What's the Best Way to Get the Index of a Vector Element: Iterator Subtraction or `std::distance`?

Vectors and Container Iterator Arithmetic

When traversing a vector, obtaining the current index of the pointed-at element is a common necessity. This article explores the advantages and drawbacks of two common approaches: subtracting iterators and using the std::distance function.

Method 1: Subtracting Iterators

Subtracting iterators calculates the distance between two iterator positions. For vectors, its syntax is it - vec.begin(). However, this method has a potential issue: if the container later changes its underlying data structure (e.g., from a vector to a list), the subtraction operation may fail. This could result in undefined behavior or compilation errors.

Method 2: Using std::distance

std::distance takes two iterators as arguments and computes the distance between them. For vectors, its syntax is std::distance(vec.begin(), it). Compared to iterator subtraction, this method has the advantage of being agnostic to the container's underlying structure. As a result, it can handle different container implementations without modification.

Considerations

While std::distance provides enhanced flexibility, some programmers prefer the simplicity and familiarity of iterator subtraction. Ultimately, the choice between these two methods depends on the specific circumstances of your code.

If you anticipate changing the container's underlying data structure during iteration, std::distance is the preferred option. However, if you ensure that the container's structure remains constant and favor maximum performance, iterator subtraction may be sufficient.

Additional Options

If you don't traverse the container randomly, maintaining a separate loop counter can be an efficient alternative to either method. For example:

for (int i = 0; i < vec.size(); i++) {
  // Access element at index i without using iterators
  vec[i];
}

Remember, it is a common name for a container iterator, adhering to the naming convention std::container_type::iterator it;.

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