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

If you can afford for c++14, you can express the conditional branches as generic lambdas. The benefit is that they capture surrounding variables and the solution doesn't require extra member functions.

template  struct tag {};

template 
auto static_if(tag, T t, F f) { return t; }

template 
auto static_if(tag, T t, F f) { return f; }

template 
auto static_if(T t, F f) { return static_if(tag{}, t, f); }

template 
auto static_if(T t) { return static_if(tag{}, t, [](auto&&...){}); }

// ...

ConditionalData data;        
static_if([&](auto& d)
{
    d.setData(3);
})(data);

DEMO

In c++17 you can just say:

if constexpr (hasdata)
{
    data.setData(3);
}

DEMO 2