C++: function object

整理自：Thinking in C++, Volume 2

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1. Intro

在 C++: Generic Algorithm Examples 我们写过一个叫做 gt15(int x) 的函数，如果能把这个函数封装成一个对象 Gt(15) 明显会更灵活。这样的对象我们称为 function object，而实现的手段就是重载 operator() (the function-call operator):

#include <iostream>
using namespace std;

class Gt {
    int than;
public:
    Gt(int than) : than(than) {}  // 注意冒号后面是 member initializer list
    bool operator()(int x) const {  // 注意语法 
        return x > than;
    }
};

int main() {
    Gt gt15(15);
    
    cout << gt15(1) << endl; // output: 0 (for false)
    cout << gt15(20) << endl; // output: 1 (for true)
}

注意重载 operator() 的语法：

• operator() 这个整体表示 () 这个 operator，相当于 python 的 __call__
• operator()(int x) 就类似 __call__(x: int)
• 如果是无参的 opeartor，你要写成 operator()()。

我们进一步观察，其实可以把 int than; 变成 template<T>。这里我们不示范了，因为 lib 已经有写了，但是又稍微有点不同：

template <class T> 
struct greater : binary_function <T,T,bool> {
	bool operator() (const T& x, const T& y) const {return x>y;}
};

首先它是一个 struct；然后它不像我们的 Gt 可以初始化一个参数，而且只能像 greater<int>(15, 20) 这样传两个参数进去才能用。这个 greater 无法直接用于 C++: Generic Algorithm Examples 的场景，所以我们需要进一步封装：

bind2nd(greater<int>(), 15));

• 需要注意的是，greater<int>() 这是在调用构造器生成一个 greater<int> 对象，对等于我们的 Gt gt15(15);
• 我们经常会看到直接在这个 temporary 上调用函数，比如 greater<int>()(37, 15)，这一句是对等于我们的 gt15(37)
  • 所以不要把 greater<int>() 当成是调用 operator()

bind2nd 的意思是 “bind the 2nd parameter”，所以这一句的效果就是把 greater<int>() 这个 BinaryFunction 对象（虽然是个 struct，姑且这么称呼）变成了一个 UnaryFunction 对象，功能上等同于我们的 Gt gt15(15);。

同理还有 bind1st。

这样 C++: Generic Algorithm Examples 里的例子就可以重写成：

#include <algorithm>
#include <cstddef>
#include <functional>
#include <iostream>
#include <iterator>
using namespace std;

int main() {
    int a[] = { 10, 20, 30 };
    const size_t SIZE = sizeof a / sizeof a[0];
    
	remove_copy_if(a, a + SIZE,
                   ostream_iterator<int>(cout, "\n"),
                   bind2nd(greater<int>(), 15));
    // output: 10
}

2. Classification of function objects

• Generator: Takes no arguments. The standard library provides one generator, the function rand() declared in <cstdlib>, and has some algorithms, such as generate_n(), which apply generators to a sequence.
• Unary Function: Takes a single argument. ReturnType includes void.
• Binary Function: Takes two arguments. ReturnType includes void.
• Unary Predicate: A Unary Function that returns a bool.
• Binary Predicate: A Binary Function that returns a bool.

有些算法要求 binary predicate 要满足 strict weak order 的定义，参考 Special Relations / Combinations of Properties: - It allows for a more general interpretation of “equality.” Some of the standard containers consider two elements equivalent if neither is less than the other (using operator<). This is important when comparing floating-point values, and objects of other types where operator== is unreliable or unavailable. - This notion also applies if you want to sort a sequence of data records (structs) on a subset of the struct’s fields. That comparison scheme is considered a strict weak ordering because two records with equal keys are not really “equal” as total objects, but they are equal as far as the comparison you’re using is concerned.

3. Standard function objects

Name	Type	Result produced
plus	BinaryFunction	arg1 + arg2
minus	BinaryFunction	arg1 - arg2
multiplies	BinaryFunction	arg1 * arg2
divides	BinaryFunction	arg1 / arg2
modulus	BinaryFunction	arg1 % arg2
negate	UnaryFunction	-arg1
equal_to	BinaryPredicate	arg1 == arg2
not_equal_to	BinaryPredicate	arg1 != arg2
greater	BinaryPredicate	arg1 > arg2
less	BinaryPredicate	arg1 < arg2
greater_equal	BinaryPredicate	arg1 >= arg2
less_equal	BinaryPredicate	arg1 <= arg2
logical_and	BinaryPredicate	arg1 && arg2
Logical_or	BinaryPredicate	arg1 || arg2
logical_not	UnaryPredicate	!arg1
unary_negate	UnaryLogical	!(UnaryPredicate(arg1))
binary_negate	BinaryLogical	!(BinaryPredicate(arg1, arg2))

4. Function object adaptors

比如前面提到的 bind1st 和 bind2nd 都是 function object adaptors。

此外还有一个 not1, which takes a unary function object as a parameter and invert its truth value. 比如 not1(bind1st(equal_to<int>(), 20)) 的作用就是 bind1st(not_equal_to<int>(), 20)。

需要注意的是，gt15 这样的 function name 在 remove_copy_if 操作中和 function object 起到的作用是一样的，但是 function name 不能用于 bind1st 和 bind2nd，这个时候你需要 ptr_fun() adaptor, which take a pointer to a function and turn it into a function object. ptr_fun() is not designed for a function that takes no arguments—they must only be used with unary functions or binary functions.

• ptr_fun<class Arg, class Result>(funcName) for unary functions
• ptr_fun<class Arg1, class Arg2, class Result>(funcName) for binary functions

5. Member function object adaptor

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>
using namespace std;

class Shape {
public:
    virtual void draw() = 0;
    virtual ~Shape() {}
};

class Circle : public Shape {
private:
	int id; 
public:
	Circle(int id) : Shape(), id(id) {
		
	}
public:
    void draw() {
        cout << "Circle." << id << "::Draw()" << endl;
    }
    ~Circle() {
		
    }
};

class Square : public Shape {
private:
	int id; 
public:
	Square(int id) : Shape(), id(id) {
		
	}
    void draw() {
        cout << "Square." << id << "::Draw()" << endl;
    }
    ~Square() {
		
    }
};

int main() {
    vector<Shape*> shapeVec;
    shapeVec.push_back(new Circle(1));
    shapeVec.push_back(new Square(1));
    
	for_each(shapeVec.begin(), shapeVec.end(), mem_fun(&Shape::draw));
    for (Shape* ps : shapeVec) { // for-each loop is supported in C++11
    	delete ps;
	}
	
	// vector<Shape> shapeVec2; // ERROR: C++ 中不允许建立 abstract class 的 vector，只能像上面用指针 
	vector<Circle> circleVec;
	Circle c2(2), c3(3);
	circleVec.push_back(c2);
    circleVec.push_back(c3);
    
    for_each(circleVec.begin(), circleVec.end(), mem_fun_ref(&Shape::draw));
}


// output:
/*
	Circle.1::Draw()
	Square.1::Draw()
	Circle.2::Draw()
	Circle.3::Draw()
*/

简单说就是 mem_fun 在遍历 object pointer po 时调用 po->func()，而 mem_fun_ref 是在遍历 object reference ro 时调用 ro.func()。

我觉得 for_each + mem_fun 的这个用法在 C++11 支持 for-each loop 后应该就没有啥市场了……

下面再举个 find_if + mem_fun 的例子：

typedef vector<string>::iterator StrVecIterator;

vector<string> vs;
...
StrVecIterator svi = find_if(vs.begin(), vs.end(), mem_fun_ref(&string::empty));
if (svi != vs.end()) {
	...
}