在具有 int 类型键的 std::multimap 中查找键值对

Question

I have a std::multimap<int, X*> where X is a user-defined type. I want to find a specific key-value-pair within this multimap (ie an iterator pointing to this pair).

(A) Complete Example:

#include <map>
#include <algorithm>

class X {};

int main()
{
    X myX;
    std::multimap<int, X*> myMap;
    auto it = std::find(myMap.begin(), myMap.end(), std::make_pair(5, &myX));
    
    return 0;
}

However, this does not compile (gcc 12.2.1 with -std=gnu++2a ):

no match for ‘operator==’ (operand types are ‘std::pair<const int, X*>’ and ‘const std::pair<int, X*>’)

So it seems to me somehow int gets converted to const int .

(B) Using std::find_if with a lamdba function with const int , the code compiles:

auto it = std::find_if(myMap.begin(), myMap.end(), [myX](std::pair<const int, X*>& node){ return 5 == node.first && (&myX) == node.second; } );

Questions:

1. Why is the type of the keys in the multimap const int and not int ?
2. How to fix it in a more natural way than using a (complex) lambda function like in (B) or by first looking up by key and then searching within its values (as described by Igor Tandetnik in the comments)?

Answer 1

Why is the type of the keys in the multimap const int and not int?

Because the Key in all standard maps is immutable.

How to fix it in a more natural way than using a (complex) lambda function like in (B) or by first looking up by key and then searching within its values?

Here's a simplified example that doesn't use X* but X in the map:

#include <algorithm>
#include <iostream>
#include <map>

struct X {
    bool operator==(const X& rhs) const { return m_value == rhs.m_value; }

    int m_value;
};

int main() {
    std::multimap<int, X> myMap{
        {4, {1}}, {5, {2}}, {6, {3}}, {4, {4}}, {5, {5}}, {6, {6}}
    };

    // get the range of _Key_s equal to 5
    auto [first, last] = myMap.equal_range(5);

    // the lambda - here used to find the first entry with the _Value_ `X{2}`    
    auto cond = [](auto&& pair) { return pair.second == X{2}; };

    if (auto it = std::find_if(first, last, cond); it != last)
    {
        auto& [k, v] = *it;

        std::cout << k << ' ' << v.m_value << '\n';
    }
}

Demo

---

If many Values may be found, just put the find_if in a loop:

    for(;(first = std::find_if(first, last, cond)) != last; ++first) {
        auto& [k, v] = *first;

        std::cout << k << ' ' << v.m_value << '\n';
    }

Demo