vector性能调优之resize与reserve

vector的resize与reserve

reserve()函数为当前vector预留至少共容纳size个元素的空间.(译注:实际空间可能大于size)

resize() 函数( void resize( size_type size, TYPE val ) )改变当前vector的大小为size,且对新创建的元素赋值val

（翻译：

调整容器大小以包含count元素。

如果当前大小大于count，则容器将被缩减为其第一个count元素，就像重复调用pop_back（）一样。

如果当前大小小于count，则附加元素并用值的副本初始化。）

resize和reserve函数本质都涉及了vector的内存存储空间，因为vector在内存中是连续存放的，所以当resize的空间大于现有的存储空间（capacity() 函数 返回当前vector在重新进行内存分配以前所能容纳的元素数量.）时，会重新选择更大的空间，并将所有元素复制过去。resize在初始化内存容量时有对值的初始化，所以此时push_back会产生size+1，内存容量不够，重新寻找更大的内存空间并复制所有元素，所以这个过程是很费事的。

void testResize(){
    vector<int> vector1;
    vector1.resize(10);
    vector1.push_back(1);
    vector1.push_back(2);
    vector1.push_back(3);
    cout<<"vector1的长度："<<vector1.size()<<endl;//vector1的长度：13
    for_each(vector1.begin(),vector1.end(),[](int x){cout<<x<<" ";});//0 0 0 0 0 0 0 0 0 0 1 2 3
    cout<<endl<<"当前vector在重新进行内存分配以前所能容纳的元素数量:"<<vector1.capacity()<<endl;//20
}
void testReserve(){
    vector<int> vector1;
    vector1.reserve(10);
    vector1.push_back(1);//vector1的长度：3
    vector1.push_back(2);//1 2 3
    vector1.push_back(3);
    cout<<"vector1的长度："<<vector1.size()<<endl;//vector1的长度：3
    for_each(vector1.begin(),vector1.end(),[](int x){cout<<x<<" ";});// 1 2 3
    cout<<endl<<"当前vector在重新进行内存分配以前所能容纳的元素数量:"<<vector1.capacity()<<endl;//10
}

插入测试

接下来探讨插入的效率的实例,分别尝试在插入大数据3.8GB和小数据380MB时，各种情况的实现。

（1）push_back直接插入

结论：费事，在插入的过程中，不断寻找“庇护所”，不断“迁移大本营”，舟车劳顿效率低下

void testPushBack_bigsize(){
    vector<int> vector1;
    clock_t start = clock();
    for (int i = 0; i < 1000000000; ++i) {//3814MB
        vector1.push_back(i);
    }
    cout <<"共耗时："<< (clock() - start)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：42s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:1000000000 capacity:1073741824
    clock_t start2 = clock();
    vector1.push_back(1);
    cout <<"共耗时："<< (clock() - start2)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：0s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:1000000001 capacity:1073741824
}

（2）先reserve在push_back

结论：先分配空间再进行后续处理，能够有效的减少插入时间的损耗，耗时占原插入方式的1/3到1/2之间。

void testPushBack_byReserve_bigsize(){
    vector<int> vector1;
    vector1.reserve(1000000000);//3814MB
    clock_t start = clock();
    for (int i = 0; i < 1000000000; ++i) {
        vector1.push_back(i);
    }
    cout <<"共耗时："<< (clock() - start)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：17s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:1000000000 capacity:1000000000
    clock_t start2 = clock();
    vector1.push_back(1);
    cout <<"共耗时："<< (clock() - start2)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：76s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:1000000001 capacity:2000000000
}
void testPushBack_byReserve_smallsize(){
    vector<int> vector1;
    vector1.reserve(100000000);//381MB
    clock_t start = clock();
    for (int i = 0; i < 100000000; ++i) {
        vector1.push_back(i);
    }
    cout <<"共耗时："<< (clock() - start)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：1s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:100000000 capacity:100000000
    clock_t start2 = clock();
    vector1.push_back(1);
    cout <<"共耗时："<< (clock() - start2)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：2s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:100000001 capacity:200000000
}

（2）先resize在利用坐标进行赋值（相当于插入）

结论：在分配空间时直接对空间进行初始化，赋予初值，极大提升了存储的速率。但是在resize后进行push_back是不明智的选择。

void testinsert_byResize_bigsize(){
    vector<int> vector1;
    vector1.resize(1000000000);
    clock_t start = clock();
    for (int i = 0; i < 1000000000; ++i) {
        vector1[i]=i;
    }
    cout <<"共耗时："<< (clock() - start)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：3s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:1000000000 capacity:1000000000
    clock_t start2 = clock();
    vector1.push_back(1);
    cout <<"共耗时："<< (clock() - start2)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：66s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:1000000001 capacity:2000000000
}
void testinsert_byResize_smallsize(){
    vector<int> vector1;
    vector1.resize(100000000);
    clock_t start = clock();
    for (int i = 0; i < 100000000; ++i) {
        vector1[i]=i;
    }
    cout <<"共耗时："<< (clock() - start)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：0s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:size:10000000 capacity:10000000
    clock_t start2 = clock();
    vector1.push_back(1);
    cout <<"共耗时："<< (clock() - start2)/ CLOCKS_PER_SEC <<"s"<<endl;//共耗时：2s
    cout <<"size:"<<vector1.size() << " capacity:" << vector1.capacity() << endl;//size:10000001 capacity:20000000
}

vector优化结论

防止reallocate内存，而导致的数据拷贝产生的额外耗时

vector在push_back的时候，如果空间不足，会自动增补一些空间，如果没有预留的空间可用
就直接申请另一块可用的连续的空间，把数据拷贝过去，然后删除旧空间，使用新空间
结果造成效率低下 。

可以通过以下两种组合来防止reallocate.

1. vector::resize() 使用array index，效率最高，但是需要提前知道size大小

2. vector::reserve()使用 push_back()，效率一般，较原生有一定提升。