Ensuring a static member of a class is constructed

Question

Consider the following code

#include <iostream>
using namespace std;

struct Printer{
    Printer(){
        std::cout << "Created\n";
    }
};

template<class Derived>
struct InitPrinter{
    static Printer p;
};

template<class Derived>
Printer InitPrinter<Derived>::p;


struct MyClass:InitPrinter<MyClass>{
     MyClass(){}

};

// Uncomment line below to print out created
//auto& p = MyClass::p;

int main() {
    return 0;
}

I expected that this would print out "Created", however, it does not print out anything (tested with MSVC and with ideone gcc c++11). Is this a compiler implementation issue, or is this behavior supported by the standard? If the commented out line is uncommented then it prints out as expected. Is there any way to the static Printer p to be instantiated without requiring either changes to MyClass or extra statements like the auto& p = MyClass::p?

The reason I am interested in this is I am looking to have create a templated base class, that will run some code at startup when it is derived from.

[temp.inst]/10 "An implementation shall not implicitly instantiate a function template, [...] or a static data member of a class template that does not require instantiation." And deriving from a class template specialization does not require instantiating (the definition of) its static data members. — dyp, Nov 13 '13 at 21:27
I got it working by changing auto& p to Printer p, but I guess that is not what you wanted :P — John Odom, Nov 13 '13 at 21:29
@DyP Lazy enough to read the question and look up the relevant specification... — paddy, Nov 13 '13 at 21:29
Related / solution: http://stackoverflow.com/q/6420985/420683 duplicate (of duplicates): http://stackoverflow.com/q/10333871/420683 — dyp, Nov 13 '13 at 21:34
@DyP Looking at the links you provided, it still looks like there is no standard guaranteed way to do what I want. Thanks for looking this up. — John Bandela, Nov 13 '13 at 21:43

score 2 · Accepted Answer · answered Nov 13 '13 at 21:30

The appropriate quote is [temp.inst]/2

Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; in particular, the initialization (and any associated side-effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist.

emphasis mine.

There's also [temp.inst]/1

The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions or default arguments, of the class member functions, member classes, scoped member enumerations, static data members and member templates [...]

and [temp.inst]/10

An implementation shall not implicitly instantiate a function template, [...] or a static data member of a class template that does not require instantiation.

Ensuring a static member of a class is constructed

1 Answers1