C++中关于类型转换的问题讨论

这里以signed/unsigned char, signed/unsigned short, signed/unsigned int类型为例，

讨论一下基本类型转换的基本原理，这样我们在编程中碰到由类型错误转换而引发的越界问题时，也可以比较容易诊断，不至于把BUG怀疑到机器或编译器身上:)。

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一，3种基本类型表示范围如下 （圆形示例图），其中阴影部分的弧边界包含的是没有越界的数值范围：

2，类型转换原则小结：

我们把类型转换分成两类：

1）同类型符号之间的转换：

　　*同长度类型，不做转换，如int->int，

　　*向更长度类型转换，如char->short->int

　　原则：向高字节对齐

　　　　   因为有足够位数容纳数值，所以数值会保持不变；

　　　　　若为负数时高位会被1，若为正数时高位会补0.

      *向短长度类型转换：如int ->short->char

       原则：高字节截断，低字节保留。

2）不同类型符号之间的转换：

　　*同长度类型转换：如int<->unsigned int

　　原则：重新解释至目标类型，

　　　　  这里是重新解释，换名话说，在等长字节时，有符号类型和无符号类型在内存中的十六进制表示仍然是一样的。

　　　　 当数值范围溢出时，重新解释会造成前后两者数值的会发生巨大跳变，详见示例。

　　*向更长类型转换：如char->unsigned int

　　原则：先字节对齐，再类型统一。

　　　　　如：char->unsigned int  ===>char -> int ->unsigned int

                  unsigned char -> int ===>unsigned char ->unsigned int ->int

　　这个原则会造成将负数执行”跨类型跨字节“转换时，前后两数值之间很大差距，

　　如果想让差距变得”相对“合理些（等价于重新解释后的数值），你需要手动做强制转换：先类型统一，再做字节对齐。

　　*向更短字节对齐：如int->unsigned char

　　原则：高字节截断，低字节保留

提示：

　　1）如果想要转换无错，还是规规矩矩的保证数值：不要溢出，不要截断，不要跨类型跨字节转换。

　　2）在”有符号类型“向”无符号类型“转换时，数值不要为负数，否则打印出的数值并非你想要的结果。

　　尤其是“有符号短字节类型“向”无符号长字节类型转换“时，负数会按照高字节补1的原则，你将会得到一个无敌错错值。

3，类型转换源代码

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

typedef char s8;
typedef unsigned u8;
typedef short s16;
typedef unsigned short u16;
typedef int s32;
typedef unsigned int u32;

#define NONE "33[0m"
#define BLUE "33[0;32m"
#define RED "33[0;31m"

void func_s8(s8 num)
{
    s8 s8Num     = num;
    u8 u8Num     = num;
    s16 s16Num     = num;
    u16 u16Num     = num;
    s32 s32Num     = num;
    u32 u32Num     = num;
    //signed->signed
    printf(BLUE " s8[%hhd#0x%02hhx]:" NONE "
s16(%hd#0x%04hx) 
s32(%d#0x%08x)
", 
        s8Num, s8Num, s16Num, s16Num, s32Num, s32Num);

    //signed->unsigned 
    printf(" u8(%hhu#0x%02hhx) 
u16(%hu#0x%04hx) 
u32(%u#0x%08x)
",
        u8Num, u8Num, u16Num, u16Num, u32Num, u32Num);
}

void func_u8(u8 num)
{
    s8 s8Num     = num;
    u8 u8Num     = num;
    s16 s16Num     = num;
    u16 u16Num     = num;
    s32 s32Num     = num;
    u32 u32Num     = num;
    //unsigned->unsigned
    printf(BLUE " u8[%hhu#0x%02hhx)]" NONE "
u16(%hu#0x%04hx) 
u32(%u#0x%08x)
",
        u8Num, u8Num, u16Num, u16Num, u32Num, u32Num);

    //unsigned->signed 
    printf(" s8(%hhd#0x%02hhx): 
s16(%hd#0x%04hx) 
s32(%d#0x%08x)
", 
        s8Num, s8Num, s16Num, s16Num, s32Num, s32Num);
}

void func_s16(s16 num)
{
    s8 s8Num     = num;
    u8 u8Num     = num;
    s16 s16Num     = num;
    u16 u16Num     = num;
    s32 s32Num     = num;
    u32 u32Num     = num;
    //signed->signed
    printf(BLUE "s16[%hd#0x%04hx]: " NONE "
s32(%d#0x%08x) 
 s8(%hhd#0x%02hhx)
", 
        s16Num, s16Num, s32Num, s32Num, s8Num, s8Num);

    //signed->unsigned 
    printf("u16(%hu#0x%04hx) 
u32(%u#0x%08x) 
 u8(%hhu#0x%02hhx)
",
        u16Num, u16Num, u32Num, u32Num, u8Num, u8Num);
}

void func_u16(u16 num)
{
    s8 s8Num     = num;
    u8 u8Num     = num;
    s16 s16Num     = num;
    u16 u16Num     = num;
    s32 s32Num     = num;
    u32 u32Num     = num;
    //unsigned->unsigned
    printf(BLUE "u16[%hu#0x%04hx]:" NONE "
u32(%u#0x%08x) 
 u8(%hhu#0x%02hhx)
",
        u16Num, u16Num, u32Num, u32Num, u8Num, u8Num);

    //unsigned->signed 
    printf("s16(%hd#0x%04hx) 
s32(%d#0x%08x) 
 s8(%hhd#0x%02hhx)
", 
        s16Num, s16Num, s32Num, s32Num, s8Num, s8Num);
}

void func_s32(s32 num)
{
    s8 s8Num     = num;
    u8 u8Num     = num;
    s16 s16Num     = num;
    u16 u16Num     = num;
    s32 s32Num     = num;
    u32 u32Num     = num;
    //signed->signed
    printf(BLUE "s32[%d#0x%08x]:" NONE "
s16(%hd#0x%04hx) 
 s8(%hhd#0x%02hhx)
", 
        s32Num, s32Num, s16Num, s16Num, s8Num, s8Num);

    //signed->unsigned 
    printf("u32(%u#0x%08x) 
u16(%hu#0x%04hx) 
 u8(%hhu#0x%02hhx)
",
        u32Num, u32Num, u16Num, u16Num, u8Num, u8Num);
}

void func_u32(u32 num)
{
    s8 s8Num     = num;
    u8 u8Num     = num;
    s16 s16Num     = num;
    u16 u16Num     = num;
    s32 s32Num     = num;
    u32 u32Num     = num;
    //unsigned->unsigned
    printf(BLUE "u32[%u#0x%08x]" NONE "
u16(%hu#0x%04hx) 
 u8(%hhu#0x%02hhx)
",
        u32Num, u32Num, u16Num, u16Num, u8Num, u8Num);

    //unsigned->signed 
    printf("s32(%d#0x%08x): 
s16(%hd#0x%04hx) 
 s8(%hhd#0x%02hhx)
", 
        s32Num, s32Num, s16Num, s16Num, s8Num, s8Num);
}

int main()
{
    printf(RED "######s8Num testing######
" NONE);
    s8 s8Num = 0x7F;
    func_s8(s8Num);
    s8Num = 0x80;
    func_s8(s8Num);
    s8Num = 0xFF;
    func_s8(s8Num);
    
    printf(RED "######u8Num testing######
"NONE);
    u8 u8Num = 0x7F;
    func_u8(u8Num);
    u8Num = 0x80;
    func_u8(u8Num);
    u8Num = 0xFF;
    func_u8(u8Num);

    printf(RED "######s32Num testing######
"NONE);
    s32 s32Num = 0x7FFFFFFF;
    func_s32(s32Num);
    s32Num = 0x80000000;
    func_s32(s32Num);
    s32Num = 0xFFFFFFFF;
    func_s32(s32Num);
    
    printf(RED "######u32Num testing######
"NONE);
    u32 u32Num = 0x7FFFFFFF;
    func_u32(u32Num);
    u32Num = 0x80000000;
    func_u32(u32Num);
    u32Num = 0xFFFFFFFF;
    func_u32(u32Num);
    
    printf(RED "######s16Num testing######
"NONE);
    s16 s16Num = 0x7FFF;
    func_s16(s16Num);
    s16Num = 0x8000;
    func_s16(s16Num);
    s16Num = 0xFFFF;
    func_s16(s16Num);
    
    printf(RED "######u16Num testing######
"NONE);
    u16 u16Num = 0x7FFF;
    func_u16(u16Num);
    u16Num = 0x8000;
    func_u16(u16Num);
    u16Num = 0xFFFF;
    func_u16(u16Num);
    
    return 0;
}

4，类型转换运行结果

######s8Num testing######
 s8[127#0x7f]:
s16(127#0x007f) 
s32(127#0x0000007f)
 u8(127#0x7f) 
u16(127#0x007f) 
u32(127#0x0000007f)
 s8[-128#0x80]:
s16(-128#0xff80) 
s32(-128#0xffffff80)
 u8(128#0x80) 
u16(65408#0xff80) 
u32(4294967168#0xffffff80)
 s8[-1#0xff]:
s16(-1#0xffff) 
s32(-1#0xffffffff)
 u8(255#0xff) 
u16(65535#0xffff) 
u32(4294967295#0xffffffff)
######u8Num testing######
 u8[127#0x7f)]
u16(127#0x007f) 
u32(127#0x0000007f)
 s8(127#0x7f): 
s16(127#0x007f) 
s32(127#0x0000007f)
 u8[128#0x80)]
u16(128#0x0080) 
u32(128#0x00000080)
 s8(-128#0x80): 
s16(128#0x0080) 
s32(128#0x00000080)
 u8[255#0xff)]
u16(255#0x00ff) 
u32(255#0x000000ff)
 s8(-1#0xff): 
s16(255#0x00ff) 
s32(255#0x000000ff)
######s32Num testing######
s32[2147483647#0x7fffffff]:
s16(-1#0xffff) 
 s8(-1#0xff)
u32(2147483647#0x7fffffff) 
u16(65535#0xffff) 
 u8(255#0xff)
s32[-2147483648#0x80000000]:
s16(0#0x0000) 
 s8(0#0x00)
u32(2147483648#0x80000000) 
u16(0#0x0000) 
 u8(0#0x00)
s32[-1#0xffffffff]:
s16(-1#0xffff) 
 s8(-1#0xff)
u32(4294967295#0xffffffff) 
u16(65535#0xffff) 
 u8(255#0xff)
######u32Num testing######
u32[2147483647#0x7fffffff]
u16(65535#0xffff) 
 u8(255#0xff)
s32(2147483647#0x7fffffff): 
s16(-1#0xffff) 
 s8(-1#0xff)
u32[2147483648#0x80000000]
u16(0#0x0000) 
 u8(0#0x00)
s32(-2147483648#0x80000000): 
s16(0#0x0000) 
 s8(0#0x00)
u32[4294967295#0xffffffff]
u16(65535#0xffff) 
 u8(255#0xff)
s32(-1#0xffffffff): 
s16(-1#0xffff) 
 s8(-1#0xff)
######s16Num testing######
s16[32767#0x7fff]: 
s32(32767#0x00007fff) 
 s8(-1#0xff)
u16(32767#0x7fff) 
u32(32767#0x00007fff) 
 u8(255#0xff)
s16[-32768#0x8000]: 
s32(-32768#0xffff8000) 
 s8(0#0x00)
u16(32768#0x8000) 
u32(4294934528#0xffff8000) 
 u8(0#0x00)
s16[-1#0xffff]: 
s32(-1#0xffffffff) 
 s8(-1#0xff)
u16(65535#0xffff) 
u32(4294967295#0xffffffff) 
 u8(255#0xff)
######u16Num testing######
u16[32767#0x7fff]:
u32(32767#0x00007fff) 
 u8(255#0xff)
s16(32767#0x7fff) 
s32(32767#0x00007fff) 
 s8(-1#0xff)
u16[32768#0x8000]:
u32(32768#0x00008000) 
 u8(0#0x00)
s16(-32768#0x8000) 
s32(32768#0x00008000) 
 s8(0#0x00)
u16[65535#0xffff]:
u32(65535#0x0000ffff) 
 u8(255#0xff)
s16(-1#0xffff) 
s32(65535#0x0000ffff) 
 s8(-1#0xff)