std :: find Object by Member [英] std::find Object by Member

问题描述

情况

我在使用STL时遇到了一个速度提升，看起来像一个正常情况，简化为：

  class Person {
 string Name; 
 int年龄
}; 
 
矢量< Person>人; 
 AddPeople（people）; 
 
 string s（Bob）; 
 find（people.begin（），people.end（），s）; 
  

问题

不幸的是找到想比较整个班级。

问题

一个更好或更合适的方式做这个STL方式？建议的问题没有帮助，但我设法找到了几个相关 问题< a>但无直接解决方案。

---

解决方法

有一些潜在的解决方法：

1. Forgo 查找完全（混乱，但可以重构）：

     bool bBob = false ; 
    for（UINT i = 0; i< people.size（）; i ++）{
    if（people [i] .Name == s）
    bBob = true; 
    break; 
   } 
     




2. 提供转换运算符不能用于 find ）：

     $ b string Name; 
    int年龄
    operator string（）{return Name;} 
   }; 
    
   人b（Bob，99）; 
    string s（Bob）; 
    b == s; //不工作
    string（b）== s; // works，but not good for find（）
     




3. 定义一个独立的等式运算符，有效但全局暴露）：

     BOOL操作符==（Person l，string r）{
    return l .Name == r; 
   } 
     




4. 定义成员相等运算符be first）：

     class Person {
    string Name; 
    int年龄
    bool operator ==（string s）{return Name == s;} 
   }; 
    
   人b（Bob，99）; 
    string s（Bob）; 
    b == s; // works 
    s == b; // does not work，but not a problem for find（）
     

b $ b

看起来＃4是最好的候选人，但没有一个是理想的或感觉STL，有些有问题。

解决方案

有没有更好或更合适的方法来做这个STL方式？

您可以使用 std :: find_if （由C ++ 11 lambdas支持）：

  std :: string name =Bob 
 // ... 
 std :: find_if（std :: begin（people），std :: end（people），
 [& p.Name == name;} 
  

注意，调用它的STL方式是C ++标准库，而不是STL（标准模板库）。STL是C ++标准库的容器和算法库的强大灵感，但两个东西不一样，参见这是关于StackOverflow的问答有关详细信息。

编辑：

由于您使用的编译器不支持lambdas，因此您可以定义自己的functor谓词：

  struct person_has_name 
 {
 person_has_name（std :: string const& n）：name（n）{} 
 bool operator Person const& p）{return p.Name == name;} 
 private：
 std :: string name; 
}; 
  pre> 
 
 
使用 std :: find_if 这样：
  std :: string name =Bob; 
 // ... 
 std :: find_if（people.begin（），people.end（），person_has_name（name））; 
  
 
Scenario

I’ve run into a speedbump while using the STL with what seems like a normal scenario, simplified here:
class Person {
  string Name;
  int    Age;
};

vector<Person> people;
AddPeople(people);

string s("Bob");
find(people.begin(), people.end(), s);

Problem
Unfortunately find wants to compare the entire class.



Question
Is there a better or more appropriate way to do this the "STL way"? The suggested questions weren’t helpful, but I managed to find a couple of related questions but no direct solution.





Work-arounds/Tests

There’s some potential work-arounds:

Forgo find altogether (cluttered, but could be refactored):
bool bBob = false;
for (UINT i = 0; i < people.size(); i++) {
  if (people[i].Name == s)
  bBob = true;
  break;
}

Provide conversion operator (implicit conversion doesn’t work; explicit can’t be used in find):
class Person {
  string Name;
  int    Age;
  operator string() {return Name;}
};

Person b ("Bob", 99);
string s ("Bob");
       b  == s;     //doesn’t work
string(b) == s;     //works, but no good for find()

Define a standalone equality operator (simple, effective, but globally exposed):
BOOL operator==(Person l, string r) {
  return l.Name == r;
}

Define a member equality operator (makes comparison order dependent; object must be first):
class Person {
  string Name;
  int    Age;
  bool operator==(string s) {return Name == s;}
};

Person b ("Bob", 99);
string s ("Bob");
b==s;               //works
s==b;               //doesn’t work, but not a problem for find()

It looks like #4 is the best candidate, but none seem ideal or feel "STL", and some have problems.
解决方案

  
Is there a better or more appropriate way to do this the "STL way"?
You can use std::find_if (powered by C++11 lambdas):
std::string name = "Bob";
// ...
std::find_if(std::begin(people), std::end(people), 
    [&] (Person const& p) { return p.Name == name; }
Notice, that calling it "STL way" is inappropriate. This is the C++ Standard Library, not the STL ("Standard Template Library"). The STL served as a strong inspiration for the Containers and Algorithms Library of the C++ Standard Library, but the two things are not the same. See this Q&A on StackOverflow for further information.

EDIT:

Since you are using a compiler that does not support lambdas, you can define your own functor predicate:
struct person_has_name
{
    person_has_name(std::string const& n) : name(n) { }  
    bool operator () (Person const& p) { return p.Name == name; }
private:
    std::string name;
};
And use it with std::find_if this way:
std::string name = "Bob";
// ...
std::find_if(people.begin(), people.end(), person_has_name(name));


                        
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