c++可变模板删除函数逻辑

我已经能够进一步与我的可变模板从我之前的问题。我现在有一个新问题。在这个代码示例中:

#include <iostream>
#include <cstddef>
constexpr std::uint32_t Flag0   = 0x0001;
constexpr std::uint32_t Flag1   = 0x0002;
constexpr std::uint32_t Flag2   = 0x0004;
constexpr std::uint32_t FlagAll = 0xFFFF;
template<std::uint32_t...Cs>
struct flags_tag {constexpr flags_tag(){}; };
template<std::uint32_t...Cs>
struct make_flags{ using type=flags_tag<Cs...>; };
template<std::uint32_t...Cs>
using make_flags_t=typename make_flags<Cs...>::type;
template<std::uint32_t value>
class pValue_t
{
    template<std::uint32_t StateMask, class flags>
    friend class Compound;    
};
template<> class pValue_t<Flag0>
{ 
public:
    pValue_t() : 
        m_pValue0(reinterpret_cast<void*>(0xFFFFFFFF))
    {} 
protected:
    void* m_pValue0;
};
template<> class pValue_t<Flag1>
{ 
public:
    pValue_t() : 
        m_pValue1(reinterpret_cast<void*>(0xDEADBEEF))
    {}
protected:
    void* m_pValue1;
};
template<> class pValue_t<Flag2>
{ 
public: 
    pValue_t() : 
        m_pValue2(reinterpret_cast<void*>(0xCAFEBABE))
    {}
protected:
    void* m_pValue2;
};
template<std::uint32_t StateMask, class flags>
class Compound;
template<std::uint32_t StateMask, std::uint32_t...Cs>
class Compound< StateMask, flags_tag<Cs...> >:
  public pValue_t<Cs>...
{       
public:
    void print()
    { 
        if (IsStateValid(Flag0))
        { 
            std::cout << this->m_pValue0 << 'n';
        }
        if ((StateMask & Flag1) == Flag1)
        {
            std::cout << this->m_pValue1 << 'n';
        }
        // *** THIS IS THE PROBLEM STATEMENT ***
        if (IsStateValid(Flag2))
        {
            std::cout << this->m_pValue2 << 'n';
        }
    }
    static bool IsStateValid(std::uint32_t stateMask)
        { return ((StateMask & stateMask) == stateMask); }
    uint32_t m_stateMask;
};
using my_type = Compound< Flag0 | Flag1, make_flags_t<Flag0, Flag1>>;
int main() {
  my_type test;
  test.print();
}

print函数包含对m_pValue2的引用，当StateMask包含Flag2时，该引用有效。

现在，编译器警告它找不到m_pValue2。我希望编译器删除引用m_pValue2的代码块，当StateMask(在编译时已知)不包含Flag2(当IsStateValid()为false)。

准确的错误如下:

main.cpp: In instantiation of 'void Compound<StateMask, flags_tag<Cs ...> >::print() [with unsigned int StateMask = 3u; unsigned int ...Cs = {1u, 2u}]':
main.cpp:95:18:   required from here
main.cpp:80:27: error: 'class Compound<3u, flags_tag<1u, 2u> >' has no member named 'm_pValue2'
             std::cout << this->m_pValue2 << 'n';

我希望这是可能的。在其他模板编程中，我使用IsStateValid()编译出与StateMask不匹配的代码段。但是，我从来没有试图编译掉可能丢失的成员变量。

有人有什么想法吗?

void print()
{
    printImpl<Flag0>();
    printImpl<Flag1>();
    printImpl<Flag2>();
}
template <uint32_t F>
void printImpl() {
    printImpl<F>(std::is_base_of<pValue_t<F>, Compound>{});
}
template <uint32_t F>
void printImpl(std::true_type ) {
    // we DO have this flag
    pValue_t<F>::print();
}
template <uint32_t F>
void printImpl(std::false_type ) {
    // we do NOT have this flag
    // so do nothing
}

template<> class pValue_t<Flag2>
{ 
public: 
    pValue_t() : 
        m_pValue2(reinterpret_cast<void*>(0xCAFEBABE))
    {}
    void print() {
        std::cout << m_pValue2 << 'n';
    }
protected:
    void* m_pValue2;
};

static constexpr bool IsStateValid(std::uint32_t stateMask)
    { return ((StateMask & stateMask) == stateMask); }
template <typename A = void,
          typename T = typename std::enable_if<IsStateValid(Flag2), A>::type>
void blub(int x=0) {
    std::cout << this->m_pValue2 << 'n';
}
template <typename A = void, 
          typename T = typename std::enable_if<!IsStateValid(Flag2), A>::type>
void blub(long x=0) {
}

blub();