c++ - 如何安全地从内存中清除使用 new 关键字创建的对象（带有属性）？

Question

据我所知，每个“新”调用都需要对该对象进行相应的“删除”调用。那么这真的正确吗？：

using namespace std;

class Box {
   public:
      double length;
      char letters_in_box[80];
};

int main() {
   Box *b = new Box;
   b->length = 2.0;
   b->letters_in_box = "Hello world";

   //Some code with b

   delete b;

   return 0;
}

与“length”双精度和“letters_in_box”数组相关的内存是否被清除？

标签： c++classnew-operatordelete-operator

Answer 1

是的。当您删除b它时，它也会删除letters_in_box数组。

但是，对于您，b->letters_in_box = "Hello world";您将收到一个编译错误：“错误 C3863：数组类型 'char [80]' 不可分配”

#include <memory> // For 'memcpy_s' (since C11)

class Box
{
public:
    double length;
    char letters_in_box[80];
};

int main()
{
    Box* b = new Box;

    b->length = 2.0;
    // b->letters_in_box = "Hello world"; ** Compile Error C3863: array type 'char [80]' is not assignable **
    memcpy_s(b->letters_in_box, sizeof(b->letters_in_box), "Hello world", sizeof("Hello world"));

    // Some code with b

    delete b;
}

现代 C++

new比smart pointers更好的做法，例如，在出现异常的情况下，您不必为 delete 操心：

#include <memory> // For 'std::unique_ptr' and for 'memcpy_s'

class Box
{
public:
    double length;
    char letters_in_box[80];
};

constexpr char my_text[] = "Hello world"; 

int main()
{
    auto b = std::make_unique<Box>(); // No need to delete

    b->length = 2.0;
    memcpy_s(b->letters_in_box, sizeof(b->letters_in_box), my_text, sizeof(my_text));

    // Some code with b
}

另外，（而不是 C 数组）我更喜欢使用C++ 数组：

#include <array>  // For 'std::array'
#include <memory> // For 'std::unique_ptr' and for 'memcpy_s' 

class Box
{
public:
    double length;
    std::array<char, 80> letters_in_box;
};
   
constexpr char my_text[] = "Hello world";

int main()
{
    auto b = std::make_unique<Box>(); // No need to delete

    b->length = 2.0;
    memcpy_s(&b->letters_in_box, b->letters_in_box.size(), my_text, sizeof(my_text));

    //Some code with b
}

--

最后一条评论：如果没有使用约束char[]，我会std::string改用：

#include <string> // For 'std::string'
#include <memory> // For 'std::unique_ptr' 

class Box
{
public:
    double length;
    std::string letters_in_box;
};

int main()
{
    auto b = std::make_unique<Box>(); // No need to delete

    b->length = 2.0;
    b->letters_in_box = "Hello world";

    //Some code with b
}