auto with ternary operator and templates

Question

I have a template class

template <typename T>
class foo;

There are 2 valid values for T, corresponding to:

using fooT1 = class foo<T1>;
using fooT2 = class foo<T2>;

I wanted to write code which looks like:

const auto* fooPtr = useFooT1 ? getFooT1Ptr() : getFooT2Ptr();

because the code using fooPtr in this function doesn't depend on whether fooPtr is of type fooT1 or fooT2

However, I get the following compiler error:

error: conditional expression between distinct pointer types ...

I understand that as per the C++ standard, there should be a common type that both can be casted to, so this approach may not work.

What's a good way to achieve this functionality without replicating a lot of code?

Answer 1

Indeed, C++ is statically typed, so the type of a variable can't depend on a runtime condition.

Instead, put the generic code into a template:

template <typename T> doStuff(foo<T> * f) {
    // stuff that works with any `foo` type
}

and call a different specialisation depending on the run-time variable

if (useFooT1) {
    doStuff(getFooT1Ptr());
} else {
    doStuff(getFooT2Ptr());
}