Artem Zefirov Artem Zefirov - 3 months ago 13
C++ Question

c++ fatal error C1001 on using variadic template

Hi everybody. Compiling my code I came upon the next error:

fatal error C1001: An internal error has occurred in the compiler.
When I pass only one parameter to "exec" function, everything is ok. But when I pass more then one parameters it issues an error. I use vs 2015 and visual c++ compiler respectively.

#define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>
//using namespace std;

template <class T>
void param_push_(T arg, char ** param) {
sprintf(*param, "%15.15e", arg);
}

template <class T, class... args>
void param_push_(T first, args... args, char ** param) {
static int param_No = 0;
sprintf(param[param_No++], "%15.15e", first);
param_push(args..., param[param_No]);
}

template <class ... param_types>
void exec(const char * command, const param_types& ...param_values) {

int arg_count = sizeof...(param_values);
char ** params = new char*[arg_count];
for (int i = 0; i < arg_count; ++i) {
params[i] = new char[22 * sizeof(char)];
}

param_push_(param_values..., params); //cast parameters to (char *)

for (int i = 0; i < arg_count; ++i) {
delete[] params[i];
}
delete[] params;

}

int main()
{
double e_restriction = 0.55300000000000000000124124;
double M_restriction = 5;

exec("SELECT \"M\",e from orbital WHERE e < $1::double precision AND \"M\" < $2::double precision", e_restriction,M_restriction);

return 0;
}

Answer

Try this - it works.

#include <stdio.h>
#include <iostream>

template <typename ...> struct param_push;

template <class H, class... T>
struct param_push<H,T...> {
   void operator() (H head, T... tail, char** param) {
     sprintf(param[0], "%15.15e", head);
     param_push<T...> one_less;
     one_less(tail..., &param[1]);
   }
};

template <>
struct param_push<> {
  void operator() (char** param) {
    // does nothing, no more args to sprintf
  }
};

template <class ... param_types>
void exec(const char * command, const param_types& ...param_values) {

  int arg_count = sizeof...(param_values);
  char ** params = new char*[arg_count];
  for (int i = 0; i < arg_count; ++i) {
    params[i] = new char[22 * sizeof(char)];
  }

  // param_push_(param_values..., params); //cast parameters to (char *)
  struct param_push<param_types...> functor;
  functor(param_values..., params);

  for (int i = 0; i < arg_count; ++i) {
    std::cout << params[i] << " will be deleted" << std::endl;
    delete[] params[i];
  }
  delete[] params;

}

int main()
{
  double e_restriction = 0.55300000000000000000124124;
  double M_restriction = 5;

  exec("SELECT \"M\",e from orbital WHERE e < $1::double precision AND \"M\" < $2::double precision", e_restriction,M_restriction);

  return 0;
}

(and now, homework for Arty Zefirov: find the reason why when using template struct functors the technique works, why it fails for templated functions and update your question with the findings)

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