c++ - 带模板的 typedef

Question

template<class T> struct A {
    typedef int Int;
    A::Int b; // Line 1 (fails)
    Int c; // Line 2 (compiles)
};

int main(){    
   A<int> x;
   x.c = 13;
}

错误

error: ISO C++ forbids declaration of ‘Int’ with no type
error: extra qualification ‘A<T>::’ on member ‘Int’
error: expected ‘;’ before ‘b’

第 1 行失败但第 2 行编译。为什么？

Answer 1

你需要一个typename

typename A::Int b;

typename 关键字是必需的，因为成员是使用限定名称 A::Int 引用的。

Int c 没问题，因为在这种情况下没有使用限定名称。

14.6/6

Within the definition of a class template or within the definition of a member of a class template, the keyword typename is not required when referring to the unqualified name of a previously declared member of the class template that declares a type. The keyword typename shall always be specified when the member is referred to using a qualified name, even if the qualifier is simply the class template name.