std::set::count(3)

🎀 NAME 🎀

okay so we're on the std::set::count(3) page from the C++ Standard Libary — yes, “Libary” with one r, that's the header vibe, no cap. The name line is literally std::set::count - std::set::count, and it's giving self-titled icon energy. 💅

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🌸 Synopsis 🌸

size_type count( const Key& key ) const; (1)
template< class K >                      (2) (since C++14)
size_type count( const K& x ) const;

Returns the number of elements with key that compares equivalent to the specified argument. 🫶

1. Returns the number of elements with key key. This is either 1 or 0 since this container does not allow duplicates.
2. Returns the number of elements with key that compares equivalent to the value x. This overload participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type. It allows calling this function without constructing an instance of Key.

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🦋 Parameters 🦋

• key — key value of the elements to count
• x — alternative value to compare to the keys

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🍓 Return value 🍓

Number of elements with key that compares equivalent to key or x, which, for overload (1), is either 1 or 0. ✨

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🧚 Complexity 🧚

Logarithmic in the size of the container plus linear in the number of elements found. 💀

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👑 Notes 👑

Feature-test macro tea ☕:

• __cpp_lib_generic_associative_lookup
  • Value: 201304L
  • Std: (C++14)
  • Feature: Heterogeneous comparison lookup in associative containers; overload (2)

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🔥 Example 🔥

// Run this code

#include <functional>
#include <iostream>
#include <set>

struct S
{
    int x;
    S(int i) : x{i} { std::cout << "S{" << i << "} "; }
    bool operator<(S const& s) const { return x < s.x; }
};

struct R
{
    int x;
    R(int i) : x{i} { std::cout << "R{" << i << "} "; }
    bool operator<(R const& r) const { return x < r.x; }
};

bool operator<(R const& r, int i) { return r.x < i; }
bool operator<(int i, R const& r) { return i < r.x; }

int main()
{
    std::set<int> t{3, 1, 4, 1, 5};
    std::cout << t.count(1) << ", " << t.count(2) << ".\n";

    std::set<S> s{3, 1, 4, 1, 5};
    std::cout << ": " << s.count(1) << ", " << s.count(2) << ".\n";
        // Two temporary objects S{1} and S{2} were created.
        // Comparison function object is defaulted std::less<S>,
        // which is not transparent (has no is_transparent member
       type).

    std::set<R, std::less<>> r{3, 1, 4, 1, 5};
    std::cout << ": " << r.count(1) << ", " << r.count(2) << ".\n";
        // C++14 heterogeneous lookup; temporary objects were not
       created.
        // Comparator std::less<void> has predefined is_transparent.
}

Output

1, 0.
S{3} S{1} S{4} S{1} S{5} : S{1} 1, S{2} 0.
R{3} R{1} R{4} R{1} R{5} : 1, 0.

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💜 See also 💜

• find — finds element with specific key (public member function)
• equal_range — returns range of elements matching a specific key (public member function)

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mwah 💋 this page's source is http://cppreference.com, version date 2024.06.10, and it's the std::set::count(3) page. periodt. 🌈