How to generate a random variable or class name and use it later?

Question

A way to create random (actually sequentially numbered) variable names is demonstrated in the question How to generate random variable names in C++ using macros?.

However, it is not explained how such a generated name can be accessed later. Imagine a class declaration with a constructor. You need the same random name for the class name and the c-tor. Always using the generator macro just generates a new identifer and the compilation fails. Here's an example:

#define CONCAT_(x,y) x##y
#define CONCAT(x, y) CONCAT_(x, y)

#define DESCRIBE_IMPL CONCAT(DescribeImpl, __COUNTER__)
class DESCRIBE_IMPL {
public:
  DESCRIBE_IMPL() {}
};

I tried storing the current counter value somehow (e.g. enum { COUNTER = __COUNTER__ }, but using this enum always leads to the name DescribeImplCOUNTER instead of the variant with the counter value.

The Real Problem

What I'm trying to solve with this approach is a case where I have multiple definitions of the same class in different cpp files (generated from macros) and the linker complains about multiple symbols (ODR violation). I cannot change this approach without significantly change the usage pattern.

This trick is only viable for things where the name is a throwaway. What are you really trying to solve? You seem to be asking about your perceived solution. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Apr 2, 2019 at 11:27
What is the real problem you try to solve with a solution like this? For "sequentially numbered" variables, one usually uses arrays or similar data-structures. Having a set of otherwise equal variables or classes clutters up the code, making it hard to read, understand and maintain. Using macros to "create" these symbols does't make it better. — Some programmer dude
– Some programmer dude, Commented Apr 2, 2019 at 11:28
Youy guys are right, this is part of a bigger problem and I tried to leave out as much irrelevant as possible. This approach is one attempt to solve the real problem. Let me add some information in my question. — Mike Lischke
– Mike Lischke, Commented Apr 2, 2019 at 11:31
The built-in non-standard macro __COUNTER__ is per translation unit, so if you use the macro in different translation units it will not help. I think you should delete this question, and try to come up with an minimal reproducible example for a new question where you ask about your real and underlying problem instead. — Some programmer dude
– Some programmer dude, Commented Apr 2, 2019 at 11:44
Have you tried putting your class in your cpp files in the anonymous namespace? — Alan Birtles
– Alan Birtles, Commented Apr 2, 2019 at 11:54

PeterT · Accepted Answer · 2019-04-03 04:28:44Z

0

Since no one else wants to post the answer from the comments. (Examples from here)

The issues is that this violates the ODR:

// a.cpp
struct S {
  int a;
};

// b.cpp
class S {
public:
  int a;
};

One solution is put the definitions into an anonymous namespace in each file, preventing the linker from trying to re-conciliate the name with another translation unit.

// a.cpp
namespace {
struct S {
  int a;
};
}

// b.cpp
namespace {
class S {
public:
  int a;
};
}

answered Apr 3, 2019 at 4:28

PeterT

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