reserve函数
语法:
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void reserve(size_type __n); |
reserve() 设置Vector最小的元素容纳数量
源代码
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/** * @brief Attempt to preallocate enough memory for specified number of * elements. * @param n Number of elements required. * @throw std::length_error If @a n exceeds @c max_size(). * * This function attempts to reserve enough memory for the * %vector to hold the specified number of elements. If the * number requested is more than max_size(), length_error is * thrown. * * The advantage of this function is that if optimal code is a * necessity and the user can determine the number of elements * that will be required, the user can reserve the memory in * %advance, and thus prevent a possible reallocation of memory * and copying of %vector data. */ void reserve(size_type __n); |
测试代码
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// 梁笔记 // https://zouzhongliang.com #include <iostream> #include <vector> using namespace std; int main() { vector<int> v1; vector<int> v2(3, 2); v1.push_back(1); v1.push_back(2); v1.push_back(3); v1.push_back(4); v1.push_back(5); cout<<"v1向量骨元素数："<<v1.size()<<endl; cout<<"v2向量骨元素数："<<v2.size()<<endl; cout<<"v1向量容量："<<v1.capacity()<<endl; v1.reserve(10); v1.push_back(5); v1.push_back(5); v1.push_back(5); v1.push_back(5); v1.push_back(5); v1.push_back(5); cout<<"v1向量骨元素数："<<v1.size()<<endl; cout<<"v1向量容量："<<v1.capacity()<<endl; } |
测试结果：
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v1向量骨元素数：5 v2向量骨元素数：3 v1向量容量：8 v1向量骨元素数：11 v1向量容量：20 |
注：设置reserve意义在于，向量内元素数量多少与向量容量之间的关系。reserve值设越大，向量容量越大，容量越大于向量尺寸（元素数量）。