std::bsearch(3)

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, /* c-compare-pred */* comp
   ); (1)
   void* bsearch( const void* key, const void* ptr, std::size_t
   count,
                  std::size_t size, /* compare-pred */* comp );
   extern "C" using /* c-compare-pred */ = int(const void*,
   const void*); (2) (exposition only*)
   extern "C++" using /* compare-pred */ = int(const void*,
   const void*);
   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 <array>
 #include <cstdlib>
 #include <iostream>
 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;
         if (p)
             std::cout << " found at position " << (p - arr.data()) << '\n'
         else
             std::cout << " not found\n";
     }
 }

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

See also
   qsort sorts a range of elements with unspecified type
               (function)
   equal_range returns range of elements matching a specific key
               (function template)
   C documentation for
   bsearch