template<typename T>
class drake::never_destroyed< T >
Wraps an underlying type T such that its storage is a direct member field of this object (i.e., without any indirection into the heap), but unlike most member fields T's destructor is never invoked.
This is especially useful for function-local static variables that are not trivially destructable. We shouldn't call their destructor at program exit because of the "indeterminate order of ... destruction" as mentioned in cppguide's Static and Global Variables section, but other solutions to this problem place the objects on the heap through an indirection.
Compared with other approaches, this mechanism more clearly describes the intent to readers, avoids "possible leak" warnings from memory-checking tools, and is probably slightly faster.
Example uses:
The singleton pattern:
class Singleton {
public:
static Singleton& getInstance() {
static never_destroyed<Singleton> instance;
return instance.access();
}
private:
friend never_destroyed<Singleton>;
Singleton() = default;
};
A lookup table, created on demand the first time its needed, and then reused thereafter:
enum class Foo { kBar, kBaz };
Foo ParseFoo(const std::string& foo_string) {
using Dict = std::unordered_map<std::string, Foo>;
std::initializer_list<Dict::value_type>{
{"bar", Foo::kBar},
{"baz", Foo::kBaz},
}
};
return string_to_enum.
access().at(foo_string);
}
In cases where computing the static data is more complicated than an initializer_list, you can use a temporary lambda to populate the value:
const std::vector<double>& GetConstantMagicNumbers() {
std::vector<double> prototype;
std::mt19937 random_generator;
for (int i = 0; i < 10; ++i) {
double new_value = random_generator();
prototype.push_back(new_value);
}
return prototype;
}()};
return result.access();
}
Note in particular the () after the lambda. That causes it to be invoked.
