C++ 2D Vector赋值后显示异常，同逻辑2D Array正常

问题现象 #

• 从一维vector<int>复制数据到二维vector<vector<double>>时，循环内部输出数据完全正确，但循环结束后部分位置数据异常（如[0][6]、[0][7]、[1][6]、[1][7]），最后一行始终正常；修改nSets参数后，前几行末尾2个位置仍会损坏。
• 相同复制逻辑下，固定大小的二维数组int nArray[3][8]所有数据输出均正常。

复现代码 #

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

int nCol = 8;
int nSize = 4;
int nLookBack = 2;
int nSets = 3;
int nInputs = 4;
int nArray[3][8];


int main()
{
    // 创建数据集
    vector<int> nData(80);
    for(int i = 0; i < 80; i++)
    {
        nData[i] = i+1;
    }

    // 展示原始数据
    for(int i = 0 ; i < 80; i+=nCol)
    {
        for(int j = 0; j < nCol; j++)
        {
            cout << nData[i+j] << "\t";
        }
        cout << endl;
    }

    cout << endl << endl;

    // +-----------------------------------------------------

    vector<vector<double>> nVector( nSets , vector<double> (nInputs));

    // 从nVector复制数据到2D vector和2D array
    cout << "循环内部输出:"  << endl;

    for(int i = 0; i < nSets  ; i++)
    {
        for(int j = 0; j< nLookBack ; j++)
        {
            // 复制数据到2D vector
            nVector[i] [(j * nSize ) + 0 ] = nData[ ((i+j) * nCol) + 0] ;
            nVector[i] [(j * nSize ) + 1 ] = nData[ ((i+j) * nCol) + 1] ;
            nVector[i] [(j * nSize ) + 2 ] = nData[ ((i+j) * nCol) + 2] ;
            nVector[i] [(j * nSize ) + 3 ] = nData[ ((i+j) * nCol) + 3] ;

            // 循环内展示2D vector数据
            cout << "[" << i << "][" << (j * nSize ) + 0 << "] = " << nVector[i] [(j * nSize ) + 0 ]  << "\t" ;
            cout << "[" << i << "][" << (j * nSize ) + 1 << "] = " << nVector[i] [(j * nSize ) + 1 ]  << "\t" ;
            cout << "[" << i << "][" << (j * nSize ) + 2 << "] = " << nVector[i] [(j * nSize ) + 2 ]  << "\t" ;
            cout << "[" << i << "][" << (j * nSize ) + 3 << "] = " << nVector[i] [(j * nSize ) + 3 ]  << "\t" ;

            // 复制数据到2D array
            nArray[i] [(j * nSize ) + 0 ] = nData[ ((i+j) * nCol) + 0] ;
            nArray[i] [(j * nSize ) + 1 ] = nData[ ((i+j) * nCol) + 1] ;
            nArray[i] [(j * nSize ) + 2 ] = nData[ ((i+j) * nCol) + 2] ;
            nArray[i] [(j * nSize ) + 3 ] = nData[ ((i+j) * nCol) + 3] ;
        }
        cout << endl;
    }

    // +-----------------------------------------------------

    // 展示2D vector数据
    cout << endl << "二维vector输出:"  << endl;
    for(int i = 0; i < nSets ; i++)
    {
        for(int j = 0; j < (nLookBack * nSize); j++)
        {
            cout  << " [" << i << "][" << j << "] = " << nVector[i][j] << "\t" ;
        }
        cout << endl;
    }

    // +-----------------------------------------------------

    // 展示2D array数据
    cout << endl << "二维数组输出:"  << endl;
    for(int i = 0; i < nSets ; i++)
    {
        for(int j = 0; j < (nLookBack * nSize); j++)
        {
            cout  << " [" << i << "][" << j << "] = " << nArray[i][j] << "\t" ;
        }
        cout << endl;
    }

    return 0;
}

输出对比 #

循环内部输出 #

[0][0] = 1  [0][1] = 2  [0][2] = 3  [0][3] = 4  [0][4] = 9  [0][5] = 10 [0][6] = 11 [0][7] = 12
[1][0] = 9  [1][1] = 10 [1][2] = 11 [1][3] = 12 [1][4] = 17 [1][5] = 18 [1][6] = 19 [1][7] = 20
[2][0] = 17 [2][1] = 18 [2][2] = 19 [2][3] = 20 [2][4] = 25 [2][5] = 26 [2][6] = 27 [2][7] = 28

二维Vector输出 #

[0][0] = 1  [0][1] = 2  [0][2] = 3  [0][3] = 4  [0][4] = 9  [0][5] = 10     [0][6] = 9  [0][7] = 10
[1][0] = 9  [1][1] = 10     [1][2] = 11     [1][3] = 12     [1][4] = 17     [1][5] = 18     [1][6] = 17     [1][7] = 18
[2][0] = 17     [2][1] = 18     [2][2] = 19     [2][3] = 20     [2][4] = 25     [2][5] = 26     [2][6] = 27     [2][7] = 28

二维数组输出 #

[0][0] = 1  [0][1] = 2  [0][2] = 3  [0][3] = 4  [0][4] = 9  [0][5] = 10     [0][6] = 11     [0][7] = 12
[1][0] = 9  [1][1] = 10     [1][2] = 11     [1][3] = 12     [1][4] = 17     [1][5] = 18     [1][6] = 19     [1][7] = 20
[2][0] = 17     [2][1] = 18     [2][2] = 19     [2][3] = 20     [2][4] = 25     [2][5] = 26     [2][6] = 27     [2][7] = 28

问题原因 #

核心问题是二维Vector的初始化大小错误：

• 代码中初始化二维Vector时，每个子Vector的大小被设置为nInputs=4，但实际需要存储的列数是nLookBack * nSize = 2*4=8。
• 后续访问索引4-7的位置属于数组越界访问，这是C++中的未定义行为——写入越界位置会覆盖相邻内存区域的数据，循环内部刚写入时内存未被改写所以输出正常，但后续操作会破坏这些越界存储的数据，导致最终读取异常。
• 二维数组是固定大小8列，不存在越界问题，因此数据始终正确。

解决方法 #

修改二维Vector的初始化代码，将子Vector的大小设置为实际需要的列数nLookBack * nSize：

// 计算实际需要的列数
int requiredCols = nLookBack * nSize;
vector<vector<double>> nVector(nSets, vector<double>(requiredCols));

或者直接在初始化时计算：

vector<vector<double>> nVector(nSets, vector<double>(nLookBack * nSize));

内容的提问来源于stack exchange，提问作者Osiris