects b and c share the single static data member v.

An implicitly instantiated template is in the same namespace where you defined the template.

If a function template or a member function template specialization is involved with overload resolution, the compiler implicitly instantiates a declaration of the specialization.

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