Pointer

Basic Pointer

A pointer is a variable declared to store a memory address. Its declaration will also tell you the type of data it points to. You can retrieve the memory address of your variables, then mess with them.

To declare a pointer you can use int *pt or int* pt, note that this two syntax will do the same thing — make a pointer pointing to type int. However, I personally prefer the first one since it indicates that pt is a pointer, pointing to int, the second one indicates that pt is an int pointer, which is certainly not the case.

The type int just tell the compiler how to read the data, for example, if you declare a variable with type short (which is 2 bytes), they just store binary data into memory and read exactly 2 bytes, we can typecast it to int, then we read 4 bytes next time(may lead to memory corruption though). Type is just ways to read data!

As a result, every pointer is 8 bytes(in 64 bits) since they refer to memory location, can read it with the type you specify. For example, int *pt means that pt is a location, read that location as int.

Also, int* pt, not_a_pointer; is a common pitfall. Then you can see why I prefer the first declaration!

Now we know that a pointer is nothing special but a memory location, but the location itself is not enough.
To get the value inside the location, we need * again, however, this time we are not declaring pointers, we dereference it. This means, we set/get value of it.
Get the memory location of a value is called reference, get the value of the location is called dereference.

Pitfall comes again! We need to first make sure that it is pointing to a valid address!

// BUGGY CODE START
int *x;
*x = 1;
// seg fault
// BUGGY CODE END
 
int var = 1;
int *pt = NULL; 
// initialize it with NULL so it won't mess useful address up
// The default value of a pointer is undefined
// or use nullptr in C++
 
pt = &var;
// here we point pt to address of var
 
*pt = 2; // set value
// we change the value of that address, so now both var and *p is 2
 
printf("%d", var);
printf("%d", *pt); // get value
printf("%p\n", pt);
printf("%p\n", &var); // get address
 
// 2
// 2
// 0x16d10b26c
// 0x16d10b26c

In the example above, we use & to get the address of a variable, this is called reference a variable.

NULL (in C) or nullptr (in C++) is a safe way to initialize a pointer, the former is a int, with value set to 0, the second is a pointer points to nothing

More Than Basic

• Pointer arithmetic (If you were to design this, what would you do? Think about it!)
  • pt++ vs *pt++
• Arrays are just pointer (since they are memory)
• Pointer to pointer (And more, aka 2d-array)
• malloc and free, new and delete, new [] and delete[] (Best and basic practice)
• Memory is the key!
• Stack and heap. See also Memory and Memory Allocation in C
• const pointer. See also: C++ Const
• Complex declaration: cdecl: C gibberish ↔ English
• C++ Smart Pointer

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2022-07-28 Update
其实要分辨指针，要（1）分清楚星号所在的地方，是个左值（left value），还是个右值（right value），（2）分清楚指针所指的类型，加上一些内存的通识就能理解了。

General Pointer

• * 在 int 后还是数值前

As far as C goes they both do the same thing. It is a matter of preference. int* i shows clearly that it is an int pointer type. int *i shows the fact that the asterisk only affects a single variable. So int *i, j and int* i, j would both create i as an int pointer and j as an int.

int *i, j 和 int* i, j 效果都一样，一个指针和一个整数

• 两指针可以相减，不可相加。若要进行相减运算，则两指针必须指向同一数组，相减结果为相距的数组元素个数

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Pointer to Function

URL: How do function pointers in C work? - Stack Overflow

• Use Case

int (*functionPtr)(int, int);
functionPtr = &addInt;
 
int add2to3(int (*functionPtr)(int, int)) {
    return (*functionPtr)(2, 3);
}
// function as argument

• Via typedef

typedef int (*myFuncDef)(int, int);
// note that the typedef name is indeed myFuncDef
 
myFuncDef functionFactory(int n) {
    printf("Got parameter %d", n);
    myFuncDef functionPtr = &addInt;
    return functionPtr;
}

• C++ this pointer