C ++中双/浮动类型的二进制序列化的可移植性 [英] Portability of binary serialization of double/float type in C++

问题描述

C ++标准没有讨论float和double类型的底层布局，只讨论它们应该代表的值的范围。 （对于签名的类型也是如此，这是两个赞美或别的东西）

The C++ standard does not discuss the underlying layout of float and double types, only the range of values they should represent. (This is also true for signed types, is it two's compliment or something else)

我的问题是：用于串行化/反序列化POD类型的技术是什么并以便携式方式浮动？目前看来，唯一的办法就是将值直接表示（如123.456），所有体系结构中的ieee754布局不是标准的。

My question is: What the are techniques used to serialize/deserialize POD types such as double and float in a portable manner? At the moment it seems the only way to do this is to have the value represented literally(as in "123.456"), The ieee754 layout for double is not standard on all architectures.

推荐答案

BrianBeej JorgensenHall在他的网络编程指南要打包的代码 float （或者$ code> double ）到 uint32_t （分别为 uint64_t ），以便能够通过网络在两台可能不会都同意他们的代表。它有一些限制，主要是不支持NaN和无穷大。

Brian "Beej Jorgensen" Hall gives in his Guide to Network Programming some code to pack float (resp. double) to uint32_t (resp. uint64_t) to be able to safely transmit it over the network between two machine that may not both agree to their representation. It has some limitation, mainly it does not support NaN and infinity.

这是他的包装功能：

#define pack754_32(f) (pack754((f), 32, 8))
#define pack754_64(f) (pack754((f), 64, 11))

uint64_t pack754(long double f, unsigned bits, unsigned expbits)
{
    long double fnorm;
    int shift;
    long long sign, exp, significand;
    unsigned significandbits = bits - expbits - 1; // -1 for sign bit

    if (f == 0.0) return 0; // get this special case out of the way

    // check sign and begin normalization
    if (f < 0) { sign = 1; fnorm = -f; }
    else { sign = 0; fnorm = f; }

    // get the normalized form of f and track the exponent
    shift = 0;
    while(fnorm >= 2.0) { fnorm /= 2.0; shift++; }
    while(fnorm < 1.0) { fnorm *= 2.0; shift--; }
    fnorm = fnorm - 1.0;

    // calculate the binary form (non-float) of the significand data
    significand = fnorm * ((1LL<<significandbits) + 0.5f);

    // get the biased exponent
    exp = shift + ((1<<(expbits-1)) - 1); // shift + bias

    // return the final answer
    return (sign<<(bits-1)) | (exp<<(bits-expbits-1)) | significand;
}

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