使用std::function改善模板的低效性

泛型编程中，模板会根据传入类型的不同，生成多种实例，相对低效。

模板编程：

#include <iostream>

using namespace std;

//未使用函数包装器
template <typename T,typename F>
T use_f(T v, F f)
{
    static int count = ;
    count++;
    cout<<"use_f count = "<<count<<": &count"<<&count<<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(int argc, char *argv[])
{
    double y = 1.2;
    cout<<"Function pointer dub:"<<endl;
    cout<<" --"<<use_f(y,dub);
    cout<<"Function pointer square:"<<endl;
    cout<<" --"<<use_f(y,square);
    cout<<"Function pointer Fp:"<<endl;
    cout<<" --"<<use_f(y,Fp());
    cout<<"Function pointer Fq:"<<endl;
    cout<<" --"<<use_f(y,Fq());
    cout<<"Function pointer lambada1:"<<endl;
    cout<<" --"<<use_f(y,[](double u){return u*u;});
    cout<<"Function pointer lambada2:"<<endl;
    cout<<" --"<<use_f(y,[](double u){return u+2.5;});
    return ;
}

模板编程的优势是高度抽象，将算法统一封装，但是其根据类型实例化的特性也造成了其低效性。

以上执行结果，通过查看静态模板函数中，静态数据的地址，其产生了5个函数实例。代码量受传入类型的影响产生了巨大的增量。

通过std::function来减少模板的实例化：

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

//使用函数包装器
template <typename T,typename F>
T use_f(T v, F f)
{
    static int count = ;
    count++;
    cout<<"use_f count = "<<count<<": &count"<<&count<<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(int argc, char *argv[])
{
    double y = 1.2;
    //    cout<<"Function pointer dub:"<<endl;
    //    cout<<" --"<<use_f(y,dub);
    //    cout<<"Function pointer square:"<<endl;
    //    cout<<" --"<<use_f(y,square);
    //    cout<<"Function pointer Fp:"<<endl;
    //    cout<<" --"<<use_f(y,Fp());
    //    cout<<"Function pointer Fq:"<<endl;
    //    cout<<" --"<<use_f(y,Fq());
    //    cout<<"Function pointer lambada1:"<<endl;
    //    cout<<" --"<<use_f(y,[](double u){return u*u;});
    //    cout<<"Function pointer lambada2:"<<endl;
    //    cout<<" --"<<use_f(y,[](double u){return u+2.5;});

    std::function<double(double)> fdub     = dub;
    std::function<double(double)> fsquare  = square;
    std::function<double(double)> fFp      = Fp();
    std::function<double(double)> fFq      = Fq();
    std::function<double(double)> lambada1 = [](double u){return u*u;};
    std::function<double(double)> lambada2 = [](double u){return u+2.5;};

    cout<<"Function pointer dub:"<<endl;
    cout<<" --"<<use_f(y,fdub);
    cout<<"Function pointer square:"<<endl;
    cout<<" --"<<use_f(y,fsquare);
    cout<<"Function pointer Fp:"<<endl;
    cout<<" --"<<use_f(y,fFp);
    cout<<"Function pointer Fq:"<<endl;
    cout<<" --"<<use_f(y,fFq);
    cout<<"Function pointer lambada1:"<<endl;
    cout<<" --"<<use_f(y,lambada1);
    cout<<"Function pointer lambada2:"<<endl;
    cout<<" --"<<use_f(y,lambada2);

    return ;
}

所有的静态变量只有一个地址，也就是说模板 函数只产生了一个实例，其类型只需要匹配是一个std::function<double(double)>传入的对象即可，模板的效率大为提高。

但是看来好像代码量并未减少甚至还有些许增多,下面我们通过优化来解决这个问题。

熊掌与鱼：

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

//模板中使用函数包装器
template <typename T>
T use_f(T v, std::function<T(T)> f)
{
    static int count = ;
    count++;
    cout<<"use_f count = "<<count<<": &count"<<&count<<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(int argc, char *argv[])
{
    double y = 1.2;
    //这里需要<double>使得std::function<T(T)>实例化为具体的对象
    cout<<"Function pointer dub:"<<endl;
    cout<<" --"<<use_f<double>(y,&dub);
    cout<<"Function pointer square:"<<endl;
    cout<<" --"<<use_f<double>(y,square);
    cout<<"Function pointer Fp:"<<endl;
    cout<<" --"<<use_f<double>(y,Fp());
    cout<<"Function pointer Fq:"<<endl;
    cout<<" --"<<use_f<double>(y,Fq());
    cout<<"Function pointer lambada1:"<<endl;
    cout<<" --"<<use_f<double>(y,[](double u){return u*u;});
    cout<<"Function pointer lambada2:"<<endl;
    cout<<" --"<<use_f<double>(y,[](double u){return u+2.5;});

    return ;
}

实例化与代码量都减少了，鱼和熊掌兼得。