有一种不用分支的方法，但不太好。

sign = -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));

http://graphics.stanford.edu/~seander/bithacks.html

那一页上还有很多其他有趣的、过于聪明的东西……

似乎大多数答案都忽略了最初的问题。

在C/ c++中是否存在标准符号函数(signum, sgn) ?

在标准库中没有，但是有一个copysign可以通过copysign(1.0, arg)以几乎相同的方式使用，在boost中有一个真正的符号函数，它也可能是标准的一部分。

    #include <boost/math/special_functions/sign.hpp>

    //Returns 1 if x > 0, -1 if x < 0, and 0 if x is zero.
    template <class T>
    inline int sign (const T& z);

int sign(float n)
{     
  union { float f; std::uint32_t i; } u { n };
  return 1 - ((u.i >> 31) << 1);
}

这个函数假设:

浮点数的二进制32表示 在使用命名联合时对严格的混叠规则做出例外的编译器

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.)

有一种不用分支的方法，但不太好。

sign = -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));

http://graphics.stanford.edu/~seander/bithacks.html

那一页上还有很多其他有趣的、过于聪明的东西……

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。