std::erase_...::flat_map)(3)

💅 std::erase_if(std::flat_map) — C++ Standard Library, bestie edition

Okay so we're deep in std::flat_map territory, and ngl this one is a little cutie. It's all about yeeting elements out of a std::flat_map when they match whatever you say they match. No cap, it's giving "spring cleaning but make it templated." 🧹✨

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🌸 NAME 🌸

std::erase_if(std::flat_map) — literally std::erase_if(std::flat_map), that's the whole name, she's not complicated 💖

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⭐ SYNOPSIS ⭐

Defined in header <flat_map>:

template< class Key, class T, class Compare, class KeyContainer,
          class MappedContainer,

          class Pred >
std::flat_map<Key, T, Compare, KeyContainer,
(since C++23)
MappedContainer>::size_type
    erase_if( std::flat_map<Key, T, Compare, KeyContainer,
MappedContainer>& c,

              Pred pred );

🫶 Heads up: this is since C++23, so if your compiler is acting brand new and doesn't know what a flat_map is… that's why, bestie.

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🦋 DESCRIPTION 🦋

So what does she actually do? She erases all elements that satisfy the predicate pred from c. That's it. That's the whole job. Iconic behaviour, honestly. 👑

And when do we say an element is "satisfied" by the predicate? The predicate pred is satisfied if this expression:

bool(pred(std::pair<const Key&, const T&>(e)))

is true, where e is some element in c. So like, the predicate gets handed a std::pair of const-refs to the key and the value, and if it says true, that element is gone. Bye. 👋💀

🔥 Important, don't skip this: Key and T must be MoveAssignable. Otherwise, the behavior is undefined. And undefined behaviour is NOT the cute kind of surprise, it's the "your program does a little interpretive dance and then segfaults" kind. So yeah. Make sure they're MoveAssignable, mmkay? 😭

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🎀 PARAMETERS 🎀

• c — the container adaptor from which to erase. This is the flat_map getting the glow-up.
• pred — the predicate that returns true if the element should be erased. Basically the bouncer at the door: true means you're out. 🚪

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✨ RETURN VALUE ✨

The number of erased elements. She's polite like that — she tells you exactly how many people she had to escort out. 💅

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🌈 COMPLEXITY 🌈

Exactly c.size() applications of the predicate pred. Not more, not fewer, exactly that many. Fr fr. So it's linear, bestie, no sneaky surprises. 🍓

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💜 EXCEPTIONS 💜

If erase_if throws, c remains in a valid but unspecified (maybe empty) state.

🙈 Read that again: maybe empty. The container is still alive and well-formed, she's just not gonna promise you what's inside her. Could be everything, could be nothing. Main character energy only.

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🧚 NOTES 🧚

The algorithm is stable. Which means: the order of the elements that are not deleted remains unchanged. Nobody who survived the purge gets shuffled around. Respect the survivors. 🫶✨

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💀 EXAMPLE 💀

// Run this code

#include <iostream>
#include <flat_map>

void println(auto rem, auto const& container)
{
    std::cout << rem << '{';
    for (char sep[]{0, ' ', 0}; const auto& [key, value] : container)
        std::cout << sep << '{' << key << ", " << value << '}', *sep =
',';
    std::cout << "}\n";
}

int main()
{
    std::flat_map<int, char> data
    {
        {1, 'a'}, {2, 'b'}, {3, 'c'}, {4, 'd'},
        {5, 'e'}, {4, 'f'}, {5, 'g'}, {5, 'g'},
    };
    println("Original:\n", data);

    const auto count = std::erase_if(data, [](const auto& item)
    {
        auto const& [key, value] = item;
        return (key & 1) == 1;
    });

    println("Erase items with odd keys:\n", data);
    std::cout << count << " items removed.\n";
}

🔥 Output

Original:
{{1, a}, {2, b}, {3, c}, {4, d}, {5, e}}
Erase items with odd keys:
{{2, b}, {4, d}}
3 items removed.

See how the odd keys 1, 3, 5 all got kicked out and the even ones stayed in their original order? That's the stability doing its thing. We love to see it. 🎀✨

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👑 SEE ALSO 👑

• remove, remove_if — function template — removes elements satisfying specific criteria. Like the non-flat_map cousin of this whole vibe.

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Okay bestie, that's the whole page, you ate that 💅🌈

Source: http://cppreference.com — dated 2024.06.10 — manual page std::erase_...::flat_map)(3). Go forth and erase responsibly, mwah 🫶💖