# C/C++ const
>When it modifies a data declaration, the const keyword specifies that the object or variable isn't modifiable.
https://learn.microsoft.com/en-us/cpp/cpp/const-cpp?view=msvc-170
https://learn.microsoft.com/zh-tw/cpp/cpp/const-cpp?view=msvc-170
## basic
```cpp=
//const declar
int i = 42;
const int j = i;
int k = j;
const int t; // error unintilized const
//const and pointer
const int x = 8;
int y = 18;
const int *xptr1 = &x; //1 point to const int
int* const xptr2 = &x; //2 the pointer is const but invalid conversion from ‘const int*’ to ‘int*
int const *xptr3 = &x; //3 same as 1
int* const yptr1 = &y; //4 is ok the pointer is const
const int* const yptr2 = &y;//5 const point to const int
*xptr1 = 10; //is invalid
xpt1 = &i; //is ok
*yptr1 = 10; //is ok
yptr1 = &i; //is invalid
//const and reference
const int &r1 = i;
const int &r2 = 52;
int &r3 = x; //error non const reference to const object
```
## top level and low level const
top level:
```cpp=
int t1 = 0;
int *const pt1 = &t1;
const int t2 = 1;
```
low level:
```cpp=
const int *pl1 = &t2;
const int &rl1 = t2; // const in reference types is always low-level
```
both top and low level:
```cpp=
const int *const ptl1 = pt1; //is top level and low level
```
| top level | low level |
| ----- | ---- |
| value is const | point or ref value is const |
| copy can ignore | copy can't ignore |
```cpp=
t1 = t2; // is ok
pl1 = ptl1;// ok: p2 has the same low-level const qualification as ptl1
int *p = pl1; // error: pl1 has a low-level const but p doesn’t
pl1 = &t1; // ok: we can convert int* to const int*
pl1 = &t2;
int x = *pl1; // ok *pl1 is pointer to top level const t2
int &r1 = t2; // error: can’t bind an ordinary int& to a const int object
const int &r2 = t1; // ok: can bind const int& to plain int
int y = rl1; //ok rl1 is reference to t2
```
c++ primer section 2.4.2(P62)
## const and function
- const in parmeter
```cpp=
void function(int);
void function(const int); // redeclares Record function(int)
void function(int*);
void function(int* const); // redeclares Record function(int*)
```
>top-level const has no effect on the objects that can be passed to the function. A parameter that has a top-level const is indistinguishable from one without a top-level const
https://blog.xuite.net/tsai.oktomy/program/65131235#
```cpp=
void function(int&); // function that takes a reference to int
void function(const int&); // new function that takes a const reference
void function(int*); // new function, takes a pointer to int
void function(const int*); // new function, takes a pointer to const
```
- const in return type
```cpp=
const int& function(const int& i1, const int& i2){
return i1 >= i2 ? i1 : i2;
}
```
```cpp=
int function(const int& i1, const int& i2){
return i1 >= i2 ? i1 : i2;
}
```
```cpp=
const int function(const int& i1, const int& i2){
return i1 >= i2 ? i1 : i2;
}
```
```cpp=
int& function(const int& i1, const int& i2){
return i1 >= i2 ? i1 : i2;
//error can't convert const int to int
}
```
- const function
means function can't modify the data in object
https://stackoverflow.com/questions/751681/what-is-the-meaning-of-const-at-the-end-of-a-member-function-declaration
## const type transfer
```cpp=
int& function(const int& i1){
return const_casts<int&>(i1);
//tranfer const int& to int&
}
```
https://en.cppreference.com/w/cpp/language/const_cast
https://learn.microsoft.com/en-us/cpp/cpp/const-cast-operator?view=msvc-170
## const and auto
```cpp=
int i = 0, &r = i;
auto a = r; // a is an int (r is an alias for i, which has type int
const int ci = i, &cr = ci;
auto b = ci; // b is an int (top-level const in ci is dropped)
auto c = cr; // c is an int (cr is an alias for ci whose const is top-level)
auto d = &i; // d is an int* (& of an int object is int*)
auto e = &ci; // e is const int* (& of a const object is low-level const)
const auto f = ci; // deduced type of ci is int; f has type const int
auto &g = ci; // g is a const int& that is bound to ci
auto &h = 42; // error: we can’t bind a plain reference to a literal
const auto &j = 42; // ok: we can bind a const reference to a literal
auto k = ci, &l = i; // k is int; l is int&
auto &m = ci, *p = &ci; // m is a const int&; p is a pointer to const int
// error: type deduced from i is int; type deduced from &ci is const int
auto &n = i, *p2 = &ci;
```
c++ primer 2.5.2 (p69)
## const and pointer
```cpp=
//1
int const *h = 5;
//2
int *const h = 5;
//3
const int *h = 5; //same as 1
//4
const int const *h = 5; //error duplicate
const int* const h = 5; //is ok
```
```cpp=
const int z = 2;
const int *pz = &z;
const int **ppz1 = &pz;
int const **ppz2 = &pz; //same as ppz1
int x = 3;
int * const px = &x;
int * const *ppx = &px;
//*ppx value can't be modify and ppx only can point to the pointer which has const value
int y = 4;
int *py = &y;
int ** const ppy = &py;
// ppy value can't be change but *ppy value can be change
*ppy = &x; //is ok
```
```mermaid
classDiagram
pz --|> z
ppz --|> pz
class z{
value:10
addr()0x0001
}
class pz{
value:0x0001
addr()0x0020
}
class ppz{
value:0x0020
addr()0x0030
}
```
https://jonny.vip/2018/03/12/cplusplus-%E5%AD%B8%E7%BF%92%E7%AD%86%E8%A8%98-%E9%A0%82%E5%B1%A4-const-%E9%82%84%E6%98%AF%E5%BA%95%E5%B1%A4-const/
# constexpr
>A constant expression is an expression whose value cannot change and that can
be evaluated at compile time.
c++ primer section 2.4.4(P64)
```cpp=
constexpr int mf = 20; // 20 is a constant expression
constexpr int limit = mf + 1; // mf + 1 is a constant expression
constexpr int sz = size(); // ok only if size is a constexpr functio
```
### constexpr Functions
>A constexpr function is a function that can be used in a constant expression,
>A constexpr function is defined like any other function but must
meet certain restrictions: The return type and the type of each parameter in a
must be a literal type , and the function body must contain exactly
one return statement
###### tags: `C++`
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