C ++创建动态类型

Question

I have the following situation: Depending on some parameter that my function takes it have to create different types:

I want to do something like this:

if(variant==1){
    #define my_type int;
}
else{
    #define my_type double;
}
cout<<sizeof(my_type);

and then use my_type in my further code.

So, that in case of variant=1 sizeof(my_type) gives 4 and for variant=2 it gives 8.

How can this be done? Either in this manner or another.

Thanks.

Answer 1

I agree with @Magnus Hoff in that what you asked cannot be done. But there are two approximations.

Option 1: make variant a macro.

#ifdef variant
#  define my_type int
#else
#  define my_type double
#endif

Option 2: use template function.

Instead of

void func(int variant) {
  if (variant==1)
    #define my_type int
  else
    #define my_type double
  my_type ...
}

do this:

template<typename my_type> void func() {
  my_type ...
}        

Answer 2

Replace this:

if(variant==1){
    #define my_type int;
}
else{
    #define my_type double;
}
cout<<sizeof(my_type);

… with this:

template< class Type >
void foo()
{
    // ...
    cout<<sizeof(Type);
}

// ...
if( variant==1 )
{
    foo<int>();
}
else
{
    foo<double>();
}

Note that a runtime value can't affect compile time decisions. Without a time travel device.