C++使用两种memory: heap和stack。Stack存储每个函数的信息，包括instructions, variables defined in that function。如果函数1 call 函数2, 函数2的信息存到stack顶部。函数return时，它的所有信息被pop。Heap存global variables, C++允许程序员自己在heap上分配内存, 即new(), new总是返回指向分配在heap的内存的指针。程序员必须自己释放掉不再需要的内存, 即delete。

const int *p 意思是p所指向的东西是constant

int x = 17;

const int *p = &x;

x = 27; // OK

*p = 27; // error: read-only variable is not assignable

p = new int; // OK

int * const q 意思是指针q是constant

int y = 18;

int * const q = &y;

y = 28; // OK

*q = 28; // OK

q = new int; // error: read-only variable is not assignable

const int * const r 意思是both

int y = 18;

int * const q = &y;

y = 28; // OK

*q = 28; // error: read-only variable is not assignable

q = new int; // error: read-only variable is not assignable

Don't dereference uninitialized pointers

int *p; *p = 17;

// pointer p was never allocated memory. may compile, and will certainly crash

Don't dereference NULL pointers

int *p = &x; p = NULL; *p = 17; // may compile, and will certainly crash

Don't dereference deleted pointers

delete p; *p = 33;

// The memory has not been returned to the heap just yet, so 33 is stored.

Watch out for memory leaks

int *p = new int(82); p = NULL; p = new int(17);

// associated memory was never returned to the heap

不要滥用delete

int x = 55; int *p = &x; delete p;

// this memory did not come from the heap

Don't call delete twice on the same block of memroy

int *a = new int(99);

int *b = a;

delete a;

delete b;

// this compiles, but will crash