1.int（整型数据）：
1.1 进制的表示：十进制，八进制，16进制，二进制
int number1 = 15; // Decimal
int number2 = 017; // Octal
int number3 = 0x0F; // Hexadecimal
int number4 = 0b00001111; // Binary
上面几种表示方式都表示15这个数字，用cout输出得到相同的结果
1.2 声明和初始化：int类型占据4个字节（32位）
声明并且初始化一个整数类型可以使用大括号，括号和=。
利用大括号{}进行声明和初始化：
int elephant_count; //Variable may contain random garbage value . WARNING
int lion_count{};//Initializes to zero
int dog_count {10}; //Initializes to 10
int domesticated_animals { dog_count + cat_count };
int narrowing_conversion {2.9};//Compiler error
利用括号()进行声明和初始化：
int test(); //error
int orange_count(10);
int fruit_count (apple_count + orange_count);
int narrowing_conversion_functional (2.9); //Information lost.
利用=进行初始化：
int truck_count = 7;
int vehicle_count = bike_count +truck_count;
int narrowing_conversion_assignment = 2.9; //information lost
通过对比可以发现，{}来初始化整型数据是最安全的，对于没有赋初值的整型数据会自动初始化为0，对于赋值为浮点数的数据类型会报编译器错误。
1.3 long/short, signed/unsigned 标识符
unsigned int range 0~2^n-1 4 bytes
signed int range -2^(n-1)~2^(n-1)-1 4 bytes
//unsigned int value4{-5}; // Compiler error.
// short 关键字占据2 Bytes
short short_var {-32768} ; // 2 Bytes
short int short_int {455} ; //
signed short signed_short {122}; //
signed short int signed_short_int {-456}; //
unsigned short int unsigned_short_int {456};
// 没有short或者long关键字都占据4 Bytes
int int_var {55} ; // 4 bytes
signed signed_var {66};//
signed int signed_int {77};//
unsigned int unsigned_int{77};
// long关键字一般占据4 Bytes
long long_var {88}; // 4 OR 8 Bytes
long int long_int {33};
signed long signed_long {44};
signed long int signed_long_int {44};
unsigned long int unsigned_long_int{44};
// long long关键字占据8 Bytes
long long long_long {888};// 8 Bytes
long long int long_long_int {999};
signed long long signed_long_long {444};
signed long long int signed_long_long_int{1234};
unsigned long long int unsigned_long_long_int{1234};