Defined in header <cmath> | ||
---|---|---|
#define MATH_ERRNO 1 | (since C++11) | |
#define MATH_ERREXCEPT 2 | (since C++11) | |
#define math_errhandling /*implementation defined*/ | (since C++11) |
The macro constant math_errhandling
expands to an expression of type int
that is either equal to MATH_ERRNO
, or equal to MATH_ERREXCEPT
, or equal to their bitwise OR (MATH_ERRNO | MATH_ERREXCEPT
).
The value of math_errhandling
indicates the type of error handling that is performed by the floating-point operators and functions:
Constant | Explanation |
---|---|
MATH_ERREXCEPT | indicates that floating-point exceptions are used: at least FE_DIVBYZERO , FE_INVALID , and FE_OVERFLOW are defined in <cfenv> . |
MATH_ERRNO | indicates that floating-point operations use the variable errno to report errors. |
If the implementation supports IEEE floating-point arithmetic (IEC 60559), math_errhandling & MATH_ERREXCEPT
is required to be non-zero.
The following floating-point error conditions are recognized:
Condition | Explanation | errno | floating-point exception | Example |
---|---|---|---|---|
Domain error | the argument is outside the range in which the operation is mathematically defined (the description of each function lists the required domain errors) |
EDOM |
FE_INVALID |
std::acos(2) |
Pole error | the mathematical result of the function is exactly infinite or undefined |
ERANGE |
FE_DIVBYZERO |
std::log(0.0) , 1.0/0.0 |
Range error due to overflow | the mathematical result is finite, but becomes infinite after rounding, or becomes the largest representable finite value after rounding down |
ERANGE |
FE_OVERFLOW |
std::pow(DBL_MAX,2) |
Range error due to underflow | the result is non-zero, but becomes zero after rounding, or becomes subnormal with a loss of precision |
ERANGE or unchanged (implementation-defined) |
FE_UNDERFLOW or nothing (implementation-defined) |
DBL_MIN/2 |
Inexact result | the result has to be rounded to fit in the destination type | unchanged |
FE_INEXACT or nothing (unspecified) |
std::sqrt(2) , 1.0/10.0 |
Whether FE_INEXACT
is raised by the mathematical library functions is unspecified in general, but may be explicitly specified in the description of the function (e.g. std::rint
vs std::nearbyint
).
Before C++11, floating-point exceptions were not specified, EDOM
was required for any domain error, ERANGE
was required for overflows and implementation-defined for underflows.
#include <iostream> #include <cfenv> #include <cmath> #include <cerrno> #include <cstring> #pragma STDC FENV_ACCESS ON int main() { std::cout << "MATH_ERRNO is " << (math_errhandling & MATH_ERRNO ? "set" : "not set") << '\n' << "MATH_ERREXCEPT is " << (math_errhandling & MATH_ERREXCEPT ? "set" : "not set") << '\n'; std::feclearexcept(FE_ALL_EXCEPT); errno = 0; std::cout << "log(0) = " << std::log(0) << '\n'; if(errno == ERANGE) std::cout << "errno = ERANGE (" << std::strerror(errno) << ")\n"; if(std::fetestexcept(FE_DIVBYZERO)) std::cout << "FE_DIVBYZERO (pole error) reported\n"; }
Possible output:
MATH_ERRNO is set MATH_ERREXCEPT is set log(0) = -inf errno = ERANGE (Numerical result out of range) FE_DIVBYZERO (pole error) reported
(C++11) | floating-point exceptions (macro constant) |
macro which expands to POSIX-compatible thread-local error number variable (macro variable) |
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