Const Map Access: Why Use `at()` Instead of `operator[]`?

Accessing Elements in a Const Map: Operator[] vs. at()

When working with constant maps, accessing elements using operator[] may fail. However, using at() is a viable alternative. Why is this the case?

In a non-const map, operator[] performs two functions:

• If the key exists, it returns a reference to the associated value.
• If the key doesn't exist, it constructs a default-constructed value associated with the key and returns a reference to it.

However, in a const map, operator[] is not allowed to modify the underlying data structure. Hence, it can only perform the first function and throws an error when trying to insert a new element using the second function.

In contrast, at() is a method introduced in C 11 specifically for accessing elements in a const map. It provides several benefits:

• It throws a std::out_of_range exception if the key doesn't exist, making it consistent with other containers like vector and deque.
• It has a const overload, allowing its use on const maps.

For these reasons, at() is the recommended method for accessing elements in a const std::map. The example code:

#include <iostream>
#include <map>

int main()
{
    std::map<int, char> A;
    A[1] = 'b';
    A[3] = 'c';

    const std::map<int, char>& B = A;

    std::cout << B.at(3) << std::endl; // it works
    std::cout << B[3] << std::endl;    // it does not work
}

Will output:

c
error: can't access elements with operator[] in a const std::map

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