如何将不同的实例应用于同一对象的名称？

Question

int lim;    
do{
      cin>>lim;
      switch(lim)
      {
      case 1: {mpa<1,int,const char*> C; }break;
      case 2: {mpa<2,int,const char*> C; }break;
      case 3: {mpa<3,int,const char*> C; }break;
      case 4: {mpa<4,int,const char*> C; }break;
      case 5: {mpa<5,int,const char*> C; }break;
      default: cout<<"Incorrect number, please repeat\n";
      }
 }while(lim<1 || lim>5);

I want to pass a value(1,2,3,4 or 5) to the template int a,typename T1,typename T2. I need different instantiations of this template. But object C will be destroyed after switch. How to apply different instantiations to the same name of object?

Answer 1

In this case, I think the best option is to rethink class mpa to not require that first parameter at compile time. Seriously, everyone hates rewriting things, but that's what you should do.

Since I know you'll ignore that, here's a workaround:

Make a class mpa_base<class, class> , that mpa inherits from, and has all the same functions, but they're all virtual (ESPECIALLY the destructor). Then, you can do this:

typedef mpa_base<int, const char*> mpa_int_pchar;
std::unique_ptr<mpa_int_pchar> C; //this is a smart pointer to a mpa object
int lim;    
do{
      cin>>lim;
      switch(lim)
      {
      case 1: C.reset(new mpa<1,int,const char*>()); break;
      case 2: C.reset(new mpa<2,int,const char*>()); break;
      case 3: C.reset(new mpa<3,int,const char*>()); break;
      case 4: C.reset(new mpa<4,int,const char*>()); break;
      case 5: C.reset(new mpa<5,int,const char*>()); break;
      default: cout<<"Incorrect number, please repeat\n";
      }
 }while(lim<1 || lim>5);

Answer 2

Refactor the common part in another function and then you can easily do this. Also, always have the input from the stream as part of the loop check or make it a break-condition.

template<class MPA>
void foo(MPA const& mpa){
  // whatever you want to do, do it here
}

int lim = 0; 
do{
  if(!(cin >> lim)){  // broken input
    cin.clear(); // clear error flags
    break; // bail out
  }
  switch(lim)
  {
  case 1: { foo(mpa<1, int, char const*>(/*params*/)); }break;
  case 2: { foo(mpa<2, int, char const*>(/*params*/)); }break;
  case 3: { foo(mpa<3, int, char const*>(/*params*/)); }break;
  case 4: { foo(mpa<4, int, char const*>(/*params*/)); }break;
  case 5: { foo(mpa<5, int, char const*>(/*params*/)); }break;
  default: cout<<"Incorrect number, please repeat\n";
  }
}while(lim < 1 || lim > 5);