4

我正在尝试使用以下代码在一个类上检测运算符，该代码不适用于 sfinae，任何专家都对我所缺少的内容有任何想法。

当您在尝试检测它的类型上删除运算符 + 时，编译器也会死亡

    template<class T1, class T2>
        class has_addition_operator
        {
        private:

            typedef char no;

            static auto has(T1* a, T2* b) -> decltype( *a + *b);

            static char has(...);

        public:
            enum{
                value = (sizeof( has(new T1(), new T2())) != sizeof(no))
            };

        };


struct point{
        int x, y;

        point operator + (point const & o){

            point r = *this;
            r.x += o.x;
            r.y += o.y;
            return r;

        }

    };

    bool has = liboperator::has_addition_operator<point,point>::value;

编译器具有以下输出：

1>------ Build started: Project: liboperator, Configuration: Debug Win32 ------
1>  liboperator.cpp
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2784: 'std::_String_iterator<_Mystr> std::operator +(_String_iterator<_Mystr>::difference_type,std::_String_iterator<_Mystr>)' : could not deduce template argument for 'std::_String_iterator<_Mystr>' from 'point'
1>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(420) : see declaration of 'std::operator +'
1>          c:\projects\liboperator\liboperator\liboperator\liboperator.cpp(26) : see reference to class template instantiation 'liboperator::has_addition_operator<T1,T2>' being compiled
1>          with
1>          [
1>              T1=point,
1>              T2=point
1>          ]
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2784: 'std::_String_const_iterator<_Mystr> std::operator +(_String_const_iterator<_Mystr>::difference_type,std::_String_const_iterator<_Mystr>)' : could not deduce template argument for 'std::_String_const_iterator<_Mystr>' from 'point'
1>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring(288) : see declaration of 'std::operator +'
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2784: 'std::move_iterator<_RanIt> std::operator +(_Diff,const std::move_iterator<_RanIt> &)' : could not deduce template argument for 'const std::move_iterator<_RanIt> &' from 'point'
1>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(1947) : see declaration of 'std::operator +'
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2784: 'std::_Array_iterator<_Ty,_Size> std::operator +(_Array_iterator<_Ty,_Size>::difference_type,std::_Array_iterator<_Ty,_Size>)' : could not deduce template argument for 'std::_Array_iterator<_Ty,_Size>' from 'point'
1>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(1801) : see declaration of 'std::operator +'
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2784: 'std::_Array_const_iterator<_Ty,_Size> std::operator +(_Array_const_iterator<_Ty,_Size>::difference_type,std::_Array_const_iterator<_Ty,_Size>)' : could not deduce template argument for 'std::_Array_const_iterator<_Ty,_Size>' from 'point'
1>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(1662) : see declaration of 'std::operator +'
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2784: 'std::reverse_iterator<_RanIt> std::operator +(_Diff,const std::reverse_iterator<_RanIt> &)' : could not deduce template argument for 'const std::reverse_iterator<_RanIt> &' from 'point'
1>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(1226) : see declaration of 'std::operator +'
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2784: 'std::_Revranit<_RanIt,_Base> std::operator +(_Diff,const std::_Revranit<_RanIt,_Base> &)' : could not deduce template argument for 'const std::_Revranit<_RanIt,_Base> &' from 'point'
1>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(1031) : see declaration of 'std::operator +'
1>c:\projects\liboperator\liboperator\liboperator\has_addition_operator.h(15): error C2676: binary '+' : 'point' does not define this operator or a conversion to a type acceptable to the predefined operator
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
4

2 回答 2

8

SFINAE 代表“替换失败不是错误”。请注意粗体部分 - 在您的检查器功能中没有什么可以替代的

static auto has(T1* a, T2* b) -> decltype( *a + *b);

因为它本身不是模板。当您实例化 trait 类时，它的签名是已知的，并且它被实例化的任何类型都应该具有operator+.

您需要自己制作has模板。像这样的东西应该可以工作（未经测试）：

template<typename U1, typename U2>
static constexpr auto has(int*)
    -> decltype( std::declval<U1>() + std::declval<U2>(), yes() )

yes与大小不同的类型的 typedef在哪里sizeof(no)？上面的写法，编译器没有机会推断U1，U2所以你需要明确地指定它们。因此，您也需要将回退函数设为模板。

于 2013-06-13T09:08:26.777 回答
2

问题在于“有运算符”的情况：

static auto has(T1* a, T2* b) -> decltype( *a + *b);

您是在告诉编译器，总是has有一个接受 aT1*和 aT2*并返回...表达式的类型的重载T1 + T2。但是只有当加法运算符可用时，此表达式才具有类型。

这会让你进入一个循环：为了确定一个是否operator+存在（填充value），编译器必须确定operator+... 的返回类型，它可能不存在！

于 2013-06-13T08:55:18.463 回答