C++ construct that behaves like the __COUNTER__ ma

问题:

This question already has an answer here:

• Does C++ support compile-time counters? 7 answers

I have a set of C++ classes and each one must declare a unique sequential id as a compile-time constant. For that I'm using the __COUNTER__ built-in macro which translates to an integer that is incremented for every occurrence of it. The ids need not to follow a strict order. The only requirement is that they are sequential and start from 0:

class A {
public:
    enum { id = __COUNTER__ };
};

class B {
public:
    enum { id = __COUNTER__ };
};

// etcetera ...

My question is: Is there a way to achieve the same result using a C++ construct, such as templates?

回答1:

Here is a possible way to do it using __LINE__ macro and templates:

template <int>
struct Inc
{
    enum { value = 0 };
};

template <int index>
struct Id
{
    enum { value = Id<index - 1>::value + Inc<index - 1>::value };
};

template <>
struct Id<0>
{
    enum { value = 0 };
};

#define CLASS_DECLARATION(Class) \
template <> \
struct Inc<__LINE__> \
{ \
    enum { value = 1 }; \
}; \
 \
struct Class \
{ \
    enum { id = Id<__LINE__>::value }; \
private:

Example of using:

CLASS_DECLARATION(A)
    // ...
};

CLASS_DECLARATION(B)
    // ...
};

CLASS_DECLARATION(C)
    // ...
};

See live example.

回答2:

Is a explicit chaining acceptable?

class A {
public:
    static const unsigned int id = 1;
};

class B {
public:
    static const unsigned int id = A::id+1;
};

The advantage of this approach is that you always get the same Id and you know what it is no matter what your compiler is. While with __LINE__ or __COUNTER__ approach may not be so predicatable. The disadvantage is that with chaining your class must always know the previous one on the chain.

Playing with templates (and C++11):

template <typename... T>
class Identificable;

template <>
class Identificable<> {
public:
    static const unsigned int id = 1;   
};

template <typename Prev>
class Identificable<Prev> {
public:
    static const unsigned int id = Prev::id+1;
};

class A : public Identificable<> {
public:
};

class B : public Identificable<A> {
public:
};

回答3:

Standard C++ has the __LINE__ macro.

That is, __LINE__ is a "C++ construct", as requested, in contrast to __COUNTER__, which isn't.

__LINE__ differs from Visual C++’s __COUNTER__ in that at least earlier versions of Visual C++ produced garbled expansions of __LINE__ when a certain compilation option was used.

---

Depeneding on your needs you may however be able to simply use type_info instances for identification. C++11 added general support for comparing type_info, called std::typeindex. This means you can use standard collections.

回答4:

Is there a way to achieve the same result using a C++ construct, such as templates?

Yes, there is :-) The basic idea is to use chaining allocated IDs, which avoids the use of __COUNTER__, __LINE__ or other approaches proposed earlier and does not require injecting "extra" info into the type definition.

Here is a brief description of the solution proposed in v1 for the counter implemented on C++03, using template metaprograming. Two template specializations ID_by_T and T_by_ID are used to define links type <=> ID at compile time. The type's ID is a enum constant. If the link was not defined, ID_by_T<type>::ID returns -1, and T_by_ID<undefinedID>::type returns the null_t predefined type. The macro DEF_TYPE_ID(type_name) generates a new ID at the point of the definition the type <=> ID link.

This approach is based on a macro redefinition: When a macro is undefined using #undef, its value is expanded into the C++ code. For instance:

DEF_TYPE_ID(int)
#undef  PREV_TYPE
#define PREV_TYPE int

Macro DEF_TYPE_ID uses the following call to the previous definition of macro PREV_TYPE: ID_T_pair<type_name, ID_by_T<PREV_TYPE>::ID+1>. That is why I said about chaining allocated IDs.