std::vector 未设置大小且未正确分配数据 -> 下标范围错误

Question

Debug Assertion Failed!

Program: File: c:\program files (x86)\microsoft visual studio17\community\vc\tools\msvc.15.26726\include\vector Line: 1742

Expression: vector subscript out of range

For information on how your program can cause an assertion failure, see the Visual C++ documentation on asserts.

(Press Retry to debug the application) Sandbox.exe has triggered a breakpoint.

我正在尝试为另一个项目编写可变维度（如 1 种类型以包含任何 n 乘 m 矩阵）矩阵库，因此我想创建和修改“2D”数字数组。为此，我使用了 std::vector 中的 std::vector。

我知道错误是说我试图访问的索引超出范围。即尝试访问 3x1 数组的值 [5][8]。查看调试信息，Multiply 函数会标记错误，问题本身可能与“2D 向量”data 和 out.data 的分配有关，这可能意味着这是构造函数的错误，对吧？（但是最初创建的矩阵是正确的..？）

这是我的代码（忽略缺少干净的命名空间/类型定义 - 我会在重构时整理所有这些，我喜欢这样保留它所以我知道 where/what 一切都是）

//Maths.h

#pragma once
#include "ckpch.h" // C/C++ standard libraries
#include "Core.h" // API  (__declspec(im/export) = CK_API)

namespace Maths {

    struct CK_API Matrix {

        int Rows, Cols;

        std::vector<std::vector<float>> data;

        Matrix(int rows, int cols);

        Matrix(int rows, int cols, float *values);

        ~Matrix();

        Matrix Multiply(const Matrix& mat) const ;
        friend CK_API Matrix operator*(Matrix& left, const Matrix& right);

        friend CK_API std::ostream& operator<<(std::ostream& os, const Matrix& mat);
    };

}

//Maths.cpp

 #include "ckpch.h"
 #include "Maths.h"
 #include "Log.h" // Loads log info

namespace Maths {

    Matrix::Matrix(int rows, int cols) {
        Rows = rows;
        Cols = cols;

        std::vector<std::vector<float>> data(rows, std::vector<float>(cols, 0.0f));
        data.resize(rows, std::vector<float>(cols, 0.0f));

    }

    Matrix::Matrix(int rows, int cols, float* values) {

        Rows = rows;
        Cols = cols;
        std::vector<std::vector<float>> data(rows, std::vector<float>(cols, 0.0f));
        data.resize(rows, std::vector<float>(cols, 0.0f));
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                data[i][j] = values[j + i * cols];
            }
        }

    }

    Matrix::~Matrix() {
        this->data.clear();

    }

    Matrix Matrix::Multiply(const Matrix& mat) const {
        int inR1 = this->Rows; // Matrix 1 Rows
        int inR2 = mat.Rows;   // Matrix 2 Rows
        int inC1 = this->Cols; // Matrix 1 Columns 
        int inC2 = mat.Cols;   // Matrix 2 Columns

        // (n x m) * (n' x m') --> (n x m')

        int outR = this->Rows;
        int outC = mat.Cols;

        Matrix out = Matrix(outR, outC);

        if (this->Cols == mat.Rows) {
            for (int i = 0; i < inR1; i++) {
                for (int j = 0; j < inC2; j++) {
                    float sum = 0.0f;
                    for (int off = 0; off < inR1; off++) {
                        sum += this->data[off][j] * mat.data[i][off];
                    }
                    out.data[i][j] = sum;
                }
            }
            return out;
        } else {
            CK_WARN("Matrix 1 Column and Matrix 2 Row dimension mismatch! ({0} =/= {1})", Cols, mat.Rows);
        }
    }

    Matrix operator*(Matrix& left, const Matrix& right) { return left.Multiply(right); }

    std::ostream& operator<<(std::ostream& os, const Matrix& mat){

        os << mat.Rows << " x " << mat.Cols << " - Matrix: " << std::endl;

        for (int i = 0; i < mat.Rows; i++) {
            for (int j = 0; j < mat.Cols; j++) {
                os << mat.data[i][j] << ", ";
            }
            os << std::endl;
        }
        return os;
    }
}

// SandboxApp.cpp

float val1[2] = {
                1.0f, 2.0f
    };
    Maths::Matrix mat1 = Maths::Matrix(1, 2, val1);

    float val2[4] = {
                        1.0f, 2.0f,
                        -1.0f, 3.0f
    };
    Maths::Matrix mat2 = Maths::Matrix(2, 2, val2);

    Maths::Matrix mat3 = mat1 + mat2;
    Maths::Matrix mat4 = mat1 * mat2;

    std::cout << mat1 << std::endl;
    std::cout << mat2 << std::endl;
    std::cout << mat3 << std::endl;
    std::cout << mat4 << std::endl;

由于调用了 functions/methods 等，我认为所有代码都是相关的 - 我删除了所有其他内容（函数重载、加法、减法等）

Answer 1

这一行

std::vector<std::vector<float>> data(rows, std::vector<float>(cols, 0.0f));

创建一个名为 data 的新局部变量，它与您的属性同名 Matrix::data

这称为阴影，非常糟糕，因为您对这个本地所做的任何操作都不会触及该属性。

一个体面的编译器会发出警告告诉你这件事。

Could you suggest the best fix? Would it be just deleting that line?

是的。

如果您使用 std::vector::push_back() 而不是赋值，那么您不需要调整大小。对于非常大的数据集，这会有点慢。

std::vector 未设置大小且未正确分配数据 -> 下标范围错误

std::vector size not being set and data isn't assigned properly -> subscript range error

c++

matrix

size

stdvector

subscript