Question

我正在尝试使用以下代码检测运算符以及不与sfinae合作的任何专家对我缺少的内容有任何想法。

当你在

上试图检测它的类型上删除操作符+时，编译器也会死掉

    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 ==========

Answer 1

SFINAE代表“替换失败不是错误”。注意粗体部分 - 在你的检查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。当你实例化trait类时，它的签名已经是已知的，并且它被实例化的任何类型都应该有operator+。

您需要自己制作has模板。这样的事情应该有效（未经测试）：

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

其中yes是大小不同于sizeof(no)的类型的typedef。编写上述内容的方式，编译器无法推断U1和U2，因此您需要明确指定它们。因此，您还需要将后备功能作为模板。

Answer 2

问题在于“有运营商”案例：

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

您告诉编译器总是重载has，它接受T1*和T2*并返回...类型表达式T1 + T2。但只有当加法运算符可用时，此表达式才有类型。

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

检测操作员+

2 个答案: