Intra class data member initialization of static const and static constexpr

Reference resources ：https://blog.csdn.net/qq_34801642/article/details/104948850

2. example
   2.1 static Data member
   #include
   using namespace std;
   class A
   {
   public:
   // Static member of integer type
   static bool b;
   static char c;
   static int i;
   // Floating point data members
   static float f;
   static double d;

   // static int i1 = 1; // error ： Members with in class initializers must be constants
   // static double d1 = 3.14; // error ： Members with in class initializers must be constants
   };

bool A::b = true;
char A::c = ‘a’;
int A::i = 1;
float A::f = 1.5;
double A::d = 2.5;

int main()
{
cout << A::b << endl;
cout << A::c << endl;
cout << A::i << endl;
cout << A::f << endl;
cout << A::d << endl;
return 0;
}

summary ：

General static Data members can only be declared inside a class , Define and initialize outside the class .

2.2 static const Data member
#include
using namespace std;
class A
{
public:
// Static member of integer type
static const bool b1;
static const char c1;
static const int i1;
// Floating point data members
static const float f1;
static const double d1;

// Static member of integer type 
static const bool b2 = false;
static const char c2 = 'b';
static const int i2 = 2;
// Floating point data members 
// static const float f2 = 3.5;  //  error ："const float"  A member of a type cannot contain an in class initializer 
// static const double d2 = 4.5; //  error ："const double"  A member of a type cannot contain an in class initializer 

// char m1[i1];//  error ：i1 The constant of has not been initialized 
char m2[i2];

};

const bool A::b1 = true;
const char A::c1 = ‘a’;
const int A::i1 = 1;
const float A::f1 = 1.5;
const double A::d1 = 2.5;

const bool A::b2;
const char A::c2;
const int A::i2;

int main()
{
cout << A::b1 << endl;
cout << A::c1 << endl;
cout << A::i1 << endl;
cout << A::f1 << endl;
cout << A::d1 << endl;
cout << “---------------------------------------” << endl;
cout << A::b2 << endl;
cout << A::c2 << endl;
cout << A::i2 << endl;
return 0;
}

summary ：

static const Data members can be declared inside a class , Define and initialize outside the class . When declared inside a class ,static const The data member has not been initialized , Not really const.
static const Integer data members can be declared and initialized within a class , Define... Outside the class . When declared and initialized within a class ,static const Data members are real const.
If at compile time static const A data member can be replaced by its value （ For example, it indicates the number of arrays ）, It does not need to be defined . If it cannot be replaced （ As a parameter ）, Must contain a definition statement . It is recommended to define once outside the class regardless of whether it is replaceable or not .
if static const The data member is not an integer （bool、char、int、short、long etc. ）, Cannot initialize within a class .

2.3 static constexpr Data member
#include
using namespace std;
class A
{
public:
// Static member of integer type
// static constexpr bool b1; // error ：constexpr Static data member declaration requires an in class initializer
// static constexpr char c1; // error ：constexpr Static data member declaration requires an in class initializer
// static constexpr int i1; // error ：constexpr Static data member declaration requires an in class initializer
// Floating point data members
// static constexpr float f1; // error ：constexpr Static data member declaration requires an in class initializer
// static constexpr double d1; // error ：constexpr Static data member declaration requires an in class initializer

// Static member of integer type 
static constexpr bool b2 = false;
static constexpr char c2 = 'b';
static constexpr int i2 = 2;
// Floating point data members 
static constexpr float f2 = 3.5;
static constexpr double d2 = 4.5;

// char m1[i1]; //  error ：i1 The constant of has not been initialized 
char m2[i2];

};

// constexpr bool A::b1 = true;
// constexpr char A::c1 = ‘a’;
// constexpr int A::i1 = 1;
// constexpr float A::f1 = 1.5;
// constexpr double A::d1 = 2.5;
constexpr bool A::b2;
constexpr char A::c2;
constexpr int A::i2;
constexpr float A::f2;
constexpr double A::d2;

int main()
{
cout << A::b2 << endl;
cout << A::c2 << endl;
cout << A::i2 << endl;
cout << A::f2 << endl;
cout << A::d2 << endl;
return 0;
}

summary ：

static constexpr Data members must be declared and initialized within a class . When declared and initialized within a class ,static constexpr Data members are real const.
If at compile time static constexpr A data member can be replaced by its value （ For example, it indicates the number of arrays ）, It does not need to be defined . If it cannot be replaced （ As a parameter ）, Must contain a definition statement . It is recommended to define once outside the class regardless of whether it is replaceable or not .
if static constexpr Even if the data member is not an integer , You can also perform in class initialization .