c++ - 优先队列错误

Question

我遇到了麻烦，每当我尝试编译这个程序时，我都会在包含 q.pop 的函数中遇到错误。如果该行被注释掉，它可以正常编译，但它仍然会给出错误的值。由于某种原因，它总是错误的。

错误是:
priority_queue_demo.cpp: In function 'int main()':
priority_queue_demo.cpp:54:12: error: invalid conversion from 'int' to 'const char*' [-fpermissive]
/usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/basic_string.tcc:214:5: error: initializing argument 1 of 'std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]' [-fpermissive]
priority_queue_demo.cpp: In function 'bool contains(const std::priority_queue<T>&, T) [with T = int, typename std::vector<T, std::allocator<_Tp1> >::value_type = int]':
priority_queue_demo.cpp:43:47: instantiated from here
priority_queue_demo.cpp:27:3: error: passing 'const std::priority_queue<int>' as 'this' argument of 'void std::priority_queue<_Tp, _Sequence, _Compare>::pop() [with _Tp = int, _Sequence = std::vector<int, std::allocator<int> >, _Compare = std::less<int>]' discards qualifiers [-fpermissive]


#include <iostream>
#include <string>
#include <sstream>

#include <queue>

using namespace std;

template <typename T>
ostream &operator <<(ostream &os, priority_queue<T> &q) {
        os << "{";
        while (!q.empty()) {
               os << q.top() << (!q.empty() ? ", " : "");
               q.pop();
        }

        os << "}";

        return os;
}

template <typename T>
bool contains(const priority_queue<T> &q, T val) {
        for (int i = 0; i < q.size(); i++) {
                if (q.top() == val) return true;
                q.pop();
        }

        return false;
}

int main() {
        cout << boolalpha;

        priority_queue<int> qi;

        for (int i = 0; i < 20; i++)
                qi.push(i);

        cout << qi << endl;

        cout << "qi contians 15: " << contains(qi, 15) << endl;
        cout << "qi contians 23: " << contains(qi, 23) << endl;

        const int ARR_SIZE = 4;
        string arr[ARR_SIZE] = {"cat", "dog", "cow", "elephant"};

        cout << endl;

        priority_queue<string> qs;

        for(int i = 0; i < ARR_SIZE; i++)
                qs.push(i);

        cout << qs << endl;

        while(!qs.empty()) {
                qs.pop();
                cout << qs << endl;
        }


        return 0;
 }

Answer 1

问题是您传递的是 const引用队列，但是 pop()显然不是 const操作，因为它会改变队列。

你应该传递一个非常量引用:

bool contains(priority_queue<T>& q, T val) { .... }

如您所见，您无法在不更改队列的情况下真正检查队列的内容。

下一个错误是您将一个整数插入 std::string 的队列中:

qi.push(i);

你可能是说

qi.push(arr[i]);

注意 ostream& operator<< 的重载对于队列清空队列，所以任何尝试使用 contains将队列打印到 std::cout 之后无论队列的原始内容如何，​​都会产生 false。

查看演示 here .