Cleanest way for conditional code instantiation in C++ template

Question

I\'m trying to get the following C++ code running:

#include 

template class ConditionalData {
};

template 

        

Answer 1

This is a common pattern, so there's actually a paper to add constexpr_if to C++. If that makes it in to future versions, it would allow you to keep your code pretty much as-is.

template class A {
public:
    A() {
        ConditionalData data;
        constexpr_if (hasdata) {
        //^^^^^^^^^^ instead of plain if
            data.setData(3);
        }
    }
};

For now, you'll need to make do with one of the other answers.

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Edit: This was added to C++17 and called if constexpr