无效使用非静态成员函数

问题描述

我有这样的东西：

class Bar
      {
      public:
        pair<string,string> one;
        std::vector<string> cars;
        Bar(string one, string two, string car);
      };

class Car
      {
      public:
        string rz;
        Bar* owner;
        Car(string car, Bar* p);
      };

class Foo
       {
         public:
          Foo  ( void );
          ~Foo  ( void );
          int Count ( const string & one, const string &  two) const;
          int comparator (const Bar & first, const Bar & second) const;
            std::vector<Bar> bars;
       };

int Foo::comparator(const Bar & first, const Bar & second) const{
  return first.name < second.name;
}

int Foo::Count  ( const string & one, const string & two ) const{
  int result=0;
  Bar mybar = Bar( one, two, "" );
  std::vector<Bar>::iterator ToFind = lower_bound(bars.begin(), bars.end(), mybar, comparator);
  if (ToFind != bars.end() && ToFind->one == mybar.one ){
    result = ...
  }
  return result;
}

方法 Foo :: Count 应该使用 std :: lower_bound（）在 vector< Bar> 字符串。
现在不起作用的部分。至 lower_bound（）我提供了方法 comparator（）。我认为它没问题，但g ++说：

The method Foo::Count should use std::lower_bound() to find element in vector<Bar> according to pair of two strings.Now the part which doesn't work. To lower_bound() I'm providing method comparator(). I thought it was okay, but g++ says:

c.cpp: In member function ‘int Foo::Count(const string&, const string&) const’:
c.cpp:42:94: error: invalid use of non-static member function
std::vector<Bar>::iterator ToFind = lower_bound(bars.begin(), bars.end(), mybar, comparator);

方法 Count（）必须停留 const ...

And the method Count() must stay const...

我对C ++很陌生，因为我不得不学习它。

I'm quite new to C++ because I'm forced to learn it.

有什么想法？

Any ideas?

推荐答案

c> Foo :: comparator static或将其包装在 std :: mem_fun 类对象中。这是因为lower_bounds期望比较器是一个具有调用操作符的对象类，如函数指针或函子对象。另外，如果您使用的是C ++ 11或更高版本，那么您也可以像所建议的那样使用lambda函数。 C ++ 11也有 std :: bind 。

You must make Foo::comparator static or wrap it in a std::mem_fun class object. This is because lower_bounds expects the comparor to be a class of object that has a call operator, like a function pointer or a functor object. Also, if you are using C++11 or later, you can also do as dwcanillas suggests and use a lambda function. C++11 also has std::bind too.

示例：

// Binding:
std::lower_bounds(first, last, value, std::bind(&Foo::comparitor, this, _1, _2));
// Lambda:
std::lower_bounds(first, last, value, [](const Bar & first, const Bar & second) { return ...; });

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