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Preface

Const is a C language keyword that restricts a variable not to be changed. Using const in certain programs can improve the robustness of programs. In addition, when watching other people's code, a clear understanding of the role of const is helpful to understand other people's programs.

Brief introduction of 01const

The following is a brief description of const, which is basically textbook knowledge. A variable modified by const whose value is stored in a read-only segment and cannot be changed. Is called a read-only variable. For information about what is a data segment and what is a code segment, please see my previous article, "memory allocation in C language".

Int const a Teng Const int a

Both statements above can declare an as an integer and its value cannot be modified. You can choose either of these two ways.

Constants can be initialized when defined.

Int const a = 15

It's interesting when pointers are combined with constants, because there are two things that can become constants, pointers and the entities it points to. Generally speaking, people often encounter it when they take the second-level computer examination in the university and when they are interviewed.

Int * a

An is a very common pointer to an integer.

Int const * a

In this case, it is a pointer to the integer constant. That is, you can change the value of the pointer, but not the value it points to.

Int * const a

In this case, an is a constant pointer to an integer. This pointer is constant and its value cannot be modified, but you can change the value of the integer it points to.

Int const * const a

At this time, neither the pointer itself nor the value it points to is constant and is not allowed to be modified.

So here comes the problem, like the priority of the operator in C language, which is very difficult to remember, in actual development, we directly use the () symbol to solve the priority problem. The combination of the above pointer and const is so troublesome, what is the purpose of learning?

1. The reasonable use of the keyword const can make the compiler naturally protect those parameters that do not want to be changed from unintentional code modification. In short, this reduces the occurrence of bug.

2, it is based on the above reasons, some excellent open source code will make use of const this attribute, in-depth understanding, it is convenient for us to read and understand some excellent open source code.

The application of 02 constant

The above briefly describes the definition of const in the textbook, and now I talk about the application of const in my daily development.

In the development of single chip microcomputer

Const defines a constant, in MCU development, a constant constint a = 5 defined outside the function; it is stored in the internal Flash of MCU, students who do not understand please see the previous article "memory allocation of C language in STM32". So when combined with pointers mentioned above, are they also stored in the internal Flash? Let's verify it.

Int data = 0x1234 int const * a = & data;int * const b = & data;int const * const c = & data;int main (void) {int data1 = 0x1234; a = & data1; data1 = * b; data1 = * c; while (1);}

Their memory allocation is as follows

B and c are assigned to the internal flash and an is assigned to the ram. In fact, this is also very easy to understand, according to the above definition of const, single-chip microcomputer in the allocation, can not be modified variables, that is, read-only variables into the flash, can read and write variables into the ram, we will understand this carefully.

Constant as a parameter of a function

Non-pointer parameters (that is, value-passing parameters) will not be modified to the original value, const is meaningless to it, so we only discuss the case where the parameter is a pointer plus const.

As you can see above, there are three cases of pointer plus const. Here we first discuss int const * a; that is, you can change the value of the pointer, but not the value it points to.

Int fun (int * p) {if (* p = = 0xA5) {return*p;} else {pairing; return*p;}}

The above is a simple example, that is, passing in a pointer, the function reads what the pointer points to and executes different commands. Similar to serial port reception, a function processes the data internally, but cannot be modified, and the data received by the serial port may be useful elsewhere.

In the above example, there is no problem, because the code is all under control. You know whether to write inside the function or not. But there is a more standard way to write it.

Int fun (int const * p) {if (* p = = 0xA5) {return * p;} else {pairing; return * p;}}

The way to write it here is to clearly show its own design intention, and the content pointed to by the pointer can not be modified inside the function, but can only be read.

If you try to modify it, the compiler will report an error directly. But the inner part of the function can also bypass the modified data, as follows

Int fun (const int * p) {int * p2 = p; / * A duplicate pointer bypasses the const limit * / * p2 + = 1; return*p;}

Is there a corresponding usage scenario for int * const a;? As follows

In this interface design, if the function tries to change the value of the pointer, that is, the position the pointer points to, the compiler will report an error directly.

However, the example here is realistic, because even if the const modification of p2 is removed, the compiler will report waring directly, because p2 is the input parameter. Here is just a simple example, as long as you understand what it means.

In daily development, the input parameter is intconst * a; there are many usage scenarios.

Add const to the app in C++

Application syntax can be used in C++. What is an application is no longer expanded here. For example, references in C++ function parameters are often modified with const, as shown below.

Void find (constint & x) {.}

Finally, take two commonly used standard C library function declarations, both of which are examples of using const.

1. String copy function: char*strcpy (char*strDest,constchar * strSrc)

two。 Returns the string length function: intstrlen (constchar * str)

03#define and const

# define precompilation and const are somewhat "confused" in some cases, as follows

# define MAX_NUM 5int const max_num = 5 * void fun () {if (len > MAX_NUM) if (len > max_num)}

Both 5 and 6 lines of the above code will work. Then let's analyze the differences in detail.

1. The data of # define is a macro definition, which occupies code segment space (corresponding to single-chip microcomputer: internal flash). Const defines a data type, which occupies a data segment (corresponding to single-chip microcomputer: internal ram).

2. As mentioned above, # define is a macro definition that is replaced directly during the precompilation phase, while const is a data type.

# define MAX_NUM 5int const max_num = 5int data [Max _ NUM]; intdata2 [max _ num]

Line 4 above is compiled, because max_num is a data type variable of int, and the length of the array definition cannot be a variable. In fact, in the first example in more chapters, only to determine the length, # define is more appropriate, because macro definitions can be used wherever literal constants are allowed.

3. Define is just a simple string substitution with no type checking. And const has the corresponding data type, which needs to be judged, which can avoid some low-level errors. Define is just a simple string substitution that leads to a boundary effect.

Such as definition.

# define A 1#define B A+3#define C A/B3

So how much is c?

4, const can not be redefined, can not define two the same, and define is more awesome, it through the undef to cancel the definition of a symbol, and then redefine. It can also be used to determine whether a macro definition exists, often used in header files to prevent header files from being repeatedly referenced.

# ifndef GRAPHICS_H / / prevent graphics.h from being repeatedly referenced # defineGRAPHICS_H. The code... # endif

5. The const constant can be debugged, but define cannot be debugged. It has been replaced in the pre-compilation phase, and it is not available when debugging.

The above is about the content of this article on "how to use Const keywords in C language". I believe we all have some understanding. I hope the content shared by the editor will be helpful to you. If you want to know more about the relevant knowledge, please pay attention to the industry information channel.

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