bind綁定

Prerequisites:

語法

function<函式傳回值類型(函式參數1類型 參數名1,函式參數2類型 參數名2)> func2 = bind(函式名, placeholders::_1, placeholders::_2);

function<void(int, const string&)> func2 = bind(print, placeholders::_1, placeholders::_2);

placeholders::_1 對映函式參數名1

placeholders::_2 對映函式參數名2

函式bind

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#include <iostream>
#include <functional>
using namespace std;
void print(int code, const string& msg) {
  cout << "Error code = " << code << " , Msg = " << msg << endl;
}
int main() {
  //設定bind與綁定的參數個數
  function<void(int, const string&)> func = bind(print, placeholders::_1, placeholders::_2);
  //呼叫函式
  func(400, "Page not found.");
  return 0;
}

Error code = 400 , Msg = Page not found.

參數對調

以下語法把函式參數對調

  function<void(const string&,int)> func = bind(print,  placeholders::_2,placeholders::_1);

呼叫函式時也要對調

	func("Page not found.", 400);

完整程式碼

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#include <iostream>
#include <functional>
using namespace std;
void print(int code, const string& msg) {
  cout << "Error code = " << code << " , Msg = " << msg << endl;
}
int main() {
  //設定bind與綁定的參數個數
  function<void(const string&,int)> func = bind(print,  placeholders::_2,placeholders::_1);
  //呼叫函式
  func("Page not found.", 400);
  return 0;
}

Error code = 400 , Msg = Page not found.

參數數量可自訂

使用bind，可以預設參數值，限制傳進參數的個數。

  function<void(const string&)> func = bind(print, 400, placeholders::_1);

呼叫函式只傳一個參數

func("Page not found.");

完整程式碼

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#include <iostream>
#include <functional>
using namespace std;
void print(int code, const string& msg) {
  cout << "Error code = " << code << " , Msg = " << msg << endl;
}
int main() {
  //設定bind與綁定的參數個數
  function<void(const string&)> func = bind(print, 400, placeholders::_1);
  //呼叫函式
  func("Page not found.");
  return 0;
}

Error code = 400 , Msg = Page not found.

bind預設傳值

以下的程式碼，即便之後把error_code改成500，顯示仍為400

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#include <iostream>
#include <functional>
using namespace std;
void print(int code, const string& msg) {
  cout << "Error code = " << code << " , Msg = " << msg << endl;
}
int main() {
  int error_code = 400;
  //設定bind與綁定的參數個數
  function<void(const string&)> func = bind(print, error_code, placeholders::_1);
  //修改成500
  error_code = 500;
  //呼叫函式
  func("Page not found.");
  return 0;
}

Error code = 400 , Msg = Page not found.

bind傳參考

語法

function<void(const string&)> func = bind(print, ref(error_code), placeholders::_1);

使用ref(), 可以傳參考，也就可以修改變數中的值。

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#include <iostream>
#include <functional>
using namespace std;
void print(int code, const string& msg) {
  cout << "Error code = " << code << " , Msg = " << msg << endl;
}
int main() {
  int error_code = 400;
  //設定bind與綁定的參數個數
  function<void(const string&)> func = bind(print, ref(error_code), placeholders::_1);
  error_code = 500;
  //呼叫函式
  func("Page not found.");
  return 0;
}

Error code = 500 , Msg = Page not found.

參數傳的比函式參數還多

print函式只有2個參數，但有一個需求需要傳3個參數，怎麼修改？

在function的函式參數多增加一個，呼叫函式的參數也多增加一個

  function<void(int, const string&, int)> func = bind(print, placeholders::_1, placeholders::_2);
  func(400, "Page not found.", 1000);

完整程式碼

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#include <iostream>
#include <functional>
using namespace std;
void print(int code, const string& msg) {
  cout << "Error code = " << code << " , Msg = " << msg << endl;
}
int main() {
  //設定bind與綁定的參數個數
  function<void(int, const string&, int)> func = bind(print, placeholders::_1, placeholders::_2);
  //呼叫函式
  func(400, "Page not found.", 1000);
  return 0;
}

Error code = 400 , Msg = Page not found.

物件成員函式與bind

  • function<參數類型1, …> 第1個參數要填入類別參考&
  • 等於號(=)指派對映的函式，必須把記憶體位址傳進去，所以使用&取位址運算子+類別名+::範圍運算子+函式名
  • 注意！bind的參數有三個，分別是物件參考student&，整數int，字串參考string&
  function<void(Student&,int, const string&)> func =  bind(&Student::print, placeholders::_1, placeholders::_2, placeholders::_3);

呼叫function

必須把物件代入第1個參數

  func(student, 500, "Server error.");

完整程式碼

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#include <iostream>
#include <functional>
using namespace std;
class Student {
public:
  void print(int code, const string& msg) {
    cout << "Error code = " << code << " , Msg = " << msg << endl;
  }
};
int main() {
  //建立物件
  Student student;
  //設定bind與綁定的參數個數
  function<void(Student&,int, const string&)> func = bind(&Student::print, placeholders::_1, placeholders::_2,placeholders::_3);
  //呼叫函式
  func(student, 400, "Page not found.");
  return 0;
}

Error code = 400 , Msg = Page not found.

類別靜態函式

語法

只需要函式前面加上類別名::

function<void(int, const string&)> func = bind(Student::print, placeholders::_1, placeholders::_2);

完整程式碼

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class Student {
public:
  static void print(int code, const string& msg) {
    cout << "Error code = " << code << " , Msg = " << msg << endl;
  }
};
int main() {
  //設定bind與綁定的參數個數
  function<void(int, const string&)> func = bind(Student::print, placeholders::_1, placeholders::_2);
  //呼叫函式
  func(400, "Page not found.");
  return 0;
}

lambda

bind的第1個參數代入lambda的變數名，也可以直接把lambda函式放進bind的第一個參數。

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int main() {
  //lambda
  auto print = [](int code, const string& msg) {
    cout << "Error code = " << code << " , Msg = " << msg << endl;
  };
  //設定bind與綁定的參數個數
  function<void(int, const string&)> func = bind(print, placeholders::_1, placeholders::_2);
  //呼叫函式
  func(400, "Page not found.");
  return 0;
}

物件函式

bind的第1個參數代入student物件。

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class Student {
public:
  void operator()(int code, const string& msg) {
    cout << "Error code = " << code << " , Msg = " << msg << endl;
  }
};
int main() {
  //建立物件
  Student student;
  //設定bind與綁定的參數個數
  function<void(int, const string&)> func = bind(student, placeholders::_1, placeholders::_2);
  //呼叫函式
  func(400, "Page not found.");
  return 0;
}

Error code = 400 , Msg = Page not found.

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