How to detect if a class has member variables?

问题

Problem

I would like to detect if a class has member variables and fail a static assert if they do. Something like:

struct b {
    int a;
}
static_assert(!has_member_variables<b>, "Class should not contain members"). // Error.

struct c {
    virtual void a() {}
    void other() {}
}
static_assert(!has_member_variables<c>, "Class should not contain members"). // Fine.

struct d : c {
}
static_assert(!has_member_variables<d>, "Class should not contain members"). // Fine.

struct e : b {
}
static_assert(!has_member_variables<e>, "Class should not contain members"). // Error.

struct f : c {
    char z;
}
static_assert(!has_member_variables<f>, "Class should not contain members"). // Error.

Is there a way to achieve this with SFINAE template? This class may have inheritance or even multiple inheritance with virtual functions (no members in the base classes though).

Motivation

I have a pretty simple setup as follows:

class iFuncRtn {
    virtual Status runFunc(Data &data) = 0;
};

template <TRoutine, TSpecialDataType>
class FuncRoutineDataHelper : public iFuncRtn {
    Status runFunc(Data &data) {
        static_assert(!has_member_variables<TRoutine>, "Routines shouldnt have data members!");
        // Prepare special data for routine
        TSpecialDataType sData(data);
        runFuncImpl(sData);
}

class SpecificRtn : 
    public FuncRoutineDataHelper<SpecificRtn, MySpecialData> {
    virtual Status runFuncImpl(MySpecialData &sData) {
        // Calculate based on input 
        sData.setValue(someCalculation);
    }
};

The FunctionalityRoutines are managed and run on a per tick basis. They are customized and can perform a wide variety of tasks such as contacting other devices etc. The data that is passed in can be manipulated by the routine and is guaranteed to be passed in on each tick execution until the functionality is finished. The right type of data is passed in based on the DataHelper class. I wan't to discourage future people from mistakenly adding data to the functionality routines as it is very unlikely to do what they expect. To force this, I was hoping to find a way with static assert.

回答1:

You can solve this by depending on the compiler doing empty base class optimizations, by checking if a class derived from your T has the same size as an empty class with virtual functions:

template<typename T, typename... BaseClasses>
class IsEmpty
{
    // sanity check; see the updated demo below
    static_assert(IsDerivedFrom<T, BaseClasses...>::value);

    struct NonDerived : BaseClasses... { virtual ~NonDerived() = default; };
    struct Derived : T { virtual ~Derived() = default; };

public:
    inline static constexpr bool value = (sizeof(NonDerived) == sizeof(Derived));
};

This should work with both single and multiple inheritance. However, when using multiple inheritance, it's necessary to list all base classes, like that:

static_assert(IsEmpty<Derived, Base1, Base2, Base3>::value);

Obviously, this solution rules out final classes.

Here's the updated demo.

~~Here's the original demo. (doesn't work with multiple inheritance)~~

回答2:

You will have to mark the classes in some way or another. Pick a way you are comfortable with, a property or some kind of type integer member with an enum. Whoever makes sub-classes will have to follow your convention to make it work.

All other answers here will be some variant of this.

Any answer that uses a sizeof could not guarantee this will work between platforms, compilers, or even classes on the same platform and compiler, due to easily being able to fit a new member inside the default class member alignment, where the sizes of sizeof could easily end up the same for a sub-class.

Background:

As stated in your code and question, all of that is just plain and basic C ad C++ code, and is resolved entirely at compile time. The compiler will tell you if a member exists or not. After its compiled it's a mash of efficient, nameless, machine code with no hints or help for that kind of thing by itself.

Any name you use for a function or data member effectively disappears, as you know it and see it there, after compile and there is no way to lookup any member by name. Each data member is known only by its numerical offset from the top of the class or struct.

Systems like .Net, Java, and others are designed for reflection, which is the ability to remember class members by name, where you can find them at runtime when you program is running.

Templates in C++, unless mixed mode C++ on something like .Net, are also all resolved at compile time, and the names will also all be gone, so the templates by themselves buy you nothing.

Languages like Objective-C also are written to not fail necessarily if certain types of special members are missing, similar to what you are asking, but under the covers its using a lot of supporting code and runtime management to keep track independently, where the actual function itself and its code are still unware and rely on other code to tell them if a member exists or to not fail on null member.

In pure C or C++ you will need to just make your own system, and be literal about tracking dynamically what does what. You could make enums, or lists or dictionaries of name strings. This is what is normally done, you just have to leave hints for yourself. A class cannot be compiled in a way that gives implicit visibility to future sub-classes by definition, without using some form if RTTI.

Its common to put a type member on a class for this very reason, which could be a simple enum. I would not count on sizes or anything that might be platform dependent.

来源：https://stackoverflow.com/questions/52207616/how-to-detect-if-a-class-has-member-variables

标签

c++

templates

sfinae