Add/Remove data members with template parameters?

问题

Consider the following code :

template<bool AddMembers> class MyClass
{
    public:
        void myFunction();
        template<class = typename std::enable_if<AddMembers>::type> void addedFunction();

    protected:
        double myVariable;
        /* SOMETHING */ addedVariable;
};

In this code, the template parameter AddMembers allow to add a function to the class when it's true. To do that, we use an std::enable_if.

My question is : is the same possible (maybe with a trick) for data members variable ? (in a such way that MyClass<false> will have 1 data member (myVariable) and MyClass<true> will have 2 data members (myVariable and addedVariable) ?

回答1:

A conditional base class may be used:

struct BaseWithVariable    { int addedVariable; };
struct BaseWithoutVariable { };

template <bool AddMembers> class MyClass
    : std::conditional<AddMembers, BaseWithVariable, BaseWithoutVariable>::type
{
    // etc.
};

回答2:

First off, your code just won't compile for MyClass<false>. The enable_if trait is useful for deduced arguments, not for class template arguments.

Second, here's how you could control members:

template <bool> struct Members { };

template <> struct Members<true> { int x; };

template <bool B> struct Foo : Members<B>
{
    double y;
};

来源：https://stackoverflow.com/questions/12117912/add-remove-data-members-with-template-parameters