switch namespace by if condtion

in my C++ program I have several namespaces that contain several pointers with identical names. I then want a function to choose a namespace according to a parameter. I.e. something like:

#include <iostream>

namespace ns1{
double x[5]={1,2,3,4,5};
}
namespace ns2{
double x[5]={6,7,8,9,10};
}

int main(){
  int b=1;
  if(b==1){
    using namespace ns1;
  }
  if(b==2){
    using namespace ns2;
  }
  std::cout << x[3] << std::endl;
}

However, this doesn't work since the compiler complains that x isn't known in that scope. I guess the problem is that "using namespace ..." is only valid within the if-statement. I think that it should be possible to switch namespaces somehow, but cannot find out how... Do you know how to do this without casting all variable separately?

int main(){
开发者_开发知识库  int b=1;
  double *x;
  if(b==1){
    x = ns1::x;
  }
  if(b==2){
    x = ns2::x;
  }
  std::cout << x[3] << std::endl;
}

Cheers, Pascal

---

Namespaces are a compile-time feature in C++, you can't manipulate them dynamically in runtime. The best advice I could give you is try a different approach - namespaces are probably not meant to do what you want to do. So ask SO what you really want to do and you'll likely get a good answer without bending the system.

---

The namespace is used at compilation time. The if state ist executed at run time.

You can't switch the compiler behavior at run time.

---

I fail to see why you don't want this:

double* dispatch(int b)
{
    switch(b)
    {
        case 1: return ns1::x;
        case 2: return ns2::x;
        default: return 0;
    }
}

int main()
{
    int b=1;
    std::cout << dispatch(b)[3] << std::endl;
}

If this is going to grow, consider using classes and polymorphism instead of namespaces.

---

In the simplest state you have "static" data but want to decide which piece of static data to use.

struct space
{
  double x[5];
};

space space1 = space{ {1,2,3,4,5} };
space space2 = space{ {6,7,8,9,10} };

int main( int argc, char * argv[] )
{
   space * s;
   if( argc == 1 )
       s = &space1;
   else
       s = &space2;
}

In reality of course your spaces will more than just one member and you will populate them in a different manner but it is one way you can chose which instance you use.

You might use polymorphism to implement functions but it depends a lot on what you need it for.

---

I don't believe namespaces are something that is determined during run time... therefore you won't be able to set it up as such.

A quick google of "changing namespace at runtime" returned http://www.velocityreviews.com/forums/t288720-changing-namespace-at-runtime.html

Hope that helps.

---

the using namespace directives basically is telling the compiler to "try to use this when resolving symbols", which doesn't happen at runtime.

I don't see any problem with the second approach....

---

As mentioned before it won't work... I would suggest that you use polymorphism if you have many variables that you want to put in your container class to achieve the intended result:

// Header file
class s
{
  public:
    double x[5];
}

class s1: public s 
{
  public:
    s1() { /* Assign x here */ };        
}

class s2: public s
{
  public:
    s2() { /* Assign x here */ };
}

// CPP file
int main()
{
  int b = 1;
  s* sptr;

  if (b == 1)
    sptr = (s*)new s1();
  else
    sptr = (s*)new s2();

  std::cout << sptr->x[3] << std::endl;
  delete sptr;
} 

If you only have a few variables you could use pointers.

int main()
{
  int b = 1;
  double *px;
  double s1x = {1, 2, 3, 4, 5};
  double s2x = {6, 7, 8, 9, 10};

  if (b == 1)
    px = &s1x;
  else
    px = &s2x;

  std::cout << px[3] << std::endl;
}