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.
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