Visual Studio 隐式静态成员初始化失败

Question

采用以下片段

#include <iostream>
using namespace std;

template<int i, typename T = int> struct A
{
    T num = i;
    A<i, T>()
    {
        cout << "Instantiated a A<" << i << ">" << endl;
    }
};

template<int i, int i2> struct B
{
    static A<i> a;
    static A<i * i2> a2;
};
template<int i, int i2> A<i> B<i, i2>::a{};
template<int i, int i2> A<i * i2> B<i, i2>::a2{};

template<typename T> struct C
{
    static void doSomething()
    {
        cout << "Have a A<" << T::a.num << "> and a A<" << T::a2.num << "> in C" << endl;
    }
};

int main() {
    typedef C<B<2, 2>> c;
    cout << "Typedefined a C\nCalling static member function to initialize C<B<2, 2>>'s B<2, 2>'s A<>s" << endl;
    c::doSomething();
    return 0;
}

现在使用 gcc，然后编译（C++11 和 C++14）和 instantiates a and a2 as expected。

感谢 WhozCraig，这也可以用 clang 编译。

然而，使用 Visual C++（2015 版）时，出现解析错误。

main.cpp(37) error C2143: syntax error: missing ';' before '<end Parse>'

后面是一些注释

main.cpp(19): note: while compiling class template static data member 'A<2,int> B<2,2>::a'
main.cpp(26): note: see reference to class template instantiation 'B<2,2>' being compiled
main.cpp(25): note: while compiling class template member function 'void C<B<2,2>>::doSomething(void)'
main.cpp(33): note: see reference to function template instantiation 'void C<B<2,2>>::doSomething(void)' being compiled

main.cpp(33): note: see reference to class template instantiation 'C<B<2,2>>' being compiled

这是怎么回事？

Answer 1

我能够减少测试用例，确定问题（这似乎是 Visual C++ 编译器中的错误）并找到解决方法：

#include <iostream>
using namespace std;

template<int i> struct A
{
    int num = i;
    A() {}
};

template<int i> struct B
{
    static A<i> a;
};

// MSVC doesn't like this syntax
//                         |||
//                         vvv
template<int i> A<i> B<i>::a{};

// To fix the error, rewrite the above line in one of the below ways:
//
// template<int i> A<i> B<i>::a;
// template<int i> A<i> B<i>::a = {};

int main() {
    typedef B<2> B2;
    cout << B2::a.num << endl;
    return 0;
}

Visual Studio 隐式静态成员初始化失败

Failed Implicit Static Member Initialization by Visual Studio

c++

visual-c++

initialization

c++14

static-members