CODEWITHSUNDEEP
c++iteratoriteration
mrchance
mrchance

Reputation: 1175

C++ idiomatic way of iterating over a container that itself is being modified

I find myself iterating over a container X that is being modified inside the loop. But in each iteration, the unmodified X is needed:

for (int x : X) { // wrong, container X in iteration should be the "original"
    modify_X();  // X modified here
}

A solution is to iterate over a copy

containerT X_copy(X);
for (int x : X_copy) {
    modify_X();   
}

or

for (int x : containerT {X}) {
    modify_X();   
}

Is there an idiomatic way of doing this?

Upvotes: 1

Views: 112

Answers (1)

cigien
cigien

Reputation: 60440

The last example in your question looks the simplest. However, from C++20, you could also do this:

for (auto copy = X; int i : copy)
{
  modify_X();
}

Here's a demo.

Note that in your first snippet, it's not only logically wrong, but it also invokes undefined behavior, as you are modifying a range that you're iterating over.

Upvotes: 3

Related Questions