函數對象和函數指針之間的區別？

Question

我定義了一個類，然後在priority_queue中保存指向Foo的指針，並使用我定義的cmp函數。

但如果CMP-funtion調用函數對象時，發生錯誤：

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; 
} 

的functione對象正常運行。

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

您需要構造que變量與您希望用作比較的函數。

#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; 
}