CRTP (Curiously Recurring Template Pattern) is a C++ idiom in which a class or struct is defined as a template that inherits from the type that it is templating. One use case of CRTP is to initialize static member variables within the class.
Here's an example of how to initialize a static template member with CRTP:
c
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template <typename Derived>
struct Base {
static int value;
};
template <typename Derived>
int Base<Derived>::value = 0;
struct Derived : Base<Derived> {};
int main() {
std::cout << Derived::value << std::endl;
return 0;
}
In this example, the Base struct is defined as a template that takes a Derived type as its parameter. The Base struct has a static member variable value, which is initialized to 0. The Derived struct inherits from Base<Derived>, so it has access to the static member variable value.
By using CRTP to initialize static template members, you can avoid the issue of order of initialization of static variables, and ensure that the static members are initialized in the correct order.
It's important to note that when initializing static template members with CRTP, the static members are shared among all instances of the derived class. This means that changes made to the static members by one instance of the derived class will affect all other instances of the derived class.
Here's an example to illustrate this:
c
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#include <iostream>
template <typename Derived>
struct Base {
static int value;
};
template <typename Derived>
int Base<Derived>::value = 0;
struct Derived1 : Base<Derived1> {};
struct Derived2 : Base<Derived2> {};
int main() {
std::cout << "Before modification: " << std::endl;
std::cout << "Derived1::value = " << Derived1::value << std::endl;
std::cout << "Derived2::value = " << Derived2::value << std::endl;
Derived1::value = 1;
std::cout << "After modification: " << std::endl;
std::cout << "Derived1::value = " << Derived1::value << std::endl;
std::cout << "Derived2::value = " << Derived2::value << std::endl;
return 0;
}
In this example, two derived classes Derived1 and Derived2 are defined, both of which inherit from Base<Derived1> and Base<Derived2>, respectively. The static member value of Base is set to 0 for both Derived1 and Derived2.
After modification of the value member of Derived1, both Derived1 and Derived2 now have a value of 1. This demonstrates that the static members are shared among all instances of the derived class.
In conclusion, initializing static template members with CRTP can be a useful technique, but it's important to understand the implications of shared static members among instances of the derived class.