Is it possible to choose a C++ generic type parameter at runtime?

Question

Is there a way to choose the generic type of a class at runtime or is this a compile-time thing in C++?

What I want to do is something like this (pseudocode):

Generictype type;
if(somveval==1)
    type = Integer;
if(someval==2)
    type = String;

list<type> myList;

Is this possible in C++? and if yes, how?

Answer 1

It's a compile time thing. Template parameter types must be known to the compiler at compile-time.

That being, said, using certain template meta-programming techniques, you can choose one type or another AT compile-time, but only if all possible types are known at compile-time, and only if the condition for selecting a type can be resolved at compile time.

For example, using partial specialization you could select a type at compile time based on an integer:

template <typename T>
class Foo
{ };

template <int N>
struct select_type;

template<>
struct select_type<1>
{
    typedef int type;
};

template<>
struct select_type<2>
{
    typedef float type;
};

int main()
{
    Foo<select_type<1>::type> f1; // will give you Foo<int>
    Foo<select_type<2>::type> f2; // will give you Foo<float>
}

Answer 2

As others have also responded, the answer to your question is "No", C++ doesn't support dynamic typing at run-time. I just wanted to point out that depending on what you're trying to accomplish, you may be able to simulate this dynamic typing using a union, which is how the VARIANT type is implemented in COM.