How to design a template class with switchable member variables at compile-time in C++?

问题

| ■ Probelm definition____________________

I'm trying to design a super-flexible, but memory efficient module satisfying below properties.

1. It can switch OFF unnecessary member variables depending on the situation.
2. Which variables it will own are determined at the compile-time.

I somehow made one which determines its member list "before" the compile-time, using macros and enumerator flags. see below :

▼ TraitSwitch.h

#pragma once
// Macros to switch-off source codes themselves.
#define ON                 1
#define OFF                0

#define TRI_AREA_INFO      ON
#define TRI_CENTROID_INFO  ON
#define TRI_NORMAL_INFO    OFF // When the normal vector info is unnecessary.
...

▼ TriangleTraits.h

#pragma once
#include <cstdint>
#include "TraitSwitch.h"

enum TriangleTrait : uint8_t
{
    NONE          = 0,   // 0000 0000

#if (TRI_AREA_INFO == ON)
    AREA          = 1,   // 0000 0001
#endif

#if (TRI_CENTROID_INFO == ON)
    CENTROID      = 2,   // 0000 0010
#endif

#if (TRI_NORMAL_INFO == ON) //        | Inactive Preprocessor Block
    NORMAL_VECTOR = 4,   // 0000 0100 |
#endif
    ... // more traits

    ALL           = 255  // 1111 1111
}
// Need some additional overloaded bitwise-operators (&, |, |=, etc ...)

▼ Triangle.h

#pragma once
#include "TriangleTraits.h"

class Triangle
{
public:
    Triangle() {}
    ~Triangle() {}

#if (TRI_AREA_INFO == ON)
    double area;
#endif

#if (TRI_CENTROID_INFO == ON)
    double centroid[3]; // x, y, z
#endif

#if (TRI_NORMAL_INFO == ON) //   | Inactive Preprocessor Block
    double normal[3]; // x, y, z |
#endif
    ...

    TriangleTrait alreadyComputed; // To avoid redundant works.
    void ComputeTraits(TriangleTrait _requested)
    {
        if (((_requested & TriangleTrait::AREA) != 0) 
            && ((alreadyComputed & _requested) == 0))
        {
            this->ComputeArea();
            alreadyComputed |= TriangleTrait::AREA;
        }
        ... // do the same things for centroid, normal
    }

private:
    void ComputeArea();
    void ComputeCentroid();
    void ComputeNormal();
    ...
}

then, C++ IntelliSense on the object may show : this

▼ main.cpp

#include <iostream>
#include "Triangle.h"

int main(void)
{
    Triangle tri;
    tri.ComputeTraits(TriangleTrait::AREA | TriangleTrait::CENTROID);

    std::cout << "area : " << tri.area << "m²" << std::endl;
    std::cout << "centroid : (" 
        << tri.centroid[0] << "," 
        << tri.centroid[1] << "," 
        << tri.centroid[2] << ")" << std::endl;
    ...
}

Firstly Triangle.h looks quite ugly, and even if it looks good, this method determines class members "before" the compile-time, anyway.

| ■ Question summary____________________

"How to design a template class with switchable members, which are determined at the compile-time."

Here's the very thing what I want :

▼ main.cpp

...

int main(void)
{
    Triangle<__MACRO_DEFINED_TRAIT_SWITCH(AREA)> tri1; // This owns area info only
    tri1.area;

    Triangle<__MACRO_DEFINED_TRAIT_SWITCH(AREA | CENTROID)> tri2; // This owns area & centroid info
    tri2.area;
    tri2.centroid;

    Triangle<__MACRO_DEFINED_TRAIT_SWITCH(AREA | NORMAL)> tri3; // This owns area & normal vector info
    tri3.area;
    tri3.normal;
    ...

    Triangle<__MACRO_DEFINED_TRAIT_SWITCH(AREA | CENTROID | NORMAL)> tri4; // This owns area & centroid & normal vector info
    tri4.area;
    tri4.centroid;
    tri4.normal;
    ...
}

I guess using templates combined with macros (with Tag-dispatching method, maybe?) will do exactly what I want, but have no any clear idea.

回答1:

After a few days of distress, I found a way. So I'll answer my question myself.

The soultion was tooooo simple. Just use multiple inheritances with variadic templates. See below:

▼TriangleTraits.h

struct AREA
{
    double area;
};

struct CENTROID
{
    double centroid[3];
};

struct NORMAL
{
    double normal[3];
};
... // more traits

// Multiple Inheritances with variadic template
template<class... Traits>
struct TriangleWithTraits : Traits... 
{
};

▼main.cpp

#include "TriangleTraits.h" // Just include this one is enough

int main()
{
    TriangleWithTraits<AREA> tri1;
    tri1.area;

    TriangleWithTraits<AREA, CENTROID> tri2;
    tri2.area;
    tri2.centroid;

    TriangleWithTraits<AREA, NORMAL> tri3;
    tri3.area;
    tri3.normal;

    TriangleWithTraits<AREA, CENTROID, NORMAL> tri4;
    tri4.area;
    tri4.normal;
    tri4.centroid;
...
}

回答2:

Cannot use preprocessor to parse template parameter, preprocesssor does not know of C++.

But there's std::conditional and there's Empty Base optimization.

So you can have bases with data class parts, and std::conditional to select between them and empty base:

struct EmptyBase {};

struct BaseWithChar
{
    char a;
};

struct BaseWithLongLong
{
    long long b;
    long long c;
};

template<int Flags>
struct A
    : std::conditional<(Flags & 1), BaseWithChar, EmptyBase>::type
    , std::conditional<(Flags & 2), BaseWithLongLong, EmptyBase>::type
{
};

int main() {
    std::cout << sizeof(A<1>) << std::endl;
    std::cout << sizeof(A<2>) << std::endl;
    std::cout << sizeof(A<3>) << std::endl;
    return 0;
}

And with later C++ standards, there's std::coditional_t and [[no_unique_address]]:

using namespace std;

struct EmptyMember {};

template<int Flags>
struct A
{
    [[no_unique_address]] typename std::conditional_t<(Flags & 1), char, EmptyMember> a;
    [[no_unique_address]] typename std::conditional_t<(Flags & 2), long long, EmptyMember> b;
    [[no_unique_address]] typename std::conditional_t<(Flags & 2), long long, EmptyMember> c;
};

int main() {
    std::cout << sizeof(A<1>) << std::endl;
    std::cout << sizeof(A<2>) << std::endl;
    std::cout << sizeof(A<3>) << std::endl;
    return 0;
}

But [[no_unique_address]] requires C++20, and without [[no_unique_address]] empty member will use some space. Looks like it does not work with Visual C++, even with 2019 preview. I've posted feebdack.

With earlier C++, when you don't have std::conditional or boost::conditional, you can easily implement your own, it is just a template with true and false specialization. See "possible implementation" on cppreference page

来源：https://stackoverflow.com/questions/60733043/how-to-design-a-template-class-with-switchable-member-variables-at-compile-time