代碼

#include <iostream>#include <cstdlib>#include <memory>#include <string>#include <functional>using namespace std;class go{public:  go() {}  ~go()  {    cout << "go die./n";  }};auto d = [] ( go * gp ){  delete gp;  cout << "deletor done./n";};class go_de{public:  void operator() ( go* g )  {    d ( g );  }};int main(){  {    unique_ptr < go, go_de > b{ new go{} };//1  }  {    //unique_ptr < go, decltype (d) > b{ new go{}}; complie error //2    unique_ptr < go, decltype (d) > a{ new go{}, d };//3  }  {    unique_ptr < go, function<void(go*) > > a{ new go{}, d };//4    //i.e. unique_ptr < go, function<void(go*) > > a{ new go{}, [](go*gp) {delete gp;cout << "deletor done./n"; }};  }  system ( "pause" );  return 0;}

描述

一般的，需要給一個模板的Concept參數時，都會像代碼1的實現一樣傳入一個實現了該Concept的類型，例如go_de就實現了unique_ptr 的模板參數Deletor。

今天想嘗試一下使用lambda表達式的類型作為模板參數傳入，發現不行。原因在于

c++14 draft n4269

5.1.2 Lambda expressions

20 The closure type associated with a lambda-expression has no default constructor and a deleted copy assignment operator. It has a defaulted copy constructor and a defaulted move constructor (12.8). [ Note: These special member functions are implicitly defined as usual, and might therefore be defined as deleted. end note ]

意思就是 lambda 表達式沒有默認的構造函數，operator=也被置為deleted。只有一個默認的復制構造函數和move構造函數。很顯然，unique_ptr 的實現肯定是用到了Deletor Concept的默認構造函數的。所以編譯不通過。這個在
unique_ptr構造函數頁寫的很清楚。

2) Constructs a std::unique_ptr which owns p, initializing the stored pointer with p and value-initializing the stored deleter. Requires that Deleter is DefaultConstructible and that construction does not throw an exception.2) Constructs a std::unique_ptr which owns p, initializing the stored pointer with p and value-initializing the stored deleter. Requires that Deleter is DefaultConstructible and that construction does not throw an exception.

設想unique_ptr( pointer p, d1 );構造函數不存在，那Lambda類型就沒法作為Concept傳入了。

總結

• 想用Lambda表達式的類型作為Concept，使用類型推導關鍵字decltype
• Lambda的類型沒有default constructor、copy assignment operator.
• 寫C++庫的時候，如果用到模板和Concept技術，要考慮添加Concept對象做參數的類型的構造函數從而才能不限制Lambda表達式類型作為Concept傳入。

畢竟，C++語言設計的原則是盡量不限制C++語言的用戶的編程方式。

以上所述是小編給大家介紹的C++ 中使用lambda代替 unique_ptr 的Deleter的方法，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對VEVB武林網網站的支持！