我已经尝试了与is_default_constructible的C 03兼容的实现：
template<class = void> struct is_default_constructible;
template<> struct is_default_constructible<>
{
protected:
// Put base typedefs here to avoid pollution
struct twoc { char a,b; };
template<bool> struct test { typedef char type; };
template<class T> static T declval();
};
template<> struct is_default_constructible<>::test<true> { typedef twoc type; };
template<class T> struct is_default_constructible : is_default_constructible<>
{
private:
template<class U> static typename test<!!sizeof(::new U())>::type sfinae(U*);
template<class U> static char sfinae(...);
public:
static bool const value = sizeof(sfinae<T>(0)) > 1;
};
当我在GCC中测试它(-std = c 03)时,它返回0,因为构造函数是不可见的：
class Test { Test(); };
int main()
{
return is_default_constructible<Test>::value;
}
当我在Visual C中测试它时(不同的版本都有相同的行为),我回来1.
当我在Clang中测试它(也是-std = c 03)时,我得到：
error: calling a private constructor of class 'Test'
template<class U> static typename test<!!sizeof(::new U())>::type sfinae(U *);
^
note: while substituting explicitly-specified template arguments into function template 'sfinae'
static bool const value = sizeof(sfinae<T>(0)) > 1;
^
note: in instantiation of template class 'is_default_constructible<Test>' requested here
return is_default_constructible<Test>::value;
^
error: calling a private constructor of class 'Test'
template<class U> static typename test<!!sizeof(::new U())>::type sfinae(U *);
^
note: while substituting deduced template arguments into function template 'sfinae' [with U = Test]
static bool const value = sizeof(sfinae<T>(0)) > 1;
^
note: in instantiation of template class 'is_default_constructible<Test>' requested here
return is_default_constructible<Test>::value;
哪个编译器是正确的,为什么？
