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?"
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;
}
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