在 object 的向量上使用 find_if

Question

I have a vector of that looks like the following:

class Foo
{
    //whatever
};

class MyClass
{
    int myInt;
    vector<Foo> foo_v;
};

And let's say, in the main :

int main (void)
{
    vector<MyClass> myClass_v;
}

I want to find a object in myClass_v that has myInt == bar . I don't care about foo_v . I thought of using the std::find_if function:

std::find_if(myClass_v.begin(),myClass_v.end(),condition);

with

bool MyClass::condition(MyClass mc)
{
    if(mc.myInt==5)
        return true;
    else
        return false;
}

However the compiler says that condition() is missing arguments. Could you tell me what am I doing wrong? I thought that std::find_if would call condition(*First) , with First being a pointer to a myClass object.

Or is there another good way to do the same thing?

Answer 1

That's not how predicates work. You have to supply either a free function bool Comparator(const MyClass & m) {... } , or build a function object , a class that overloads operator() :

struct MyClassComp
{
  explicit MyClassComp(int i) n(i) { }
  inline bool operator()(const MyClass & m) const { return m.myInt == n; }
private:
  int n;
};

std::find_if(v.begin(), v.end(), MyClassComp(5));

In C++0x:

std::find_if(v.begin(), v.end(),
             [](const MyClass & m) -> bool { return m.myInt == 5; });

This captureless lambda is in fact equivalent to a free function. Here is a capturing version that mimics the predicate object:

const int n = find_me();
std::find_if(v.begin(), v.end(),
             [n](const MyClass & m) -> bool { return m.myInt == n; });

Answer 2

struct condition {
  bool operator()(const MyClass& mc) {
    return mc.myInt == 5;
  }
}

Answer 3

You can do it with a functor or a regular function that is not part of MyClass, or with a static function inside MyClass - here's an example with non-member function (basically just removing the MyClass:: part of the condition definition):

#include <algorithm>
#include <vector>

using namespace std;

class Foo
{
  //whatever
};

class MyClass
{
  public:
  int myInt;
  vector<Foo> foo_v;
};

bool condition(MyClass mc)
{
  if(mc.myInt==5)
    return true;
  else
    return false;
}


int main (void)
{
  vector<MyClass> myClass_v;
  std::find_if(myClass_v.begin(),myClass_v.end(),condition);
}

Answer 4

Besides what wrote, you can also use the member function as a predicate.

Define it as bool MyClass::condition() { return mc.myInt==5; } bool MyClass::condition() { return mc.myInt==5; } - parameter is unnecessary since it already takes object as implicit parameter.

When using it, wrap &MyClass::condition (pointer to a member function) in std::mem_fcn from functional header.

std::find_if(myClass_v.begin(), myClass_v.end(), std::mem_fcn(&MyClass::condition));

A more verbose way to do it, is to use std::function or std::bind . Replace:

    std::mem_fcn(&MyClass::condition)

with

    std::function<bool (MyClass &)>(&MyClass::condition)    , or
    std::bind(&MyClass::condition, std::placeholders::_1).

If MyClass_v has been declared as std::vector<MyClass *> myClass_v; ,

std::function<bool (MyClass &)>(&MyClass::condition) should be altered to: std::function<bool (MyClass *)>(&MyClass::condition) . For std::mem_fn and std::bind - no changes are needed.

Code:

#include <vector>
#include <functional>
#include <algorithm>
#include <iostream>

class Foo{};

struct MyClass
{
    int myInt;
    std::vector<Foo> foo_v;
    bool condition(){ return myInt==5; }
};

int main (void)
{
    std::vector<MyClass> myClass_v{ {1,{}}, {3,{}}, {5,{}}, {6,{}} };
    std::cout << std::find_if(myClass_v.begin(), myClass_v.end(), std::mem_fn(&MyClass::condition))->myInt << std::endl;
    return 0;
}