[C++]cpp小笔记3 --- C++基础类型的大小和转换

青旅半醒 2022-09-18 14:59 5阅读 0赞

### 1. 类型的大小 ###

*  A short integer is at least 16 bits wide.
 *  An int integer is at least as big as short.
 *  A long integer is at least 32 bits wide and at least as big as int.
 *  A long long integer is at least 64 bits wide and at least as big as long.

### 2. sizeof and climits ###

int days = 0
    cout << "size of days is " << sizeof(days) << endl;
    cout << "size of int is " << sizeof(int) << endl;
    
    Results:
    size of days is 4
    size of int is 4

climits中定义了c++中基础类型的最大值，最小值，大小的常量值。

CHAR_BIT          Number of bits in a char
    CHAR_MAX       Maximum char value
    CHAR_MIN        Minimum char value
    SCHAR_MAX    Maximum signed char value
    SCHAR_MIN     Minimum signed char value
    UCHAR_MAX   Maximum unsigned char value
    SHRT_MAX       Maximum short value
    SHRT_MIN        Minimum short value
    USHRT_MAX    Maximum unsigned short value

climits的使用

#include <climits>
    // use limits.h for older systems

###     ###

### 3.  无符号类型 ###

unsigned short change;
    unsigned int rovert;
    unsigned quarterback;  //unsigned int type
    unsigned long gone;
    unsigned long long lang_lang;

![Image 1][]

从这张图上可以看出

If signed integer max value is 32767. 32767 + 1, result is -32768.

If unsigned integer max value is 65535, if 65535 + 1, result is 0.

### 4. 类型转换。 ###

自动类型转换：

*  C++会自动将converts bool, char, unsigned char, signed char, and short values to int.
 *  The unsigned short type is converted to int if short is smaller than int. If the two types are the same size, unsigned short is con-  
    verted to unsigned int. This rule ensures that there’s no data loss in promoting unsigned short.
 *  When an operation involves two types, the smaller is converted to the larger.

强制转换：

int a = 0;
    double b = (double)a; 
    
    OR 
    
    double b = double(a);

double b = static_cast<double> (a);

[Image 1]: