c++中new和delete,delete[]的匹配问题(原创)
这是我在网上逛的时候发现的一个讨论, 起源是<<c>>一书中的一些例子。查了一些资料，把我的结论写在这里。先让我们看一个论述比较好的帖子，e文的，耐心点看。</c>
Mismatching scalar and vector new and delete
In a previous entry I alluded to the problems that can occur if you mismatch scalar "new" with vector <nobr>"delete[]"</nobr> or vice versa.
There is a nice description of C++ memory management in C++ Gotchas: Avoiding Common Problems in Coding and Design on www.informit.com, and I encourage you to read at least the section titled Failure to Distinguish Scalar and Array Allocation before continuing with this entry, because I'm going to use that information as a starting point.
Here's how the Microsoft C++ compiler manages vector allocation. Note that this is internal implementation detail, so it's subject to change at any time, but knowing this may give a better insight into why mixing scalar and vector new/delete is a bad thing.
The important thing to note is that when you do a scalar "delete p", you are telling the compiler, "p points to a single object." The compiler will call the destructor once, namely for the object you are destructing.
When you do <nobr>"delete[] p"</nobr>, you are saying, "p points to a bunch of objects, but I'm not telling you how many." In this case, the compiler needs to generate extra code to keep track of how many it needs to destruct. This extra information is kept in a "secret place" when the vector is allocated with <nobr>"new[]"</nobr>.
Let's watch this in action:
class MyClass { public: MyClass(); // constructor ~MyClass(); // destructor int i; }; MyClass *allocate_stuff(int howmany) { return new MyClass[howmany]; }
The generated code for the "allocate_stuff" function looks like this:
push esi mov esi, [esp+8] ;
howmany ;
eax = howmany * sizeof(MyClass) + sizeof(size_t)
lea eax, [esi*4+4]
push eax
call operator new test eax,
eax pop ecx je fail push edi
push OFFSET MyClass::MyClass
push esi lea edi, [eax+4] ;
edi = eax + sizeof(size_t) push 4 ;
sizeof(MyClass) push edi mov [eax], esi ;
howmany call `vector constructor iterator' mov eax,
edi pop edi jmp done fail: xor eax, eax done: pop esi retd 4
Translated back into pseudo-C++, the code looks like this:
MyClass* allocate_stuff(int howmany)
{
void *p = operator new( howmany * sizeof(MyClass) + sizeof(size_t));
if (p)
{size_t* a = reinterpret_cast<size_t>(p); </size_t>
*a++ = howmany; vector constructor iterator(a, sizeof(MyClass), &MyClass::MyClass);
return reinterpret_cast<myclass>(a); </myclass>
}
return NULL;
}
In other words, the memory layout of the vector of MyClass objects looks like this:
| howmany |
| MyClass[0] |
| MyClass[1] |
| ... |
| MyClass[howmany-1] |
The pointer returned by the <nobr>new[]</nobr> operator is not the start of the allocated memory but rather points to MyClass[0]. The count of elements is hidden in front of the vector.
The deletion of a vector performs this operation in reverse:
void free_stuff(MyClass* p) { delete[] p; }
generates
mov ecx, [esp+4] ;
p test ecx,
ecx je skip push 3
call MyClass::`vector deleting destructor` skip ret 4
Translated back into pseudo-C++, the code looks like this:
void free_stuff(MyClass* p)
{ if (p)
p->vector
deleting destructor(3);
}
The vector deleting destructor goes like this (pseudo-code):
void MyClass::vector deleting destructor(int flags)
{ if (flags & 2)
{ // if vector destruct
size_t* a = reinterpret_cast<size_t>(this) - 1; </size_t>
size_t howmany = *a;
vector destructor iterator(p, sizeof(MyClass), howmany,
MyClass::~MyClass);
if (flags & 1)
{ // if delete too operator
delete(a); }
} else { // else scalar destruct
this->~MyClass(); // destruct one
if (flags & 1)
{ // if delete too operator
delete(this); }
}
}
The vector deleting destructor takes some flags. If 2 is set, then a vector is being destructed; otherwise a single object is being destructed. If 1 is set, then the memory is also freed.
In our case, the flags parameter is 3, so we will perform a vector destruct followed by a delete. Observe that this code sucks the original "howmany" value out of its secret hiding place and asks the vector destructor iterator to run the destructor that many times before freeing the memory.
So now, armed with this information, you should be able to describe what happens if you allocate memory with scalar "new" and free it with vector <nobr>"delete[]"</nobr> or vice versa. Answers to come tomorrow.
Bonus exercise: What optimizations can be performed if the destructor MyClass::~MyClass() is removed from the class definition?
看完之后，两点感触，1.自己学的太肤浅了。2.看东西还是e文好。
我觉得这个帖子里可以得出这样一个结论：当delete只有一个元素的数组时delete和delete[]是没有区别的，当delete一个包含多个元素的数组时，必须采用delete[]。而delete只对堆内存有作用，delete一个变量是不起作用的，变量的内存并没有释放，值也依然存在。同时delete [] NULL和delete NULL是安全的，但是同一个非NULL地址delete两次是致命的。
在delete一个二维数组时，应该:
for(int i=0; i<10; i++)
delete p[i];
delete [] p;
不知小弟理解的对不对，望各位大虾指正。