C++ function 表达式

最新推荐文章于 2025-04-18 18:07:58 发布

dongdonglele521

最新推荐文章于 2025-04-18 18:07:58 发布

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文章标签： C++ function 包装器

本文链接：https://blog.youkuaiyun.com/dongdonglele521/article/details/104067760

本文深入探讨了C++中的function包装器，展示了如何使用它来优化函数指针、类对象和lambda表达式的实例化过程，减少重复创建，提高代码效率。

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C++ 提供了多个包装器，这些对象用于给其他编程接口提供一致或更好的接口，接下来介绍包装器function及其解决的问题。先看一组代码：

#include <iostream>


template<typename T,typename F>

T use_f(T v, F f)
{
	static int count = 0;
	count++;
	std::cout << " use of count = " << count
		<< ", &count= " << &count << std::endl;
	return f(v);
}

class Fp
{
	private:
		double z_;
	public:
		Fp(double z = 1.0) :z_(z) {}
		double operator()(double p) { return z_*p; }
};

class Fq
{
	private:
		double z_;
	public:
		Fq(double z = 1.0) :z_(z) {}
		double operator()(double p) { return z_+p; }
};

double dub(double x) { return 2.0*x; }
double square(double x) { return x*x; }

 int main()
 {
	using std::cout;
	using std::endl;

	double y = 1.21;
	cout << "Function pointer dub is :" << endl;
	cout << " " << use_f(y, dub) << endl;
	cout << "Function pointer square is :" << endl;
	cout << " " << use_f(y, square) << endl;
	cout << "Function pointer Fp is :" << endl;
	cout << " " << use_f(y, Fp(5.0)) << endl;
	cout << "Function pointer Fq is :" << endl;
	cout << " " << use_f(y, Fq(5.0)) << endl;
	cout << "Lambda experssion " << endl;
	cout << " " << use_f(y, [](double u) {return u*u; }) << endl;
	cout << "Lambda experssion " << endl;
	cout << " " << use_f(y, [](double u) {return u*2; }) << endl;
	
}

程序运行结果如下图所示：

每次调用中参数F都接受一个double类型的变量，返回一个double 类型的值，看起来只进行一次实例化，但其实不是，通过静态成员变量看出，实例化了五次。

如何解决这个问题，采用function 包装器。代码实例如下：

#include <iostream>
#include <functional>

template<typename T, typename F>

T use_f(T v, F f)
{
	static int count = 0;
	count++;
	std::cout << " use of count = " << count
		<< ", &count= " << &count << std::endl;
	return f(v);
}

class Fp
{
private:
	double z_;
public:
	Fp(double z = 1.0) :z_(z) {}
	double operator()(double p) { return z_*p; }
};

class Fq
{
private:
	double z_;
public:
	Fq(double z = 1.0) :z_(z) {}
	double operator()(double p) { return z_ + p; }
};

double dub(double x) { return 2.0*x; }
double square(double x) { return x*x; }

int main()
{
	using std::cout;
	using std::endl;
	using std::function;

	double y = 1.21;
	function<double(double)> ef1 = dub;
	function<double(double)> ef2 = square;
	function<double(double)> ef3 = Fp(5);
	function<double(double)> ef4 = Fq(5);
	function<double(double)> ef5 = [](double u) {return u*u; };
	function<double(double)> ef6 = [](double u) {return 2 * u; };

	cout << "Function pointer dub is :" << endl;
	cout << " " << use_f(y, ef1) << endl;
	cout << "Function pointer square is :" << endl;
	cout << " " << use_f(y, ef1) << endl;
	cout << "Function pointer Fp is :" << endl;
	cout << " " << use_f(y, ef3) << endl;
	cout << "Function pointer Fq is :" << endl;
	cout << " " << use_f(y, ef4) << endl;
	cout << "Lambda experssion " << endl;
	cout << " " << use_f(y, ef5) << endl;
	cout << "Lambda experssion " << endl;
	cout << " " << use_f(y, ef6) << endl;

}

运行结果如下：

通过结果看出只进行了一次实例化。

代码还可以按照这种方式写：

#include <iostream>
#include <functional>

template <typename T>

T use_f(T v, std::function <T(T)>f)
{
	static int count = 0;
	count++;
	std::cout << " use_f count = " << count
		<< ", &count = " << &count << std::endl;
	return f(v);
}

class Fp
{
private:
	double z_;
public:
	Fp(double z = 1.0) :z_(z) {}
	double operator()(double p) { return z_*p; }
};

class Fq
{
private:
	double z_;
public:
	Fq(double z = 1.0) :z_(z) {}
	double operator()(double p) { return z_ + p; }
};


double dub(double x) { return 2.0*x; }
double square(double x) { return x*x; }
int main()
{
	using namespace std;	
	double y = 1.25;
	cout << "Function pointer dub is :" << endl;
	cout << " " << use_f<double>(y, dub) << endl;
	cout << "Function pointer square is :" << endl;
	cout << " " << use_f<double>(y, square) << endl;
	cout << "Function pointer Fp is :" << endl;
	cout << " " << use_f<double>(y, Fp(5)) << endl;
	cout << "Function pointer Fq is :" << endl;
	cout << " " << use_f<double>(y, Fq(10)) << endl;
	cout << "Lambda experssion " << endl;
	cout << " " << use_f<double>(y, [](double x) {return x*x; }) << endl;
	cout << "Lambda experssion " << endl;
	cout << " " << use_f<double>(y, [](double x) {return 2 * x; }) << endl;

}

运行结果：