双重模板参数：
我们还是以前几篇中的Stack为例子i:代码如下：
template <typename T,
template <typename ELEM> class CONT = std::deque >
class Stack {
private:
CONT<T> elems; // elements
public:
void push(T const&); // push element
void pop(); // pop element
T top() const; // return top element
bool empty() const { // return whether the stack is empty
return elems.empty();
}
};
和往常一样，你可以使用class替代typename，但是CONT必须是class。
template <typename T,
template <class ELEM> class CONT = std::deque> // OK
class Stack {
…
};
but the following is not:
template <typename T,
template <typename ELEM> typename CONT = std::deque>
class Stack { // ERROR
…
};
由于上面例子中的ELEM实际中没有用到，其实是可以省略的。
我们实现一个成员函数看看：
template <typename T, template <typename> class CONT>
void Stack<T,CONT>::push (T const& elem)
{
elems.push_back(elem); // append copy of passed elem
}
但是如果试图使用上面的stack，编译器会说deque不符合CONT的要求，问题在于，std中的deque要求不只是一个参数，第二个参数是一个配置器，他虽然有预设值，但是当它被用来匹配CONT参数的时候，预设值被编译器忽略了。
对于这个问题，我们可以修改，使得ＣＯＮＴ参数要求一个带两个参数的容器。
template <typename T,
template <typename ELEM,
typename ALLOC = std::allocator<ELEM> >
class CONT = std::deque>
class Stack {
private:
CONT<T> elems; // elements
…
};
// basics/stack8.hpp
#ifndef STACK_HPP
#define STACK_HPP
#include <deque>
#include <stdexcept>
#include <allocator>
template <typename T,
template <typename ELEM,
typename = std::allocator<ELEM> >
class CONT = std::deque>
class Stack {
private:
CONT<T> elems; // elements
public:
void push(T const&); // push element
void pop(); // pop element
T top() const; // return top element
bool empty() const { // return whether the stack is empty
return elems.empty();
}
// assign stack of elements of type T2
template<typename T2,
template<typename ELEM2,
typename = std::allocator<ELEM2>
>class CONT2>
Stack<T,CONT>& operator= (Stack<T2,CONT2> const&);
};
template <typename T, template <typename,typename> class CONT>
void Stack<T,CONT>::push (T const& elem)
{
elems.push_back(elem); // append copy of passed elem
}
template<typename T, template <typename,typename> class CONT>
void Stack<T,CONT>::pop ()
{
if (elems.empty()) {
throw std::out_of_range("Stack<>::pop(): empty stack");
}
elems.pop_back(); // remove last element
}
template <typename T, template <typename,typename> class CONT>
T Stack<T,CONT>::top () const
{
if (elems.empty()) {
throw std::out_of_range("Stack<>::top(): empty stack");
}
return elems.back(); // return copy of last element
}
template <typename T, template <typename,typename> class CONT>
template <typename T2, template <typename,typename> class CONT2>
Stack<T,CONT>&
Stack<T,CONT>::operator= (Stack<T2,CONT2> const& op2)
{
if ((void*)this == (void*)&op2) { // assignment to itself?
return *this;
}
Stack<T2> tmp(op2); // create a copy of the assigned stack
elems.clear(); // remove existing elements
while (!tmp.empty()) { // copy all elements
elems.push_front(tmp.top());
tmp.pop();
}
return *this;
}
#endif // STACK_HPP
// basics/stack8test.cpp
#include <iostream>
#include <string>
#include <cstdlib>
#include <vector>
#include "stack8.hpp"
int main()
{
try {
Stack<int> intStack; // stack of ints
Stack<float> floatStack; // stack of floats
// manipulate int stack
intStack.push(42);
intStack.push(7);
// manipulate float stack
floatStack.push(7.7);
// assign stacks of different type
floatStack = intStack;
// print float stack
std::cout << floatStack.top() << std::endl;
floatStack.pop();
std::cout << floatStack.top() << std::endl;
floatStack.pop();
std::cout << floatStack.top() << std::endl;
floatStack.pop();
}
catch (std::exception const& ex) {
std::cerr << "Exception: " << ex.what() << std::endl;
}
// stack for ints using a vector as an internal container
Stack<int,std::vector> vStack;
…
vStack.push(42);
vStack.push(7);
std::cout << vStack.top() << std::endl;
vStack.pop();
}
7 42 Exception: Stack<>::top(): empty stack 7
