Calling it from C

Firstly we need to declare to C our result returning C++ function:

// Declare our C++ function's returning result type. Only needs to be done once.
// This declares an `status_result<size_t, system_code>` which is an alias to
// `basic_result<size_t, status_code<erased<intptr_t>>>`.
// The first parameter is some unique identifier for this type which will be used
// whenever we reference this type in the future.
CXX_DECLARE_RESULT_SYSTEM(to_string_rettype, size_t);

// Tell C about our extern C++ function `to_string()`
extern CXX_RESULT_SYSTEM(to_string_rettype) _Z9to_stringPcmi(char *buffer, size_t bufferlen, int v);
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Now let’s call the C++ function from C:

void print(int v)
  char buffer[4];
  CXX_RESULT_SYSTEM(to_string_rettype) res;

  res = _Z9to_stringPcmi(buffer, sizeof(buffer), v);
    printf("to_string(%d) fills buffer with '%s' of %zu characters\n", v, buffer, res.value);
  // Is the error returned in the POSIX domain and thus an errno?
    fprintf(stderr, "to_string(%d) failed with error code %d (%s)\n", v, (int) res.error.value, strerror((int) res.error.value));
  fprintf(stderr, "to_string(%d) failed with unknown error code %lld\n", v, (long long) res.error.value);

int main(void)
  return 0;
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Running this C program yields:

to_string(9) fills buffer with '9' of 1 characters
to_string(99) fills buffer with '99' of 2 characters
to_string(999) fills buffer with '999' of 3 characters
to_string(9999) failed with error code 105 (No buffer space available)