std::bsearch() algorithm

od C++98

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

// (2)
void* bsearch( const void* key, const void* ptr, std::size_t count,
               std::size_t size, c_compare_pred comp );

With the exposition-only types defined as follows:

extern "C++" using cpp_compare_pred = int(const void*, const void*);
extern "C" using c_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.

Undefined Behaviour

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	The number of elements in the array.
size	The size of each element of the array in bytes.
comp	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. Zero if the arguments are equivalent. The signature of the comparison function should be equivalent to the following: int cmp(const void *a, const void *b); The function must not modify the objects passed to it 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.

Complexity

Not specified.

Exceptions

(none)

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).

Examples

Main.cpp

#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;
        (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