依赖型基类

基类依赖模版参数

template <typename T>
class BBB{
    public:
        void one(int ){}
};

template <typename T>
class DDD : public BBB<T>
{
    public:
        void f()
        {
            one(1); // 依赖型名称实例化时查找
        }

};

编译时会有警告：

/home/ubuntu/MyRep/cpp/TestTemplate.cpp:26:4: warning: there are no arguments to ‘one’ that depend on a template parameter, so a declaration of ‘one’ must be available [-fpermissive]
   26 |    one(1); // 依赖型名称实例化时查找
      |    ^~~
/home/ubuntu/MyRep/cpp/TestTemplate.cpp: In instantiation of ‘void DDD<T>::f() [with T = int]’:
/home/ubuntu/MyRep/cpp/TestTemplate.cpp:124:6:   required from here
/home/ubuntu/MyRep/cpp/TestTemplate.cpp:26:7: warning: ‘one’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
   26 |    one(1); // 依赖型名称实例化时查找
      |    ~~~^~~
/home/ubuntu/MyRep/cpp/TestTemplate.cpp:26:7: note: declarations in dependent base ‘BBB<int>’ are not found by unqualified lookup
/home/ubuntu/MyRep/cpp/TestTemplate.cpp:26:7: note: use ‘this->one’ instead

one 函数是非依赖名称（可以理解为：没有依赖于模板参数的名称），编译器在看到时就查找，这时 BBB 还没有实例化，先进行普通查找(作用域内)，所以找不到 ‘one’ was not declared in this scope
one 同时也是非受限名称（可以理解为：前面没有 -> :: . )，普通查找没找到函数，进行ADL查找。ADL 查找会从关联类和关联名字空间查找

根据第五条，参数类型int ，返回类型 void，集合也都是空集，仍然找不到 one 函数，所以 and no declarations were found by argument-dependent lookup at the point of instantiation。 ADL : argument-dependent lookup。

代码改为:

this->one(1);

编译通过, 因为将 one 变成了依赖型名称( 根据依赖型名称定义，如果 -> 左边的表达式类型依赖于模板参数，该受限名称(one)也是依赖型名称)，而又根据 C++规则，依赖型名称在实例化时才会进行查找，这是BBB已经被实例化，找到了 one。