Apply function to variant variable using generic lambda.

apply_function.cpp

template<class F, class Ret = void>
struct Apply : boost::static_visitor<Ret>
{
    Apply(F f) : f_(std::move(f)) {}
    
    template<class T>
    Ret operator()(T &t) { return f_(t); }
    
    template<class T>
    Ret operator()(T &t) const { return f_(t); }
    
    F f_;
};
    
template<class F>
Apply<F, void> make_apply(F f)
{
    return Apply<F, void>(std::move(f));
}
    
template<class Ret, class F>
Apply<F, Ret> make_apply(F f)
{
    return Apply<F, Ret>(std::move(f));
}
    
template<class F, class Variant>
void apply_function(F f, Variant &variant)
{
    return boost::apply_visitor(make_apply(std::move(f)), variant);
}

template<class Ret, class F, class Variant>
Ret apply_function(F f, Variant &variant)
{
    return boost::apply_visitor(make_apply<Ret>(std::move(f)), variant);
}

void A::foo()
{
    apply_function([this](auto &data) { DoProcess(data); }, variant_data_); 
}

template<class Data>
void A::DoProcess(Data &data)
{
  //do something.
}