Part 1 - Chapter 2 pointer operation

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stay C In language , For any type T, Can be in T A corresponding variable containing the address of the object is generated at the memory address where it is located . This variable , It's actually a variable that points to an object , therefore , These variables are called The pointer .
Pointers are precise and efficient tools for building data structures and manipulating memory .

Chapter structure

• Pointer basis
• Storage allocation
• Arithmetic operations on data sets and pointers
• As a pointer to a function parameter
• The pointer to the pointer
• Generic pointers and type conversions
• A function pointer

Pointer basis

A pointer stores the address of the data in memory, not the data itself .
The best way to understand pointers ： Charting .

• The pointer is usually linked by position with arrows , Instead of drawing the actual address in the icon .
• When the pointer complements any data ,, That is, the pointer is set to NULL when , Use two vertical lines to show .
• Dangling pointer ： Pointer to an invalid address .

Some examples of errors that may result in dangling pointers ： Cast any integer variable into a pointer variable ; Manipulating pointers that exceed the bounds of the array ; Release one or more pointers that are still referenced .

Storage allocation

The size of pointer variables is usually related to the compiler settings and some specific C Implementation of the Type qualifiers of .

• You must remember ： When you declare a pointer , It just allocates space for the pointer itself , There is no space allocated for the data referenced by the pointer .

by Data allocation space Two methods of ：

• (1) Declare a variable directly
• (2) Dynamically allocate storage space at runtime （ for example ： Use malloc or realloc）.

When you declare a variable , What happened?

When you declare a variable , The compiler will reserve enough memory space according to the type of variable .
The storage space of variables is automatically allocated by the system , But this storage space does not exist permanently throughout the life cycle of the program .

The automatic variable is A variable whose storage space can be automatically allocated and released when entering or leaving a module or function

stay C In language , When you want to dynamically allocate storage space , You get a pointer to a heap storage space （ The first 3 Chapter ）, This storage space is managed by us , And it will always be there , Unless we explicitly release it .

As a pointer to a function parameter

stay C The function in the call language plays an important role . most important of all , Pointers support passing parameters as references to functions （ Call by reference ）.

When passing parameters by reference , When the function changes this parameter , The value of the changed parameter will always exist , Even after the function exits . When a pass function is called by value , At this point, the value change can only last until the function returns .

It is an efficient way to pass large and complex function parameters with pointers ： Because it's just passing a pointer instead of a full copy of the data into the function , This can greatly save memory space

Call the pass function by reference