这是一个分支友好的实现:

inline int signum(const double x) {
    if(x == 0) return 0;
    return (1 - (static_cast<int>((*reinterpret_cast<const uint64_t*>(&x)) >> 63) << 1));
}

除非你的数据有一半是0，否则分支预测器会选择一个最常见的分支。两个分支都只涉及简单的操作。

另外，在一些编译器和CPU架构上，完全无分支的版本可能更快:

inline int signum(const double x) {
    return (x != 0) * 
        (1 - (static_cast<int>((*reinterpret_cast<const uint64_t*>(&x)) >> 63) << 1));
}

这适用于IEEE 754双精度二进制浮点格式:binary64。

类型安全的c++版本:

template <typename T> int sgn(T val) {
    return (T(0) < val) - (val < T(0));
}

好处:

Actually implements signum (-1, 0, or 1). Implementations here using copysign only return -1 or 1, which is not signum. Also, some implementations here are returning a float (or T) rather than an int, which seems wasteful. Works for ints, floats, doubles, unsigned shorts, or any custom types constructible from integer 0 and orderable. Fast! copysign is slow, especially if you need to promote and then narrow again. This is branchless and optimizes excellently Standards-compliant! The bitshift hack is neat, but only works for some bit representations, and doesn't work when you have an unsigned type. It could be provided as a manual specialization when appropriate. Accurate! Simple comparisons with zero can maintain the machine's internal high-precision representation (e.g. 80 bit on x87), and avoid a premature round to zero.

警告:

It's a template so it might take longer to compile in some circumstances. Apparently some people think use of a new, somewhat esoteric, and very slow standard library function that doesn't even really implement signum is more understandable. The < 0 part of the check triggers GCC's -Wtype-limits warning when instantiated for an unsigned type. You can avoid this by using some overloads: template <typename T> inline constexpr int signum(T x, std::false_type is_signed) { return T(0) < x; } template <typename T> inline constexpr int signum(T x, std::true_type is_signed) { return (T(0) < x) - (x < T(0)); } template <typename T> inline constexpr int signum(T x) { return signum(x, std::is_signed<T>()); } (Which is a good example of the first caveat.)

下面重载的接受答案确实不会触发-Wtype-limits。但它确实会触发未使用的参数警告(针对is_signed变量)。为了避免这些，第二个参数不应该这样命名:

template <typename T> inline constexpr
  int signum(T x, std::false_type) {
  return T(0) < x;
}

template <typename T> inline constexpr
  int signum(T x, std::true_type) {
  return (T(0) < x) - (x < T(0));
}

template <typename T> inline constexpr
  int signum(T x) {
  return signum(x, std::is_signed<T>());
}

对于c++ 11或更高版本，可以选择。

template <typename T>
typename std::enable_if<std::is_unsigned<T>::value, int>::type
inline constexpr signum(T const x) {
    return T(0) < x;  
}

template <typename T>
typename std::enable_if<std::is_signed<T>::value, int>::type
inline constexpr signum(T const x) {
    return (T(0) < x) - (x < T(0));  
}

对我来说，它不会触发GCC 5.3.1上的任何警告。

为什么使用三元操作符和if-else，当你可以简单地这样做

#define sgn(x) x==0 ? 0 : x/abs(x)

显然，最初的帖子的问题的答案是否定的。没有标准的c++ sgn函数。

有一个C99数学库函数叫copysign()，它从一个参数取符号，从另一个参数取绝对值:

result = copysign(1.0, value) // double
result = copysignf(1.0, value) // float
result = copysignl(1.0, value) // long double

会给你一个+/- 1.0的结果，这取决于值的符号。注意，浮点零是有符号的:(+0)将产生+1，(-0)将产生-1。