NAME

std::midpoint - std::midpoint

Synopsis

Defined in header <numeric>
template< class T > (1) (since C++20)
constexpr T midpoint( T a, T b ) noexcept;
template< class T > (2) (since C++20)
constexpr T* midpoint( T* a, T* b );

Computes the midpoint of the integers, floating-points, or pointers a and b.

1) This overload participates in overload resolution only if T is an arithmetic type
other than bool.
2) This overload participates in overload resolution only if T is an object type.
Use of this overload is ill-formed if T is an incomplete type.

Parameters

a, b - integers, floating-points, or pointer values

Return value

1) Half the sum of a and b. No overflow occurs. If a and b have integer type and the
sum is odd, the result is rounded towards a. If a and b have floating-point type, at
most one inexact operation occurs.
2) If a and b point to, respectively, x[i] and x[j] of the same array object x (for
the purpose of pointer arithmetic), returns a pointer to x[i+(j-i)/2] (or,
equivalently x[std::midpoint(i, j)]) where the division rounds towards zero. If a
and b do not point to elements of the same array object, the behavior is undefined.

Exceptions

Throws no exceptions.

Notes

Overload (2) can be simply implemented as return a + (b - a) / 2; on common
platforms. However, such implementation is not guaranteed to be portable, because
there may be some platforms where creating an array with number of elements greater
than PTRDIFF_MAX is possible, and b - a may result in undefined behavior even if
both b and a point to elements in the same array.

Feature-test macro: __cpp_lib_interpolate

Example

// Run this code

#include <cstdint>
#include <limits>
#include <numeric>
#include <iostream>

int main()
{
std::uint32_t a = std::numeric_limits<std::uint32_t>::max();
std::uint32_t b = std::numeric_limits<std::uint32_t>::max() - 2;

std::cout << "a: " << a << '\n'
<< "b: " << b << '\n'
<< "Incorrect (overflow and wrapping): " << (a + b) / 2 << '\n'
<< "Correct: " << std::midpoint(a, b) << "\n\n";

auto on_pointers = [](int i, int j) {
char const* text = "0123456789";
char const* p = text + i;
char const* q = text + j;
std::cout << "std::midpoint('" << *p << "', '" << *q << "'): '"
<< *std::midpoint(p, q) << "'\n";
};

on_pointers(2, 4);
on_pointers(2, 5);
on_pointers(5, 2);
on_pointers(2, 6);
}

Output:

a: 4294967295
b: 4294967293
Incorrect (overflow and wrapping): 2147483646
Correct: 4294967294

std::midpoint('2', '4'): '3'
std::midpoint('2', '5'): '3'
std::midpoint('5', '2'): '4'
std::midpoint('2', '6'): '4'

References

* C++20 standard (ISO/IEC 14882:2020):

* 25.10.15 Midpoint [numeric.ops.midpoint]