c++ - 关于 MSVC 和 g++ 处理 C++ 模板代码的区别

Question

我偶然发现了来自 ACCU (http://accu.org/index.php/journals/1916) 的这段代码，并且有兴趣通过将 CRTP 用于一个有趣的项目来在 C++ 中应用 AOP。作者给出的代码如下:

namespace aop
{

template <class A>
class NullAspect
{};

template <template <template <class> class> class Base>
struct Decorate
{
private:
    struct None {};

    template <template <class> class ... Aspects>
    struct Apply;

    template <template <class> class T>
    struct Apply<T>
    {
        template <class E>
        using Type = T<E>;
    };

    template<template < class > class A1, template < class > class ... Aspects>
    struct Apply<A1, Aspects...>
    {
        template <class T>
        using Type = A1<typename Apply<Aspects...>::template Type<T>>; // the errors point to this line and the 'Type' refers to the 'template Type<T>'
    };

public:
    template<template <class> class ... Aspects>
    struct with
    {
        template <class T>
        using AspectsCombination = typename Apply<Aspects...>::template Type<T>;

        typedef AspectsCombination<Base<AspectsCombination>> Type;
    };
};
}

我尝试在 Microsoft VS2015 中编译它，它给了我以下一组错误:

Error   C2146   syntax error: missing '>' before identifier 'Type'  
Error   C2947   expecting '>' to terminate template-argument-list, found '<'    
Error   C2061   syntax error: identifier 'T'    
Error   C2238   unexpected token(s) preceding ';'   
Error   C1201   unable to continue after syntax error in class template definition  
Error   C2143   syntax error: missing ';' before '}'    
Error   C2238   unexpected token(s) preceding ';'   

我采用了相同的代码，检查了语法并在 g++ 中进行了编译，并且编译正常。我应该注意的 2 个编译器之间是否存在任何差异？导致 cl.exe 产生这些错误的问题是什么？它们是因为 cl.exe 如何解析任何基于模板的代码吗？需要进行哪些更改才能使此代码在 msvc 上运行？

编辑:

下面是作者提供的test.cpp完整代码，帮助大家看得更清楚:

#include <iostream>
#include <cmath>
#include "aop.h"

//#define INHERITING_CTORS  as of g++ 6.4.3, inheriting ctors was not implemented

template <typename _UnderlyingType>
struct Number
{
    template <template <class> class A = aop::NullAspect>
    class Type
    {
    public:
        typedef _UnderlyingType UnderlyingType;
        typedef A<Number::Type<A>> FullType;

        Type(UnderlyingType n)
            : n(n)
        {}

        friend std::ostream& operator<<(std::ostream& out, const Type& number)
        {
            return out << number.n;
        }
    protected:
        UnderlyingType n;
    };
};

template <class A>
class ArithmeticAspect: public A
{
public:
    typedef typename A::FullType FullType;

#ifdef INHERITING_CTORS
    using A::A;
#else
    ArithmeticAspect(typename A::UnderlyingType n)
        : A(n)
    {}

    ArithmeticAspect(const A& a)
        : A(a)
    {}
#endif

    FullType operator+(const FullType& other) const
    {
        FullType tmp(*this);
        return tmp += other;
    }

    FullType operator-(const FullType& other) const
    {
        FullType tmp(*this);
        return tmp -= other;
    }

    FullType operator+=(const FullType& other)
    {
        A::n += other.n;
        return A::n;
    }

    FullType operator-=(const FullType& other)
    {
        A::n -= other.n;
        return A::n;
    }

    // same for *, *=, /, /=
};

template <class A>
class IncrementalAspect: public A
{
public:
    typedef typename A::FullType FullType;

#ifdef INHERITING_CTORS
    using A::A;
#else
    IncrementalAspect(typename A::UnderlyingType n)
        : A(n)
    {}

    IncrementalAspect(const A& a)
        : A(a)
    {}
#endif

    FullType operator++(int)
    {
        FullType tmp(*this);
        operator++();
        return tmp;
    }

    FullType operator++()
    {
        ++A::n;
        return *this;
    }

    FullType operator--(int)
    {
        FullType tmp(*this);
        operator--();
        return tmp;
    }

    FullType operator--()
    {
        --A::n;
        return *this;
    }
};

/*
* Configurable Aspect sumExample
*/
template <unsigned int PRECISION>
struct RoundAspect
{
    template <class A>
    class Type : public A
    {
    public:
        typedef typename A::FullType FullType;

#ifdef INHERITING_CTORS
        using A::A;
#else
        Type(typename A::UnderlyingType n)
            : A(n)
        {}

        Type(const A& a)
            : A(a)
        {}
#endif

        FullType operator+(const FullType& other) const
        {
            return FullType(round(A::operator+(other).n));
        }

    private:
        static float round(float f)
        {
            const unsigned int e = std::pow(10, PRECISION);
            return float(int(f * e)) / e;
        }
    };
};

template <class A>
class LogicalAspect: public A
{
public:
    typedef typename A::FullType FullType;

#ifdef INHERITING_CTORS
    using A::A;
#else
    LogicalAspect(typename A::UnderlyingType n)
        : A(n)
    {}

    LogicalAspect(const A& a)
        : A(a)
    {}
#endif

    bool operator!() const
    {
        return !A::n;
    }

    bool operator&&(const FullType& other) const
    {
        return A::n && other.n;
    }

    bool operator||(const FullType& other) const
    {
        return A::n || other.n;
    }
};

template <class A>
class BitwiseAspect: public A
{
public:
    typedef typename A::FullType FullType;

#ifdef INHERITING_CTORS
    using A::A;
#else
    BitwiseAspect(typename A::UnderlyingType n)
        : A(n)
    {}

    BitwiseAspect(const A& a)
        : A(a)
    {}
#endif

    bool operator~() const
    {
        return ~A::n;
    }

    FullType operator&(const FullType& mask) const
    {
        return A::n & mask.n;
    }

    FullType operator|(const FullType& mask) const
    {
        return A::n | mask.n;
    }

    FullType operator<<(const FullType& bitcount) const
    {
        return A::n << bitcount.n;
    }

    FullType operator>>(const FullType& bitcount) const
    {
        return A::n >> bitcount.n;
    }

    FullType& operator>>=(const FullType& bitcount)
    {
        A::n >>= bitcount.n;
        return *static_cast<FullType*>(this);
    }
};

template <class N>
void sumExample(typename N::UnderlyingType n1, typename N::UnderlyingType n2)
{
    N a(n1);
    N b(n2);
    N c = a + b;
    std::cout << c << std::endl;
}

template <class N>
void orExample(typename N::UnderlyingType n1, typename N::UnderlyingType n2)
{
    N a(n1);
    N b(n2);
    std::cout << (a || b) << std::endl;
}

template <class N>
void bitwiseExample(typename N::UnderlyingType n1, typename N::UnderlyingType n2)
{
    N a(n1);
    N b(n2);
    std::cout << (a + ((b >>= 1) << 3)) << std::endl;
}

int main()
{

    typedef aop::Decorate<Number<unsigned int>::Type>::with<ArithmeticAspect, IncrementalAspect, LogicalAspect, BitwiseAspect>::Type IntegralNumber;
    bitwiseExample<IntegralNumber>(1, 2);
    sumExample<IntegralNumber>(1, 2);

    typedef aop::Decorate<Number<float>::Type>::with<RoundAspect<2>::Type, ArithmeticAspect, LogicalAspect>::Type FloatRoundLogicalNumber;
    orExample<FloatRoundLogicalNumber>(1, 0);

    typedef aop::Decorate<Number<int>::Type>::with<LogicalAspect>::Type IntLogicalNumber;
    orExample<IntLogicalNumber>(1, 0);

    typedef aop::Decorate<Number<float>::Type>::with<RoundAspect<2>::Type, ArithmeticAspect>::Type FloatRoundNumber;
    sumExample<FloatRoundNumber>(1.339, 1.1233);

    return 0;
}

Answer 1

恕我直言，它是模板化模板上的 >>> 综合症。在 C++11 之前，需要一个空格来分隔 >> 标记，而不是一个 >>> 标记。

从 C++ 11 n4296 草案开始，在 14.2 Names of template specializations [temp.names] §3 中说:

... Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id.

看起来 MSVC2015 还没有实现那部分标准(或者你可能忘记声明源的 C++ 版本*)

为了完整起见，在未指定 std=c++11 的情况下使用 CLang 3.4.1 进行编译时会显示此错误:

error: a space is required between consecutive right angle brackets (use '> >')
    using Type = A1<typename Apply<Aspects...>::template Type<T>>; // the er...

希望在 C++11 模式下甚至没有警告...

(*) 不幸的是，我无法访问 VS2015，Microsoft 声明他们的 C++11/14/17 Features页 Right angle brackets由 VS2013 和 2015 实现。所以我假设项目属性中的某处应该有一个代码级别指示。