c – 基于通用char []的存储并避免严格的别名相关的UB

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

• the dynamic type of the object,
• a cv-qualified version of the dynamic type of the object,
• a type similar (as defined in 4.4) to the dynamic type of the object,
• a type that is the signed or unsigned type corresponding to the dynamic type of the object,
• a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
• an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including,recursively,an element or non-static data member of a subaggregate or contained union),
• a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
• a char or unsigned char type.

鉴于突出显示的项目符号的措辞,我想出了以下alias_cast的想法：

#include <iostream>
#include <type_traits>

template <typename T>
T alias_cast(void *p) {
    typedef typename std::remove_reference<T>::type BaseType;
    union UT {
        BaseType t;
    };
    return reinterpret_cast<UT*>(p)->t;
}

template <typename T,typename U>
class Data {
    union {
        long align_;
        char data_[sizeof(T) + sizeof(U)];
    };
public:
    Data(T t = T(),U u = U()) { first() = t; second() = u; }
    T& first() { return alias_cast<T&>(data_); }
    U& second() { return alias_cast<U&>(data_ + sizeof(T)); }
};


int main() {
    Data<int,unsigned short> test;
    test.first() = 0xdead;
    test.second() = 0xbeef;
    std::cout << test.first() << "," << test.second() << "n";
    return 0;
}

(上面的测试代码,特别是Data类只是一个倾销演示的想法,所以请不要指出我应该如何使用std :: pair或std :: tuple,alias_cast模板也应该扩展到处理cv限定类型,只有满足对齐要求才能安全使用,但我希望这段代码足以证明这一想法.)

这个技巧使用g(当使用g -std = c 11 -Wall -Wextra -O2 -fstrict-aliasing -Wstrict-aliasing编译时)使警告静默,代码可以工作,但这是一个有效的方式来告诉编译器跳过严格的别名优化？

如果无效,那么在不违反别名规则的情况下,如何实现基于char数组的通用存储类呢？

编辑：
用简单的reinterpret_cast替换alias_cast,如下所示：

T& first() { return reinterpret_cast<T&>(*(data_ + 0)); }
U& second() { return reinterpret_cast<U&>(*(data_ + sizeof(T))); }

使用g编译时会产生以下警告：

aliastest-so-1.cpp: In instantiation of ‘T& Data::first() [with
T = int; U = short unsigned int]’: aliastest-so-1.cpp:28:16:
required from here aliastest-so-1.cpp:21:58: warning: dereferencing
type-punned pointer will break strict-aliasing rules
[-Wstrict-aliasing]