使用Clang中具有函数本地类型的模板的类型计数器的意外结果

Question

我根据两种类型编写了一个类模板,根据模板参数分配了一个唯一的索引:

template<typename SK,typename T>
struct Component {
    static uint const index;
};

期望是每个新类型index增加:

Component<X,A>::index; // 0
Component<X,B>::index; // 1

Component<Y,A>::index; // 0
Component<Y,B>::index; // 1
// ...etc

分配索引的完整代码如下:

using uint = unsigned int;

template<typename SK,typename T>
struct Component
{
    static uint const index;
};

template<typename SK>
class ComponentCount
{
    template<typename CSK,typename CT>
    friend struct Component;

private:
    template<typename T>
    static uint next() {
        return ComponentCount<SK>::get_counter();
    }

    static uint get_counter()
    {
        static uint counter = 0;
        return counter++;
    }
};

这在GCC(5.1)和MSVC中的预期效果如下:

// global scope
struct X {};
struct Y {};

int main()
{
    // function scope
    struct Z{};

    uint x0 = Component<X,int>::index;
    uint x1 = Component<X,double>::index;
    uint x2 = Component<X,double>::index;
    uint x3 = Component<X,std::string>::index;
    uint x4 = Component<X,int>::index;
    uint x5 = Component<X,int>::index;

    std::cout << x0 << ", " << x1 << ", " << x2 << ", "
              << x3 << ", " << x4 << ", " << x5 << std::endl;

    uint y0 = Component<Y,int>::index;
    uint y1 = Component<Y,double>::index;
    uint y2 = Component<Y,double>::index;
    uint y3 = Component<Y,std::string>::index;
    uint y4 = Component<Y,int>::index;
    uint y5 = Component<Y,int>::index;

    std::cout << y0 << ", " << y1 << ", " << y2 << ", "
              << y3 << ", " << y4 << ", " << y5 << std::endl;

    uint z0 = Component<Z,int>::index;
    uint z1 = Component<Z,double>::index;
    uint z2 = Component<Z,double>::index;
    uint z3 = Component<Z,std::string>::index;
    uint z4 = Component<Z,int>::index;
    uint z5 = Component<Z,int>::index;

    std::cout << z0 << ", " << z1 << ", " << z2 << ", "
              << z3 << ", " << z4 << ", " << z5 << std::endl;

    return 0;
}

输出是

0, 1, 1, 2, 0, 0
0, 1, 1, 2, 0, 0
0, 1, 1, 2, 0, 0

但是对于Clang(3.6.1),输出不同:

0, 1, 1, 2, 0, 0
0, 1, 1, 2, 0, 0
5, 2, 2, 3, 5, 5

具体来说,为函数本地类型(即'Z')生成的索引做了一些奇怪的事情.就像它们每次Component<Z,...>调用时增加和重新分配索引一样.

为什么会这样？这是编译器错误吗？使用带有模板的函数本地类型时是否有任何特殊注意事项(后C++ 11)？

这里有一个完整的例子:http: //coliru.stacked-crooked.com/a/7fcb989ae6eab476

==编辑==

我决定将这个问题发布给clang的bugtracker,所以如果有其他人遇到这个:

https://llvm.org/bugs/show_bug.cgi?id=24048

Answer 1

这对我来说看起来像是一个错误。我不知道 C++11 规则应该对函数局部类型和全局类型产生影响。

如果转储汇编器，您会注意到 while forX和Y值实际上被计算，因为Z它们是预先计算的。根本不生成用于计数器静态变量初始化的保护变量。

.Ltmp87:
    #DEBUG_VALUE: main:z5 <- 5
    #DEBUG_VALUE: main:z4 <- 5
    #DEBUG_VALUE: main:z3 <- 3
    #DEBUG_VALUE: main:z2 <- 2
    #DEBUG_VALUE: main:z1 <- 2
    #DEBUG_VALUE: main:z0 <- 5
    .loc    6 54 5                  # main.cpp:54:5
    movl    std::cout, %edi
    movl    $5, %esi
    .loc    6 74 5                  # main.cpp:74:5
    callq   std::basic_ostream<char, std::char_traits<char> >::operator<<(unsigned int)