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(shù)時，都會像代碼1的實現(xiàn)一樣傳入一個實現(xiàn)了該Concept的類型，例如go_de就實現(xiàn)了unique_ptr 的模板參數(shù)Deletor。

今天想嘗試一下使用lambda表達式的類型作為模板參數(shù)傳入，發(fā)現(xiàn)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?jīng)]有默認的構造函數(shù)，operator=也被置為deleted。只有一個默認的復制構造函數(shù)和move構造函數(shù)。很顯然，unique_ptr 的實現(xiàn)肯定是用到了Deletor Concept的默認構造函數(shù)的。所以編譯不通過。這個在
unique_ptr構造函數(shù)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(shù)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(shù)的類型的構造函數(shù)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腳本之家網(wǎng)站的支持！