内存泄漏在哪里？二维数组类

Question

我为一个班级提交了这个程序，错误消息说有一些内存泄漏，但我找不到它们（我什至问过另一位教授）

这是错误信息：

==27796== HEAP SUMMARY:
==27796==     in use at exit: 160 bytes in 2 blocks
==27796==   total heap usage: 192 allocs, 190 frees, 21,989 bytes allocated
==27796== 
==27796== 160 bytes in 2 blocks are definitely lost in loss record 1 of 1
==27796==    at 0x402ADFC: operator new[](unsigned int) (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27796==    by 0x804D5C2: SweeperGrid::SweeperGrid(SweeperGrid const&) (in /tmp/grading20151030-8173-zfd6af-0/sweepertest)
==27796==    by 0x804BF57: main (sweepertest.cpp:357)

以下是我使用 new 或 delete 的任何代码：

// 显式值构造函数

 SweeperGrid::SweeperGrid(const int initialRows, const int initialCols, const int density){
    if ((initialRows<5 || initialCols<5) || (density<25 || density>75)) {
        throw out_of_range("Grid not large enough (number of rows or columns cannot be fewer than 5) or density is too low or high (must be between 25% and 75%)");
    }

numRows = initialRows;
numColumns = initialCols;
numBombs = 0;

grid = new SweeperCell*[numRows];
for(int i=0; i <numRows; i++){
    grid[i] = new SweeperCell[numColumns];
}

srand(time(0));
for(int i=0; i<numRows; i++){
    for (int j=0; j<numColumns; j++){
        if(rand()%100+1<density){
            PlaceBomb(i, j);
        }
    }
}
}

// 复制构造函数

SweeperGrid::SweeperGrid(SweeperGrid const & source){
    numRows=source.GetRows();
    numColumns=source.GetColumns();
    numBombs=source.GetBombs();

grid = new SweeperCell * [numRows];
for(int i=0; i < numRows; i++){
    grid[i] = new SweeperCell[numColumns];
}

for(int i=0; i<numRows; i++){
    for (int j=0; j<numColumns; j++){
        grid[i][j] = source.At(i, j);
    }
}
}

// 析构函数

SweeperGrid::~SweeperGrid(){
for(int i=0; i<numRows; i++){
        delete [] grid[i];
}
delete [] grid;
}

// 功能：重载赋值运算符

void SweeperGrid::operator= (SweeperGrid const & source){
    numRows=source.GetRows();
    numColumns=source.GetColumns();
    numBombs=source.GetBombs();

for(int i=0; i<numRows; i++){
    delete [] grid[i];
}
delete [] grid;

grid = new SweeperCell * [numRows];
for(int i=0; i < numRows; i++){
    grid[i] = new SweeperCell[numColumns];
}

for(int i=0; i<numRows; i++){
    for (int j=0; j<numColumns; j++){
        grid[i][j] = source.At(i, j);
    }
}
}

Answer 1

您的问题是由于删除的行数与您分配的行数不同：

void SweeperGrid::operator= (SweeperGrid const & source){
    numRows=source.GetRows(); // numRows becomes the assigned number of rows here.
                              // This is a problem, because now you don't remember how many
                              // rows you need to delete!
    numColumns=source.GetColumns();
    numBombs=source.GetBombs();

    for(int i=0; i<numRows; i++){ // you delete rows here, but the number of rows found in
                                  // the NEW grid, not the old one.
        delete [] grid[i];
    }
    delete [] grid;

...
}

一个简单的修复：

void SweeperGrid::operator= (SweeperGrid const & source){

    for(int i=0; i<numRows; i++){ // you delete rows here, while you still remember 
                                  // how many you should delete
        delete [] grid[i];
    }
    delete [] grid;
    numRows=source.GetRows(); // numRows becomes the assigned number of rows here.
                              // This is not a problem, because now you don't need to
                              // remember how many rows you need to delete.
    numColumns=source.GetColumns();
    numBombs=source.GetBombs();
...
}

这类问题正是为什么手动使用 delete 和 new 是一个可怕的想法——此外，它不是异常安全的（如果在构造函数中分配一个在你已经分配了其他东西之后行失败了？你会丢失之前在构造函数中分配的每一块内存，直到你关闭程序才能看到。）

更简单的解决方案是应用Resource Acquisition Is Initialization idiom（简称RAII）并使用一个容器来控制资源，方法是在构造时分配资源并在销毁时释放资源——向量将为您完成此操作，并且像smartpointers 这样的漂亮东西也会如此。这免除了您处理每一项的责任。单身的。分配。如果。甚至。一种。单身的。一。失败，因为在 RAII 中，如果一个失败，已经分配的资源自己清理干净。

举个例子，如果我对grid 使用向量的向量，而不是使用 c 风格的动态数组来表示一堆 c 风格的动态数组，那么赋值运算符可能看起来像这样：

void SweeperGrid::operator= (SweeperGrid const & source){
    numRows=source.GetRows(); 
    numColumns=source.GetColumns();
    numBombs=source.GetBombs();

    grid = source.grid;
}