Emulating static constructors for templated classes

问题

I would like to have a templated class with a static data member, and initialize it by emulating a "static constructor." For a non-templated class, this has already been answered (see static constructors in C++? I need to initialize private static objects and What is a static constructor?). However, none of the answers seem to work for a templated class.

The following is an example that tries to adapt the "static constructor" idiom from the previous answers to a templated class. (Note that the example is simply initializing an int and could be written without such constructors; however, I require a general solution.)

#include <iostream>

struct Foo
{
    static int x;
    static struct init
    {
        init()
        {
            std::cout << "Initializing Foo..." << std::endl;
            x = 1;
        }
    } initializer;
};
int Foo::x;
Foo::init Foo::initializer;

template<int N>
struct Bar
{
    static int x;
    static struct init
    {
        init()
        {
            std::cout << "Initializing Bar..." << std::endl;
            x = N;
        }
    } initializer;
};

template<int N>
int Bar<N>::x;
template<int N>
typename Bar<N>::init Bar<N>::initializer;

int main()
{
    std::cout << Foo::x << std::endl;
    std::cout << Bar<1>::x << std::endl;
    return 0;
}

This outputs:

Initializing Foo...
1
0

But I expected it to output:

Initializing Foo...
Initializing Bar...
1
1

Is this an example of the "static initialization order fiasco?"

回答1:

No, it is not static initialization order fiasco. It is simply a result of the fact that every member of a template class is a template on it's own, and as such is not instantiated until used.

Your code never uses init member, so init is never instantiated.

However, your problem is easily solved:

#include <iostream>

template<int N>
struct Bar 
{
    static int x;
};

template<int N>
int Bar<N>::x= N;

int main()
{
    std::cout << Bar<1>::x << std::endl;
    return 0;
}

This gives you what you want in a simpler way.

回答2:

You need to explictly instantiate initializer:

[...]
template<int N>
typename Bar<N>::init Bar<N>::initializer;

template
typename Bar<1>::init Bar<1>::initializer;

int main()
{
    std::cout << Foo::x << std::endl;
    std::cout << Bar<1>::x << std::endl;
    return 0;
}

The reason is that Bar<1>::x does not depends on Bar<1>::initializer. So the compiler does not instantiate it as you do not use it. Actualy, initializer initialization does not initialize x. x is first zero initialized, then if initializer is instantiated, x is assigned a new value.

There are no risk of static initialization fiasco as long as initializer is instantiated in the same translation unit as the one where x is instantiated. So it is certainly a good idea to explictly instantiate x too.

Alternatively you could declare these variables as static locals:

#include <iostream>
template<int N>
struct Bar
{
    static int x()
      {
      static int x_val;
      static struct init
        {
        init()
          {
          std::cout << "Initializing Bar..." << std::endl;
          x_val = N;
          }
        } initializer;//indeed this circumvolution is no more needed.
      return x_val;
      }
};
int main(){
    std::cout << Bar<1>::x() << std::endl;
}

But if the initialization is not trivial, the generated code inside x() may be under optimized.

Depending on your problem, you could also define x as a wrapper around an int:

class int_inited{
  int val;
  public:
  int_inited(){
    std::cout << "Perfoming initialization" << std::endl;
    val=42;
    }
  operator int&(){
    return val;
    }
  operator const int &() const{
    return val;
    }
  };

template<class N>
struct Bar{
  static int_inited x;
  [...];

回答3:

I have found a clean solution that works for any data type. Since the assignment operation inside a template is evaluated when the compiler comes across a specific Bar<N>::x to instantiate, we can write:

template<int N>
int Bar<N>::x = init<N>();

where init() is a function templated on N that returns an int. Additionally, init() will only be called once for each value of N that the compiler instantiates.

As a more useful example, here I initialize a static std::array according to some arbitrary function:

#include <iostream>
#include <array>

template<int N>
struct Foo
{
    static std::array<double,N> x;
};

template<int N>
std::array<double,N> init()
{
    std::array<double,N> y;
    for (int i=0; i<N; ++i) {
        y[i] = (double)(i*i+i)/N;
    }
    return y;
}

template<int N>
std::array<double,N> Foo<N>::x = init<N>();

int main()
{
    const int N = 10;
    for (int i=0; i<N; ++i) {
        std::cout << Foo<N>::x[i] << std::endl;
    }
    return 0;
}

来源：https://stackoverflow.com/questions/47703975/emulating-static-constructors-for-templated-classes

标签

c++

templates

constructor

static

static-constructor