On the corners of const

About const In every corner of the world

const Keyword provides auxiliary reference information for program robustness and compilation optimization .

const It is mainly interpreted and used by the compiler , It's a static concept . That is to say , When we pass a variable const When the description is constant , Subsequent modifications to this variable , Will be checked by the compiler , Then an error .
however , Once the inspection is passed , In actual operation , It is no different from ordinary variables . such as , If we get the address of the constant by other means , You can also change its value at run time . In the memory , It is still represented as a common data variable .

const There are many descriptive articles on the Internet . Authoritative interpretation , Personally, I think it should be the standard description . The compiler shall comply with the standard requirements , For the const Carry out inspection and treatment .

const You can modify variables and functions .
Modifying variables , It can be a built-in type , It can also be a custom type .
The built-in type can be a normal variable , It can also be a pointer variable , It can also be a reference .

Modify function , You can modify the return value of a function , The parameters of the function , The function itself .
The function can be C function , It can be C++ The function in ,C++ Including static functions and member functions .

Modification of the return value and parameters of a function , Personal understanding is based on the modification of variables , Many places can be understood by referring to the modification of variables .
Modification of the function itself , Mainly for C++ in , In fact, it is decorated this The pointer . such , We understand const Member variables cannot be modified in a function , Just a simple . otherwise , Simply memorize by rote , The effect is not good .

Here we mainly look at the modification of the function return value . To elicit questions , Let's start with a few simple examples ：
Decoration of common built-in types , such as
int const cint1 = 1;
const int cint2 = 2;
The compiler will require an initial value when defining , And elsewhere in the code , You can only quote , Can't change value , Otherwise it will go wrong . Same as before , These are all compilation checks .
For the above two ways , Compilers seem to be all about const Variable to handle , There is no difference . This point , You'll see that in the back . Let me first mention .

however , For pointer variables , The above two ways of writing are different
int * const cpi1 = &xx;
const int * cpi2;
Assign values when the first one needs to be defined , Otherwise the compiler will tell you constants cpi1 Not initialized . therefore , here cpi1 Is a constant , Is not a variable , The point should be clear from the beginning .
The second one can be compiled through , explain cpi2 It's a variable , Is not a constant . however , If you try to assign a value to the content it points to , Error will be reported in compilation , Tips *cpi2 Is a constant , That is, the content pointed to by the pointer is a constant .
Because this article is to study const In every corner of the world , So let's see const Put it in the middle ：
int const * cpi3;
actually , This kind of writing is rare , It's a little nondescript , The compiler presses cpi2 Handle it the same way .

below , Let's see. const Decorating functions .
For common built-in types , We can rely on intuition , Write the following example ：
const int func();
however , Similar to variables , There are also the following possibilities ：
int const func();
As I said before , Modification of the function itself , Is placed after the function , That is, something like this ,int func() const, So here are all decorated return values .
In order to see the difference between the above two ways , We make mistakes , Let the compiler tell us where the error is , Through error messages , Further judge the difference .
How to make mistakes ？
There is a way , We declare a function pointer , Then define a function , Let the function pointer point to the function , If the two definitions are inconsistent , Error will be reported when compiling , Here is the starting point .
typedef const int (*pfunc)();
const int getVal2()  {return 123;}

pfunc xxx;
xxx = getVal2;
Because the above types are consistent , So compile through , We will declare that const Get rid of , have a look
typedef int (*pfunc)();
Compiler error
invalid conversion from ‘const int (*)()’ to ‘pfunc {aka int (*)()}’
We will define the const Move , have a look
The compiler reported the same error
invalid conversion from ‘const int (*)()’ to ‘pfunc {aka int (*)()}’
This shows two ways of writing , There is no difference in the compiler's view .
Okay , We replace the basic built-in type with a pointer , See what happens
const int * getVal2()  {return &g_var;}
The compiler error is as follows ：
invalid conversion from ‘const int* (*)()’ to ‘pfunc {aka int (*)()}’
take const Change the position ,
int const * getVal2()  {return &g_var;}
invalid conversion from ‘const int* (*)()’ to ‘pfunc {aka int (*)()}’
Errors do not change , This is consistent with the pointer variable situation described above
Let's see ,const Put it in * What happened later
int * const getVal2()  {return &g_var;}
Compiler error changed ,
invalid conversion from ‘int* const (*)()’ to ‘pfunc {aka int (*)()}’
These three （ There are essentially two kinds of ） What are the differences in writing ？

For the first and second , That is to say const In the previous case ,
int * presult = getVal2();
This writing method will report compilation errors ,invalid conversion from ‘const int*’ to ‘int*’
It returns a pointer , The data pointed to by the pointer is a constant , Do not modify , The lvalue also needs to be a pointer of the same type .

For the third way of writing , The above lvalue compilation will not be a problem .const Decorated function name , The function name itself is not modifiable , So this way of writing should be meaningless .
But the compiler will check the decoration , Where relevant , Must be of the same type .

Other uses ？ To be added .