While operator [] for accessing elements in std::map or std::unordered_map is convenient and familiar to those accustomed to working with dictionaries in other languages, it comes with some potential pitfalls and debugging nightmares in C++. Beware the brackets '[ ]', as understanding their limitations is essential to avoid unexpected results in your C++ code.

Implicit Insertion

When you use operator[] to access a key that does not exist in the map, std::map will implicitly insert a new element with that key and a default-constructed value. This can lead to unintended side effects.

std::map<char, int> my_map;
std::cout << "Size of map: " << my_map.size() << std::endl; 

int value = my_map['a']; // 'a' is not in the map, so it gets added with value 0.    
std::cout << "Value for key 'a': " << value << std::endl;
std::cout << "Size of map: " << my_map.size() << std::endl; // Size is now 1

Issue: This implicit insertion can lead to bugs if you are only intending to check for the existence of a key or to retrieve a value without modifying the map and expect a exception if key is not present.

Even if you are performing read-only operations on the map, using operator[] can unintentionally modify the map by inserting new elements.

int main() {
    std::map<char, int> my_map;

    // Read-only check
    std::cout << "Size of map: " << my_map.size() << std::endl;
    if (my_map['a'] > 0) {
        std::cout << "'a' is present with a positive value" << std::endl;
    }
    std::cout << "Size of map: " << my_map.size() << std::endl; // Size is now 1, even though we intended to just check
    std::cout<<"Value at a: "<<my_map['a']<<std::endl;
}

Issue: The map is modified even when the intention was just to check the value.

Default Constructor is called

When a new key is added implicitly using operator[], the value is default-constructed. For primitive types like int or double, this means zero initialization. However, for user-defined types, the default constructor is called, which might not be the desired behavior.

struct MyStruct {
    MyStruct() {
        data =10;
        std::cout << "Default constructor called" << std::endl;
    }
    int data;
};
int main() {
    std::map<char, MyStruct> my_map;
    MyStruct value = my_map['a']; // Default constructor for MyStruct is called
    std::cout<<"Value of data: "<<value.data<<std::endl;
    std::cout << "Size of map: " << my_map.size() << std::endl; // Size is now 1
}

Issue: This default construction might have performance implications or unintended side effects if the default constructor does more than simple initialization.

Performance Overhead

In scenarios where operator[] is used for repeated lookups and the key does not exist, it results in multiple implicit insertions with default values, which can add unnecessary performance overhead.

int main() {
    std::map<char, int> my_map;
    std::cout << "Size of map: " << my_map.size() << std::endl;
    for (char c = 'a'; c <= 'z'; ++c) {
        int value = my_map[c]; // Each lookup inserts a default value if the key does not exist
    }

    std::cout << "Size of map: " << my_map.size() << std::endl; // Size is now 26
}

Issue: This can lead to a larger map with many unnecessary default-constructed values, impacting memory usage and performance. In this example you end up 26 elements in the map.

What you should use instead

To avoid the issues with operator[], you can use the following alternatives:

at Method

at method throws an exception if the key does not exist. You should use this when you expect the key to be present.

int main() {
    std::map<char, int> my_map;
    try {
        int value = my_map.at('a'); // Throws std::out_of_range if 'a' does not exist
    } catch (const std::out_of_range& e) {
        std::cout << "Key 'a' not found" << std::endl;
    }
}

find Method

find method returns an iterator to the element or end() if the key does not exist. you can use this as well for existence checks without modifying the map.

int main() {
    std::map<char, int> my_map;
    auto it = my_map.find('a');
    if (it != my_map.end()) {
        int value = it->second;
        std::cout << "Value for key 'a': " << value << std::endl;
    } else {
        std::cout << "Key 'a' not found" << std::endl;
    }
}

Conclusion

The operator[] in std::map or in std::unordered_map offers easy element access, but it may cause unexpected results. For read-only operations or when checking for the existence of a key, it's better to use methods like at, find, or count to avoid potential issues. Understanding these nuances will help you use std::map more effectively and prevent unintended side effects in your C++ code.