C++ STL内存池的效果

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<pre name="code" class="cpp">#include<vector>#include<sys/time.h>#include<stdlib.h>#include<iostream>using namespace std;    class A{        public:        A(){            a=3;        }        private:        int a;    };int main(){    struct timeval pre;    struct timeval now;    vector<A> v;    vector<A*> vp;    gettimeofday(&pre, 0);        for(int i = 0; i < 1000000; i++)        {        A ca;        v.push_back(ca);    }    gettimeofday(&now, 0);    unsigned long tv =( now.tv_sec*1000000 + now.tv_usec \                          - pre.tv_sec*1000000 - pre.tv_usec);    cout<<tv<<endl;    gettimeofday(&pre, 0);    for(int i = 0; i<1000000; i++)    {        vp.push_back(new A());    }    gettimeofday(&now, 0);    tv =( now.tv_sec*1000000 + now.tv_usec \                          - pre.tv_sec*1000000 - pre.tv_usec);    cout<<tv<<endl;}

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1. 因为STL内存池的使用，提高了效率。

</pre></p>

#include<vector>#include<sys/time.h>#include<stdlib.h>#include<iostream>using namespace std;    class A{        public:        A(){            a=3;        }        private:        int a;       <span style="color:#cc0000;"> int b[1000];--------------------------------------------------------------------</span>    };int main(){    struct timeval pre;    struct timeval now;    vector<A> v;    vector<A*> vp;    gettimeofday(&pre, 0);        for(int i = 0; i <<span style="color:#cc0000;"> 100</span>; i++)    <span style="background-color: rgb(255, 255, 255);"><span style="color:#cc0000;">-----------------------------------------------</span></span>    {        A ca;        v.push_back(ca);    }    gettimeofday(&now, 0);    unsigned long tv =( now.tv_sec*1000000 + now.tv_usec \                          - pre.tv_sec*1000000 - pre.tv_usec);    cout<<tv<<endl;    gettimeofday(&pre, 0);    for(int i = 0; i<100; i++)    {        vp.push_back(new A());    }    gettimeofday(&now, 0);    tv =( now.tv_sec*1000000 + now.tv_usec \                          - pre.tv_sec*1000000 - pre.tv_usec);    cout<<tv<<endl;}

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2. 因为vector的push_back函数需要拷贝输入变量，当输入变量体积大时，此函数造成整体效率较低。

总结：STL中容器使用到了内存池，增加成员时从内存的角度STL做了两件事：1. 从内存池中分配内存，2. 拷贝输入变量值。

当输入变量体积大时，拷贝成为拖累。

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