Is there a standard sign function (signum, sgn) in C/C++?

The type-safe C++ version:

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

Benefits:

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.

Caveats:

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.)

I don't know of a standard function for it. Here's an interesting way to write it though:

(x > 0) - (x < 0)

Here's a more readable way to do it:

if (x > 0) return 1;
if (x < 0) return -1;
return 0;

If you like the ternary operator you can do this:

(x > 0) ? 1 : ((x < 0) ? -1 : 0)

There is a C99 math library function called copysign(), which takes the sign from one argument and the absolute value from the other:

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

will give you a result of +/- 1.0, depending on the sign of value. Note that floating point zeroes are signed: (+0) will yield +1, and (-0) will yield -1.

It seems that most of the answers missed the original question.

Is there a standard sign function (signum, sgn) in C/C++?

Not in the standard library, however there is copysign which can be used almost the same way via copysign(1.0, arg) and there is a true sign function in boost, which might as well be part of the standard.

    #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);

http://www.boost.org/doc/libs/1_47_0/libs/math/doc/sf_and_dist/html/math_toolkit/utils/sign_functions.html

Apparently, the answer to the original poster's question is no. There is no standard C++ sgn function.

Is there a standard sign function (signum, sgn) in C/C++?

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