NAME¶

std::numeric_limits::is_modulo - std::numeric_limits::is_modulo

Synopsis¶

static const bool is_modulo; (until C++11)
static constexpr bool is_modulo; (since C++11)

The value of std::numeric_limits<T>::is_modulo is true for all arithmetic types T
that handle overflows with modulo arithmetic, that is, if the result of addition,
subtraction, multiplication, or division of this type would fall outside the range
[min(), max()], the value returned by such operation differs from the expected value
by a multiple of max() - min() + 1.

is_modulo is false for signed integer types, unless the implementation defines
signed integer overflow to wrap.

Standard specializations¶

T value of std::numeric_limits<T>::is_modulo
/* non-specialized */ false
bool false
char implementation-defined
signed char implementation-defined
unsigned char true
wchar_t implementation-defined
char8_t (since C++20) true
char16_t (since C++11) true
char32_t (since C++11) true
short implementation-defined
unsigned short true
int implementation-defined
unsigned int true
long implementation-defined
unsigned long true
long long (C++11) implementation-defined
unsigned long long (C++11) true
float false
double false
long double false

Notes¶

The standard said "On most machines, this is true for signed integers." before the
resolution of LWG issue 2422. See GCC PR 22200 for a related discussion.

Example¶

Demonstrates the behavior of modulo types:

// Run this code

#include <iostream>
#include <type_traits>
#include <limits>

template<class T>
typename std::enable_if<std::numeric_limits<T>::is_modulo>::type
check_overflow()
{
std::cout << "max value is " << std::numeric_limits<T>::max() << '\n'
<< "min value is " << std::numeric_limits<T>::min() << '\n'
<< "max value + 1 is " << std::numeric_limits<T>::max()+1 << '\n';
}

int main()
{
check_overflow<int>();
std::cout << '\n';
check_overflow<unsigned long>();
// check_overflow<float>(); // compile-time error, not a modulo type
}

Possible output:¶

max value is 2147483647
min value is -2147483648
max value + 1 is -2147483648

max value is 18446744073709551615
min value is 0
max value + 1 is 0

Defect reports

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

DR Applied to Behavior as published Correct behavior
the definition of "handle
LWG 612 C++98 overflows provided a
with modulo arithmetic" was better definition
poor^[1]
is_modulo was required to be required to be false for signed
LWG 2422 C++98 true for integer types
signed integer types on most unless signed integer overflow is
machines defined to wrap

1. ↑ The definition is "adding two positive numbers can have a result that wraps
around to a third number that is less". It has the following problems:
* It does not define the wrapped value.
* It does not state whether result is repeatable.
* It does not require that doing addition, subtraction and other operations
on all values have defined behavior.

See also¶

is_integer identifies integer types
[static] (public static member constant)
is_iec559 identifies the IEC 559/IEEE 754 floating-point types
[static] (public static member constant)
is_exact identifies exact types
[static] (public static member constant)