强制转换为 bool 是检查是否存在与键匹配的 unordered_map 值的有效方法吗？ C++

Question

I have a nested unordered_map and would like to check whether a value has been created for a set of keys. Most of the solutions online like this suggest iterating through the keys to check for their existence. Is there an inherent problem with trying to access them and cast them to bool as I've done below? The output from the test leads me to believe it works.

    std::unordered_map<int, std::unordered_map<int, const char*>> myMap;
    myMap[1][2]="abc";
    myMap[2][2]="false";
    std::cout <<(bool)myMap[1][2]<< std::endl;
    std::cout <<(bool)myMap[0][0]<< std::endl;
    std::cout <<(bool)myMap[0][0]<< std::endl;
    std::cout <<(bool)myMap[1][4]<< std::endl;
    std::cout <<(bool)myMap[2][2]<< std::endl;

Output

1
0
0
0
1

Answer 1

Invoking operator[] on a non-const std::(unordered_)map will create the requested element with a default value if it does not already exist, before then returning a reference to that element. You are using that exact functionality when populating the [1][2] and [2][2] elements.

Your output contains 0's because you are requesting keys that do not exist, so the map s are inserting those elements with default values, thus you end up with a null char* pointer that you are type-casting to bool . Any non-null value is output as true/1, and any null value is output as false/0.

To simply check if an element exists, you can use the map's find() or count() method, or in C++20 the contains() method, eg:

bool keysExist(
    const std::unordered_map<int, std::unordered_map<int, const char*>> &m,
    int index1, int index2)
{
    auto iter = m.find(index1);
    return (iter != m.end()) && (iter->find(index2) != iter->end());
    // or: return (iter != m.end()) && (iter->count(index) == 1);
    // or: return (iter != m.end()) && iter->contains(index2);
}

std::unordered_map<int, std::unordered_map<int, const char*>> myMap;
myMap[1][2] = "abc";
myMap[2][2] = "false";

std::cout << keysExist(myMap, 1, 2) << std::endl;
std::cout << keysExist(myMap, 0, 0) << std::endl;
std::cout << keysExist(myMap, 0, 0) << std::endl;
std::cout << keysExist(myMap, 1, 4) << std::endl;
std::cout << keysExist(myMap, 2, 2) << std::endl;