函数对象和函数指针的区别？

Question

我定义了一个class，然后将指向Foo的指针保存在priority_queue中，并使用我定义的cmp函数。

但是如果 cmp-function 调用 function-object，就会发生错误：

class Foo
{
    friend bool cmp(Foo *, Foo *);
public:
    Foo() = default;
    Foo(int x):val(x) {}
private:
    int val;
};
bool cmp(Foo *a, Foo *b)
{
    return a->val < b->val;
}
int main()
{
    priority_queue<Foo*, vector<Foo*>, decltype(cmp)*> que;
    que.push(new Foo(5));
    que.push(new Foo(6));
    return 0;
}

函数对象正常运行。

class Foo
{
    friend struct cmp;
public:
    Foo() = default;
    Foo(int x):val(x) {}
private:
    int val;
};
struct cmp
{
    bool operator()(Foo *a, Foo *b)
    {
        return a->val < b->val;
    }
};
int main()
{
    priority_queue<Foo*, vector<Foo*>, cmp> que;
    que.push(new Foo(5));
    que.push(new Foo(6));
    return 0;
}

Answer 1

您需要使用要用作比较的函数构建 que 变量。

#include <vector>
#include <queue>

using namespace std;

class Foo
{
    friend bool cmp(Foo*, Foo*);
public:
    Foo() = default;
    Foo(int x):val(x) {}
private:
    int val;
};
bool cmp(Foo* a, Foo* b)
{
    return a->val < b->val;
}
int main()
{
    //                                                     vvv
    priority_queue<Foo*, vector<Foo*>, decltype(cmp)*> que(cmp);
    que.push(new Foo(5));
    que.push(new Foo(6));

    return 0;
}

函数对象和函数指针的区别？

the differences between function-object and function-pointer?

c++

class

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function-pointers

function-object