C++ 模板错误

Question

我正在学习C++模板，我写了一个小算法。不幸的是我遇到了一些错误 这是我的代码

    #include <iostream>
#include <iomanip>
#include <deque>
using namespace std;

template<typename T, typename S, typename W>
void push(const T& value, deque<S>& original_stack, deque<W>& min_index)
{
  original_stack.push_back(value);
  if (min_index.size() == 0)
    min_index.push_back(0);
  else
    {
      if (value < original_stack[min_index.back()])
    {
      min_index.push_back(original_stack.size() - 1);
    }
      else
    {
      min_index.push_back(min_index.back());
    }
    }
}

template<typename S, typename W>
void pop(deque<S>& original_stack, deque<W>& min_index)
{
  original_stack.pop_back();
  min_index.pop_back();
}

template<typename T, typename S, typename W>
const T& min(deque<S>& original_stack, deque<W>& min_index)
{
  return original_stack[min_index.back()];
}

int main()
{
  deque<int> data_stack;
  deque<int> min_index;
  push(3, &data_stack, &min_index);
  push(4, &data_stack, &min_index);
  push(2, &data_stack, &min_index);
  push(1, &data_stack, &min_index);
  pop(&data_stack, &min_index);
  pop(&data_stack, &min_index);
  push(0, &data_stack, &min_index);
  cout<<"min num is: "<<min(&data_stack, &min_index)<<endl;
}

当我使用 g++ 编译它时，我得到了这个：

g++ minstack.cpp -o minstack
minstack.cpp:42:3: error: no matching function for call to 'push'
  push(3, &data_stack, &min_index);
  ^~~~
minstack.cpp:7:6: note: candidate template ignored: could not match
      'deque<type-parameter-0-1, allocator<type-parameter-0-1> >' against
      'std::__1::deque<int, std::__1::allocator<int> > *'
void push(const T& value, deque<S>& original_stack, deque<W>& min_index)
     ^
minstack.cpp:43:3: error: no matching function for call to 'push'
  push(4, &data_stack, &min_index);
  ^~~~
minstack.cpp:7:6: note: candidate template ignored: could not match
      'deque<type-parameter-0-1, allocator<type-parameter-0-1> >' against
      'std::__1::deque<int, std::__1::allocator<int> > *'
void push(const T& value, deque<S>& original_stack, deque<W>& min_index)
     ^
minstack.cpp:44:3: error: no matching function for call to 'push'
  push(2, &data_stack, &min_index);
  ^~~~
minstack.cpp:7:6: note: candidate template ignored: could not match
      'deque<type-parameter-0-1, allocator<type-parameter-0-1> >' against
      'std::__1::deque<int, std::__1::allocator<int> > *'
void push(const T& value, deque<S>& original_stack, deque<W>& min_index)
     ^
minstack.cpp:45:3: error: no matching function for call to 'push'
  push(1, &data_stack, &min_index);
  ^~~~
minstack.cpp:7:6: note: candidate template ignored: could not match
      'deque<type-parameter-0-1, allocator<type-parameter-0-1> >' against
      'std::__1::deque<int, std::__1::allocator<int> > *'
void push(const T& value, deque<S>& original_stack, deque<W>& min_index)
     ^
minstack.cpp:46:3: error: no matching function for call to 'pop'
  pop(&data_stack, &min_index);
  ^~~
minstack.cpp:26:6: note: candidate template ignored: could not match
      'deque<type-parameter-0-0, allocator<type-parameter-0-0> >' against
      'std::__1::deque<int, std::__1::allocator<int> > *'
void pop(deque<S>& original_stack, deque<W>& min_index)
     ^
minstack.cpp:47:3: error: no matching function for call to 'pop'
  pop(&data_stack, &min_index);
  ^~~
minstack.cpp:26:6: note: candidate template ignored: could not match
      'deque<type-parameter-0-0, allocator<type-parameter-0-0> >' against
      'std::__1::deque<int, std::__1::allocator<int> > *'
void pop(deque<S>& original_stack, deque<W>& min_index)
     ^
minstack.cpp:48:3: error: no matching function for call to 'push'
  push(0, &data_stack, &min_index);
  ^~~~
minstack.cpp:7:6: note: candidate template ignored: could not match
      'deque<type-parameter-0-1, allocator<type-parameter-0-1> >' against
      'std::__1::deque<int, std::__1::allocator<int> > *'
void push(const T& value, deque<S>& original_stack, deque<W>& min_index)
     ^
7 errors generated.

我很困惑，谁能帮我修复这些错误，我是 C++ 模板的新手。

Answer 1

您在呼叫站点遇到问题，例如：

deque<int> data_stack;
deque<int> min_index;
push(3, &data_stack, &min_index);

如果您将上述push() 函数调用与您的push() 模板重载进行比较：

template<typename T, typename S, typename W>
void push(const T& value, deque<S>& original_stack, deque<W>& min_index)

您可以注意到第二个和第三个参数是引用（对一些deque 实例）。

但在调用站点上，您使用地址运算符 (&) 传递 deque 实例的 地址，如果将参数定义为 指针（不是引用）。

由于您选择在函数定义中使用引用（而不是指针），因此只需去掉调用站点的地址运算符即可。
这样就好了：

push(3, data_stack, min_index); // References used

其他函数调用也是如此。

---

请注意，这是一个与引用与指针相关的问题。您可以抽象出模板部分，只需考虑以下简单的 C++ 代码即可解决问题：

void f(int& n) {
    n++;
}

int main() {
    int i = 1;
    // f(&i); // <-- fails to compile
    f(i);
}

由于 f() 函数需要 reference 到 int，因此您不想在调用站点使用指针传递 int i 变量：只需传递 @ 987654333@（不是&i）。

Answer 2

不要将地址传递给函数，参数是引用而不是指针

  push(3, data_stack, min_index);
  push(4, data_stack, min_index);
  push(2, data_stack, min_index);
  push(1, data_stack, min_index);
  pop(data_stack, min_index);
  pop(data_stack, min_index);
  push(0, data_stack, min_index);

你需要明确告诉min的返回类型

cout << min<int>(data_stack, min_index) << std::endl ;

或使用：

template<typename S, typename W>
 typename deque<S>::value_type& min(deque<S>& 
                                  original_stack, deque<W>& min_index)
{
  return original_stack[min_index.back()];
}

Answer 3

original_stack.push_back(value); // T and S should have the same type or at least T should be derived from S

但这不是问题。

int main()
{
  deque<int> data_stack;
  deque<int> min_index;
  push(3, data_stack, min_index);
  push(4, data_stack, min_index);
  push(2, data_stack, min_index);
  push(1, data_stack, min_index);
  pop(data_stack, min_index);
  pop(data_stack, min_index);
  push(0, data_stack, min_index);
  cout<<"min num is: "<<min(data_stack, min_index)<<endl;
}

如您所见，我删除了所有地址运算符，因为您是通过引用传递的；当您的参数是指针时，您需要传递地址。

另外，min 的返回类型应该是 S，因为你返回栈的一个元素。 而且你甚至不需要 W，因为索引总是整数。