以下は、その条件を満たすC++関数である：
    
    
```c++
#include <map>
#include <utility>

// Define a function that takes a map as an input and returns the largest pair in the map.
std::pair<int, int> largest_pair(const std::map<int, int>& input_map) {
    // Define a variable to keep track of the current largest pair found.
    std::pair<int, int> largest{0, 0};
    // Loop over each key-value pair in the input map.
    for (const auto& [key, value] : input_map) {
        // If the current value is larger than the largest value found so far, update the largest pair.
        if (value > largest.second) {
            largest = std::make_pair(key, value);
        }
    }
    // Return the largest pair found.
    return largest;
}
```

**注意：*** 入力マップが空でないことを前提としています。入力マップが空である可能性がある場合、適切な値を返すために関数の最初に追加のチェックを追加する必要があります。
Here's the C++ function that meets the requirements:
    
    
```c++
#include <map>
#include <utility>

// Define a function that takes a map as an input and returns the largest pair in the map.
std::pair<int, int> largest_pair(const std::map<int, int>& input_map) {
    // Define a variable to keep track of the current largest pair found.
    std::pair<int, int> largest{0, 0};
    // Loop over each key-value pair in the input map.
    for (const auto& [key, value] : input_map) {
        // If the current value is larger than the largest value found so far, update the largest pair.
        if (value > largest.second) {
            largest = std::make_pair(key, value);
        }
    }
    // Return the largest pair found.
    return largest;
}
```

**Note:** We assume that the input map is not empty. If the input map can be empty, we need to add an additional check at the beginning of the function to return an appropriate value.