將浮點數轉換為 4 uint8_t

Question

我有一個float變量，需要通過 CAN 協議發送。 為此，必須將這個 32 位浮點數切成4 個uint8_t變量。

我完全不知道該怎么做。 我首先考慮將浮點數轉換為 int 但我在互聯網上找到的一些答案，使用cast或union似乎不起作用。

這是我正在嘗試做的一個簡單示例：

float f;
uint8_t ut1,ut2,ut3,ut4;

//8 first bits of f into ut1
//8 second bits of f in ut2
...

// Then I can send the uint8_t through CAN
...

Answer 1

您通常通過將浮點數轉換為 uint8_t 數組來完成此操作。

在 C 中，你可以這樣做：

uint8_t *array;
array = (unit8_t*)(&f);

在 C++ 中使用 reinterpret_cast

uint8_t *array;
array = reinterpret_cast<uint8_t*>(&f);

然后 array[0], ..., array[3] 是你的字節。

Answer 2

首先，您應該注意標准對float沒有特定的大小限制。 在某些可以想象的架構上， float可能不適合四個字節（盡管我不知道）。 在嘗試任何事情之前，您至少應該 (static_) 斷言它適合。

然后我認為最簡單的方法是斷言CHAR_BIT是8 ，並使用reinterpret_cast對unsigned char*的合法別名：

static_assert(sizeof(float) == 4);
float f = 0; // whatever value
unsigned char* float_as_char = reinterpret_cast<unsigned char*>(&f);

不過，這完全忽略了字節序問題，所以也許您真正想要的是制作字節的副本，以便您可以修復它：

static_assert(sizeof(float) == 4);
float f = 0; // whatever value
uint8_t bytes[4];
std::memcpy(bytes, &f);
// Fix up the order of the bytes in "bytes" now.

Answer 3

這是一個聯合方法，它為整數部分而不是一個數組提供單獨的名稱：

union {
    float f;
    struct {
        uint8_t ut1, ut2, ut3, ut4;
    } bytes;
} value;
value.f = 1.f;
uint8_t first = value.bytes.ut1;

我最初擔心的是，根據標准，這種union使用並不嚴格合法： C ++ Undefined對工會的行為，但ComicSansMS在對rashmatash的答案的評論中的論點是引人注目的。

Answer 4

你可以做這個非法操作：

float f = someFloatValue;
uint8_t* i = reinterpret_cast<uint8_t*>(&f);

盡管這在大多數情況下都有效，但 C++ 標准不支持它，並且編譯器可能會生成具有未定義行為的代碼。

另一種解決方案是使用聯合：

union{
    float f;
    uint8_t i[4];
}
f = someFloatValue;
// now i's contain the bit pattern of f

尚不清楚是否所有編譯器都會產生一致的結果，但它似乎比第一種方法更安全。

您還可以將f的值打包為 32 位整數。 然而，這可能會導致精度下降，但取決於您想要保持f准確程度，這將是最佳解決方案。

Answer 5

這是基於本文的基於表格的實現

它將 32 位浮點數轉換為 16 位浮點數，並將其存儲在一個無符號的 16 位值中。 這兩個 LUT 位於靜態內存中，因此可以創建這些數組，並且它可以快速處理所有極端情況。（有關詳細信息，請參閱論文）

using flt32 = float;
class flt16 {
public: // clucnky but functional and fast
flt16(flt32 in) { 
    ftoi_t fi{ in };
    half = basetable[(fi.i >> 23) & 0x1ff] + ((fi.i & 0x007fffff) >> shifttable[(fi.i >> 23) & 0x1ff]);
}

union ftoi_t{
    flt32 f;
    uint32 i;
};

uint16 half;

static constexpr uint16 basetable[512] = {
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 3072, 4096, 5120, 6144, 7168, 8192,
    9216, 10240, 11264, 12288, 13312, 14336, 15360, 16384, 17408, 18432, 19456, 20480, 21504, 22528,
    23552, 24576, 25600, 26624, 27648, 28672, 29696, 30720, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32769, 32770, 32772, 32776, 32784, 32800, 32832, 32896, 33024, 33280, 33792, 34816, 35840, 36864,
    37888, 38912, 39936, 40960, 41984, 43008, 44032, 45056, 46080, 47104, 48128, 49152, 50176, 51200,
    52224, 53248, 54272, 55296, 56320, 57344, 58368, 59392, 60416, 61440, 62464, 63488, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512
};

static constexpr uint8 shifttable[512] = {
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 
    14, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 
    13, 13, 13, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 13, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 23, 22, 21, 20, 19, 
    18, 17, 16, 15, 14, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 
    13, 13, 13, 13, 13, 13, 13, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 13
};

};