Question

I have this class:

template <typename T, uint64_t N>
struct Probe {
  static const uint64_t Counter = N;
  typedef T Type;
};

Which I utilize as:

typedef Probe <int, 0> FirstIntProbe;
typedef Probe <int, 1> SecondIntProbe;
typedef Probe <float, 2> FloatProbe;

Is it possible to create a compile time\macro method which allows me to instantiate this class without specifying the second parameter such as:

typedef Probe <int, Something?> FirstIntProbe;
typedef Probe <int, Something?> SecondIntProbe;
typedef Probe <float, Something?> FloatProbe;

I assume this is isn’t possible, but then again I’ve seen people do things in C++ I wouldn’t think was possible before.

---

Update:

• It is not necessary to increase by one, it’s just important that
  every probe have it’s own number.
• It is not needed to have unique
  number across different .cpp files\translation units.

Answer 1

You can look into using the __COUNTER__ macro, which is a compiler extension (but supported on GCC and MSVC, among others). Note that __COUNTER__ is unique only per-translation-unit, i.e. per .cpp file.

edit: Header inclusion in multiple translation units is OK. This example links and runs perfectly fine (built on GCC 4.5):

probe.h:

template <typename T, int N>
struct Probe {
    typedef T Type;
};

#define DECLARE_PROBE(type) typedef struct Probe<type, __COUNTER__>

main.cpp:

#include "test.h"

DECLARE_PROBE(int) intprobe;
DECLARE_PROBE(float) floatprobe;

int main(int argc, char** argv) {
    intprobe ip;
    floatprobe fp;
    return 0;
}

test.cpp:

#include "test.h"

DECLARE_PROBE(int) intprobe;
DECLARE_PROBE(float) floatprobe;

static intprobe ip;
static floatprobe fp;