Is it possible to generate constant value during compilation? - c++

Question

I would like my classes to be identified each type by an unique hash code. But I don't want these hashed to be generated every time a method, eg. int GetHashCode(), is invoked during runtime. I'd like to use already generated constants and I was hoping there is a way to make the compiler do some come computing and set these constants. Can it be done using templates? Could you give me some example, if it is possible.
UPDATE:
Thanks to kriss' comment I realized my question should go like this:
How to do the type checking with the lowest runtime cost possible?
I'd like to check a pointer to an object against a class type. Just the classes I implement in my libs, so I was thinking of some custom hashing, thus the original question. I did consider using typeid but I am unaware of the runtime cost of using it. I made an assumption that since typeid produces a type_info class that would be more consuming than simple comparision of unique int values.

Answer 1

You can do it with boost.MPL.

Answer 2

I would go simple route:
For classes that would be static property - so just pick a number for each class.
For instances - just use the address.

Answer 3

Static const's are evaluated at compile time - which is pretty much the basis for metaprogramming at large. Moreover, type_info::hash_code is particularly adapted for your needs, so try -
class MyClass
{
static const size_t TypeHashCode = typeid(MyClass).hash_code();
...
}
(I'm not around a compiler right now, so this may take some refining. Will try and recheck tomorrow)
EDIT: indeed, it is not only MS specific but also added only in VS2010 - but hey, at least MS agrees this is a valid need. If you don't allow both VS2010 and boost in your code - you're pretty much left with the standard compliant facilities: typeid or dynamic_cast. They do incur some overhead, but I'd take extra care to verify this overhead is indeed a worthy battle. (my money goes to - not.)

Answer 4

All such classes share something common. Then why not add a symbolic constant in a common enum for each one, you'll leave the enum give values for you, it's easier than giving explicit constants (you still have to declare each class in the enum).

Answer 5

template<class T>
struct provide_hash_code_for_class
{
public:
static uintptr_t GetHashCode()
{
return(reinterpret_cast<uintptr_t>(&unused));
}
private:
static void *unused;
};
template<class T>
void *provide_hash_code_for_class<T>::unused;
class MyClass : public provide_hash_code_for_class<MyClass>
{
};
int main()
{
std::cout << std::hex << MyClass::GetHashCode() << std::endl;
std::cout << std::hex << MyClass().GetHashCode() << std::endl;
return(0);
}
Please be aware that hash codes will change between runs so you can't rely on them for example for interprocess communication.

Answer 6

building on the simple route route by Nikolai N Fetissov:
For classes that would be static property - use the address of a function cast to an intptr_t to give a unique yet compiled-in value.
For instances - just use the address.

Answer 7

It is a pitty that there is no compile-time type hash_code supported by the standard. As a workaround, one can generate a compile time hash from the class name. An example below.
#include <stdint.h>
#include <string>
#include <vector>
#include <iostream>
#include <memory>
#include <cassert>
//Compile-time string hashing.
class HashedString
{
public:
typedef int64_t HashType;
explicit constexpr HashedString(const char* str): m_hash(hashString(str)) {}
static inline constexpr HashType hashString(const char* str)
{
return ( !str ? 0 : hashStringRecursive(5381, str));
}
static inline constexpr HashType hashStringRecursive(HashType hash, const char* str)
{
return ( !*str ? hash : hashStringRecursive(((hash << 5) + hash) + *str, str + 1));
}
const HashType m_hash;
};
struct EventBase
{
using IdType = HashedString::HashType;
virtual ~EventBase() {}
IdType getId() const { return m_eventId; } //present the runtime event id
EventBase(IdType myId) : m_eventId { myId } { }
template<class DerivedEvent>
const DerivedEvent* getAs() const
{
return dynamic_cast<const DerivedEvent*>(this);
}
protected:
const IdType m_eventId;
};
#define DEFINE_EVENT_ID(className) \
static constexpr IdType id = HashedString(#className).m_hash; \
struct SomeEvent1 : public EventBase
{
DEFINE_EVENT_ID(SomeEvent1);
SomeEvent1(int status) : EventBase(id), m_status { status } { assert(id == m_eventId); }
int m_status;
};
struct SomeEvent2 : public EventBase
{
DEFINE_EVENT_ID(SomeEvent2);
SomeEvent2() : EventBase(id) { assert(id == m_eventId); }
std::string m_s = "test event 2";
};
void testEvents()
{
std::vector<std::shared_ptr<EventBase>> events;
events.push_back(std::make_shared<SomeEvent1>(123));
events.push_back(std::make_shared<SomeEvent2>());
for (auto event : events) {
switch(event->getId()) {
case SomeEvent1::id:
std::cout << "SomeEvent1 " << event->getAs<SomeEvent1>()->m_status << std::endl;
break;
case SomeEvent2::id:
std::cout << "SomeEvent2 " << event->getAs<SomeEvent2>()->m_s << std::endl;
break;
}
}
}