How to generate random variable names in C++ using macros?

Question

I'm creating a macro in C++ that declares a variable and assigns some value to it. Depending on how the macro is used, the second occurrence of the macro can override the value of the first variable. For instance:

#define MY_MACRO int my_variable_[random-number-here] = getCurrentTime();

The other motivation to use that is to avoid selecting certain name to the variable so that it be the same as a name eventually chosen by the developer using the macro.

Is there a way to generate random variable names inside a macro in C++?

-- Edit --

I mean unique but also random once I can use my macro twice in a block and in this case it will generate something like:

int unique_variable_name;
...
int unique_variable_name;

In this case, to be unique both variable names have to be random generated.

Answer 1

Try the following:

// One level of macro indirection is required in order to resolve __COUNTER__,
// and get varname1 instead of varname__COUNTER__.
#define CONCAT(a, b) CONCAT_INNER(a, b)
#define CONCAT_INNER(a, b) a ## b

#define UNIQUE_NAME(base) CONCAT(base, __COUNTER__)

void main() {
  int UNIQUE_NAME(foo) = 123;  // int foo0 = 123;
  std::cout << foo0;           // prints "123"
}

__COUNTER__ may have portability issues. If this is a problem, you can use __LINE__ instead and as long as you aren't calling the macro more than once per line or sharing the names across compilation units, you will be just fine.

Answer 2

use __COUNTER__ (works on gcc4.8, clang 3.5 and Intel icc v13, MSVC 2015)

#define CONCAT_(x,y) x##y
#define CONCAT(x,y) CONCAT_(x,y)
#define uniquename static bool CONCAT(sb_, __COUNTER__) = false