以一个栈链表为例，新建一个模板节点类Node
C++
template<typename Type>
class Stack;//先声明Stack类以便于在Node类中声明友元类
template<typename Type>
class Node
{
public:
Node()
{
value = 0;
next = nullptr;
}
Node(Type value) :value(value) { next = nullptr; }
friend class Stack;
private:
Type value;
Node<Type>* next;
};然后写模板栈类Stack
C++
template<typename Type>
class Stack
{
private:
Node<Type>* head;
Node<Type>* foot;
public:
Stack()
{
foot = new Node<Type>();
head = foot;
}
void push(Type value)
{
Node<Type>* node=new Node<Type>(value);
node->next = head;
head = node;
}
Type pop()
{
if (foot==head)
{
throw "empty stack";
}
Node<Type>* p = head;
Type value = head->value;
head = head->next;
delete p;
return value;
}
Type read()
{
if (foot == head)
{
throw "empty stack";
}
return head->value;
}
bool empty()
{
if (foot == head)
{
return true;
}
else
{
return false;
}
}
void print()
{
Node<Type>* p = head;
while (p!=foot)
{
cout << p->value << " ";
p = p->next;
}
cout << endl;
}
};编译运行，出现C2990错误： “Stack”: 非类 模板 已经声明为类 模板，查阅微软的文档，文档描述有：
由于 Microsoft C++编译器 For Visual Studio 2005 中的重大更改，也可能会发生 C2990;编译器现在要求同类型的多个声明对于模板规范是相同的。
https://docs.microsoft.com/zh-cn/cpp/error-messages/compiler-errors-2/compiler-error-c2990?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev16.query%3FappId%3DDev16IDEF1%26l%3DZH-CN%26k%3Dk(C2990)%26rd%3Dtrue&view=vs-2019
按照文档所给实例：
C++
template<class T>
class A;
// OK
template<class T>
struct B {
template<class T>
friend class A;
};即在友元类声明的上方也需要添加模板声明，所以以下写法即可正常运行
C++
template<typename Type>
class Stack;//先声明Stack类以便于在Node类中声明友元类
template<typename Type>
class Node
{
public:
Node()
{
value = 0;
next = nullptr;
}
Node(Type value) :value(value) { next = nullptr; }
template<typename Type>
friend class Stack;
private:
Type value;
Node<Type>* next;
};