NAME

std::bsearch - std::bsearch

Synopsis

Defined in header <cstdlib>
void* bsearch( const void* key, const void* ptr, std::size_t count,

std::size_t size, /*compare-pred*/* comp ); (1)
void* bsearch( const void* key, const void* ptr, std::size_t count,

std::size_t size, /*c-compare-pred*/* comp );
extern "C++" using /*compare-pred*/ = int(const void*, const void*); //
exposition-only (2)
extern "C" using /*c-compare-pred*/ = int(const void*, const void*); //
exposition-only

Finds an element equal to element pointed to by key in an array pointed to by ptr.
The array contains count elements of size bytes each and must be partitioned with
respect to the object pointed to by key, that is, all the elements that compare less
than must appear before all the elements that compare equal to, and those must
appear before all the elements that compare greater than the key object. A fully
sorted array satisfies these requirements. The elements are compared using function
pointed to by comp.

The behavior is undefined if the array is not already partitioned in ascending order
with respect to key, according to the same criterion that comp uses.

If the array contains several elements that comp would indicate as equal to the
element searched for, then it is unspecified which element the function will return
as the result.

Parameters

key - pointer to the element to search for
ptr - pointer to the array to examine
count - number of element in the array
size - size of each element in the array in bytes
comparison function which returns a negative integer value if the first
argument is less than the second, a positive integer value if the first
argument is greater than the second and zero if the arguments are
equivalent. key is passed as the first argument, an element from the array
as the second.

The signature of the comparison function should be equivalent to the
comp - following:

int cmp(const void *a, const void *b);

The function must not modify the objects passed to it and must return
consistent results when called for the same objects, regardless of their
positions in the array.

Return value

Pointer to the found element or null pointer if the element has not been found.

Notes

Despite the name, neither C nor POSIX standards require this function to be
implemented using binary search or make any complexity guarantees.

The two overloads provided by the C++ standard library are distinct because the
types of the parameter comp are distinct (language linkage is part of its type).

Example

// Run this code

#include <iostream>
#include <cstdlib>
#include <array>

template <typename T>
int compare(const void *a, const void *b) {
const auto &arg1 = *(static_cast<const T*>(a));
const auto &arg2 = *(static_cast<const T*>(b));
const auto cmp = arg1 <=> arg2;
return cmp < 0 ? -1
: cmp > 0 ? +1
: 0;
}

int main() {
std::array arr { 1, 2, 3, 4, 5, 6, 7, 8 };

for (const int key : { 4, 8, 9 }) {

const int* p = static_cast<int*>(
std::bsearch(&key,
arr.data(),
arr.size(),
sizeof(decltype(arr)::value_type),
compare<int>));

std::cout << "value " << key;
(p) ? std::cout << " found at position " << (p - arr.data()) << '\n'
: std::cout << " not found\n";
}
}

Output:

value 4 found at position 3
value 8 found at position 7
value 9 not found