[英]Regarding differences between how MSVC and g++ treat C++ template code
我偶然發現了ACCU的這段代碼( http://accu.org/index.php/journals/1916 ),並且有興趣在C ++中使用CRTP來實現一個有趣的項目。 作者給出的代碼如下:
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;
};
};
}
我嘗試在Microsoft VS2015中編譯它,它給了我以下一組錯誤:
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 ';'
我使用相同的代碼,檢查語法並用g ++編譯它並編譯好。 我應該注意的兩個編譯器之間是否存在差異? 這會使cl.exe產生這些錯誤的問題是什么? 它們是由於cl.exe如何解析任何基於模板的代碼? 要使此代碼在msvc上運行需要進行哪些更改?
編輯:
以下是作者提供的test.cpp的完整代碼,以幫助您更清晰地了解情況:
#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;
}
恕我直言,它是模板化模板上的>>
syndrom。 在C ++ 11之前,需要一個空格來分隔>
token而不是一個>>
token。
從C ++ 11開始,n4296草案在14.2中說明了模板特化的名稱[temp.names]§3:
...類似地,第一個非嵌套>>被視為兩個連續但不同的>標記,第一個作為template-argument-list的結尾並完成template-id。
看起來MSVC2015還沒有實現那部分標准(或者您可能忘記聲明源代碼的C ++版本*)
為了完整起見,在使用CLang 3.4.1進行編譯而不指定std=c++11
它會顯示以下錯誤:
error: a space is required between consecutive right angle brackets (use '> >') using Type = A1<typename Apply<Aspects...>::template Type<T>>; // the er...
並希望在C ++ 11模式中甚至沒有警告......
(*)遺憾的是,我無法訪問VS2015,微軟在他們的C ++ 11/14/17 功能頁面中聲明了直角括號由VS2013和2015實現。所以我假設應該在項目屬性中的某處代碼級別指示。
所以在擺弄並盡可能地閱讀之后,我終於給原作者發了一封電子郵件。 Hugo Arregui先生,所有的功勞都歸功於他。
關於cl抱怨的行,修復方法如下:
template < template <class> class A1, template <class> class A2, template <class> class... Aspects>
struct Apply<A1,A2,Aspects...>
{
template<class T>
using Type = A1< typename Apply<A2, Aspects...>::template Type<T> >;
};
如果有人知道為什么Microsoft的編譯器要求我們在模板參數中擴展至少2個模板類並在此給出解釋,那將是很好的。 正在與作者交談,他也很驚訝。
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