C++二维向量的初始化方法有哪些

利用嵌套vector进行初始化：

#include <vector>std::vector<std::vector<int>> vec2d = {    {1, 2, 3},    {4, 5, 6},    {7, 8, 9}};

#include <vector>int rows = 3;int cols = 3;std::vector<std::vector<int>> vec2d;vec2d.resize(rows, std::vector<int>(cols, 0));

#include <vector>int rows = 3;int cols = 3;std::vector<std::vector<int>> vec2d(rows, std::vector<int>(cols));for (int i = 0; i < rows; i++) {    for (int j = 0; j < cols; j++) {        vec2d[i][j] = i * cols + j + 1;    }}