我正在尝试创建一种调整动态分配数组大小的方法。我能够添加行和列,但是将其调整为小于原始大小不起作用。
template <class T>
class Matrix
{
public:
// Constructors / Destructors
Matrix();
Matrix(const int rows, const int cols);
Matrix(const Matrix<T>& orig);
~Matrix();
// Display
void print() const;
// Size modification
void addRow();
void addCol();
void resize(const int rows, const int cols);
// Element access
T getCell(const int r, const int c) const;
// Operator Access
T& operator()(const int r, const int c) { return mData[r * mCols + c]; }
const T& operator()(const int r, const int c) const { return mData[r * mCols + c]; }
// Getters / Setters
int getRows() const;
int getCols() const;
private:
T* mData;
int mRows, mCols;
};
//1
template <class T>
Matrix<T>::Matrix(void){
mRows=0;
mCols=0;
mData=NULL;
}
//2
template <class T>
Matrix<T>::Matrix(const int rows, const int cols){
if(rows!=0&&cols!=0)
{
mRows=rows;
mCols=cols;
int size = rows*cols;
mData = new T[size];
for(int r=0;r<getRows();r++)
{
for(int c=0;c<getCols();c++)
{
mData[r * mCols + c]=0;
}
}
}else
{
mData=NULL;
}
}
//3
template <class T>
Matrix<T>::Matrix(const Matrix<T>& orig){
if(orig.getRows()!=0&&orig.getCols()!=0)
{
T* temp = mData;
mRows=orig.getRows();
mCols=orig.getCols();
int size = mRows*mCols;
mData = new T[size];
for(int r=0;r<getRows();r++)
{
for(int c=0;c<getCols();c++)
{
mData[r * mCols + c]=temp[r * mCols + c];
}
}
}else
{
mData=NULL;
}
}
//4
template <class T>
Matrix<T>::~Matrix()
{
delete []mData;
mData=NULL;
}
//5
template <class T>
void Matrix<T>::print() const{ //to print the mData in a Matrix form
cout << endl;
int r=0,c=0,w=10;
for(r=0;r<getRows();r++){
for(c=0;c<getCols();c++){
cout << setw(w) << mData[r * mCols + c];
}
cout << endl;
}
}
//6
template <class T>
void Matrix<T>::addRow(){
T* temp = mData;
mRows=getRows()+1;
int size = mRows*mCols;
mData = new T[size];
for(int r=0;r<getRows();r++)
{
for(int c=0;c<getCols();c++)
{
if(r==getRows()-1)
{
mData[r * mCols + c]=0;
}else
{
mData[r * mCols + c]=temp[r * mCols + c];
}
}
}
}
//7
template <class T>
void Matrix<T>::addCol(){
T* temp = mData;
mCols=getCols()+1;
int size = mRows*mCols;
mData = new T[size];
for(int r=0;r<getRows();r++)
{
for(int c=0;c<getCols()-1;c++)
{
mData[r * mCols + c]=temp[r * (mCols-1) + c];
}
}
int c=getCols()-1;
for(int r=0;r<getRows();r++)
{
mData[r * mCols + c]=0;
}
}
//8
template <class T>
void Matrix<T>::resize(const int rows, const int cols){
if(rows>=getRows())
{
while(rows!=getRows())
{
addRow();
}
}
if(cols>=getCols())
{
while(cols!=getCols())
{
addCol();
}
}
T* temp = mData;
int tempCols = getCols();
int size = rows*cols;
if(rows<getRows() || cols <getCols())
{
delete []mData;
mData=NULL;
mData= new T[size];
for(int r=0;r<getRows();r++)
{
for(int c=0;c<getCols();c++)
{
if(r<rows&&c<cols)
{
mData[r * mCols + c]=temp[r * (cols) + c];
}else
{
mData[r * mCols + c]=NULL;
}
}
}
}
}
//9
template <class T>
T Matrix<T>:: getCell(const int r, const int c) const
{
T value;
if(r<getRows()&&c<getCols()&&r>=0&&c>=0)
{
value = mData[r * mCols + c] ;
}else
{
value = 0;
}
return(value);
}
//22
template <class T>
int Matrix<T>::getRows() const{
return mRows;
}
//23
template <class T>
int Matrix<T>::getCols() const{
return mCols;
}
#endif
我实施的所有其他功能都完美无缺,我添加了植入以帮助测试。
目前,如果你有一个3,4的矩阵,你想增加到4,7它将成功地这样做。但是,如果你尝试将其截断为2,2,它仍将保持3,4,即使根据新的大小和for循环它应该更小。
所以我的问题是如果我有(3,4)Matrix M1
1 2 3 4
6 7 8 2
3 2 1 1
如果我运行m1.resize(2,7)我想得到
1 2 3 4 0 0 0
6 7 8 2 0 0 0
但目前我得到了
3801280 3802496 2 3 4 0 0
6 7 8 2 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
我实施调整大小有什么问题?
答案 0 :(得分:1)
T* temp = mData;
不执行深层复制。当你之后delete []mData;
指向的数据temp也消失了。之后解除引用temp
会触发未定义的行为。这是非常不幸的,因为这意味着你有时可能会随机获得正确的结果,并且大部分时间都会爆炸。段错误会更好。
这是一个简短的例子。我还放了不执行深层复制的行double * temp = mData;
。
#include <algorithm>
#include <iostream>
int main()
{
double * mData = new double[5];
std::fill_n(mData, 5, 1);
double * temp = mData; // Wrong! No deep copy
//double * temp = new double[5];
//std::copy(mData, mData+5, temp);
delete[] mData;
mData = new double[10];
std::fill_n(mData, 10, 0); // fill with zeros
for (int i = 0; i < 5; ++i)
mData[i] = temp[i];
for(int i = 0; i < 10; ++i)
std::cout << mData[i] << ' ';
std::cout << '\n';
delete[] temp;
delete[] mData;
}
此程序的输出可能会或可能不会很好。在所有行为未定义之后。最好在内存调试器(例如Linux上的valgrind
)中运行带有手动内存管理的程序,以检测无效操作。如果我在valgrind
中运行上面的内容,我会得到(在一些通用消息中)
==9998== Invalid read of size 8
==9998== at 0x400908: main (in /home/henri/a.out)
==9998== Address 0x5abfc80 is 0 bytes inside a block of size 40 free'd
==9998== at 0x4C2F74B: operator delete[](void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9998== by 0x4008C1: main (in /home/henri/a.out)
==9998== Block was alloc'd at
==9998== at 0x4C2E80F: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9998== by 0x40087A: main (in /home/henri/a.out)
==9998==
1 1 1 1 1 0 0 0 0 0
==9998== Invalid free() / delete / delete[] / realloc()
==9998== at 0x4C2F74B: operator delete[](void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9998== by 0x4009A9: main (in /home/henri/a.out)
==9998== Address 0x5abfc80 is 0 bytes inside a block of size 40 free'd
==9998== at 0x4C2F74B: operator delete[](void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9998== by 0x4008C1: main (in /home/henri/a.out)
==9998== Block was alloc'd at
==9998== at 0x4C2E80F: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9998== by 0x40087A: main (in /home/henri/a.out)