std::numeric_limits<T>::traps

 

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C numeric limits interface
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staticconstbool traps;		(until C++11)
staticconstexprbool traps;		(since C++11)

The value of std::numeric_limits<T>::traps is true for all arithmetic types T that have at least one value at the start of the program that, if used as an argument to an arithmetic operation, will generate a trap.

Standard specializations

T	value of std::numeric_limits<T>::traps
/* non-specialized */	false
bool	false
char	usually true
signedchar	usually true
unsignedchar	usually true
wchar_t	usually true
char8_t(since C++20)	usually true
char16_t(since C++11)	usually true
char32_t(since C++11)	usually true
short	usually true
unsignedshort	usually true
int	usually true
unsignedint	usually true
long	usually true
unsignedlong	usually true
longlong(since C++11)	usually true
unsignedlonglong(since C++11)	usually true
float	usually false
double	usually false
longdouble	usually false

Notes

On most platforms integer division by zero always traps, and std::numeric_limits<T>::traps is true for all integer types that support the value ​0​. The exception is the type bool: even though division by false traps due to integral promotion from bool to int, it is the zero-valued int that traps. Zero is not a value of type bool.

On most platforms, floating-point exceptions may be turned on and off at run time (e.g. feenableexcept() on Linux or _controlfp on Windows), in which case the value of std::numeric_limits<T>::traps for floating-point types reflects the state of floating-point trapping facility at the time of program startup, which is false on most modern systems. An exception would be a DEC Alpha program, where it is true if compiled without -ieee.

Example

Run this code

#include <iostream>#include <limits>   int main(){std::cout<<std::boolalpha<<"bool: traps = "<<std::numeric_limits<bool>::traps<<'\n'<<"char: traps = "<<std::numeric_limits<char>::traps<<'\n'<<"char16_t: traps = "<<std::numeric_limits<char16_t>::traps<<'\n'<<"long: traps = "<<std::numeric_limits<long>::traps<<'\n'<<"float: traps = "<<std::numeric_limits<float>::traps<<'\n';}

Possible output:

// GCC output: bool: traps = true char: traps = true char16_t: traps = true long: traps = true float: traps = false   // Clang output: bool: traps = false char: traps = true char16_t: traps = true long: traps = true float: traps = false

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 497	C++98	it was unclear what is returned if trapping is enabled or disabled at runtime	returns the enable status at the start of the program

See also

Floating-point environment
tinyness_before [static]	identifies floating-point types that detect tinyness before rounding (public static member constant)
has_denorm_loss [static]	identifies the floating-point types that detect loss of precision as denormalization loss rather than inexact result (public static member constant)