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What is the capture list in a lambda? Where are lambdas most commonly used? Can a lambda be recursive? What is the difference between capturing by value and by reference? How do you specify a lambda's return type? What is std::function and how does it relate to lambdas? What is an immediately invoked lambda expression (IILE) and why use it? What is a generic (template) lambda? How are lambdas implemented under the hood? What is capturing by move (init capture) and when is it needed?
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What is a mutable lambda?

Intermediate

Answer

By default, a lambda's operator() is const, so by-value captured variables can't be modified inside the body. Marking the lambda mutable removes the const, allowing modification of the by-value copies (the changes persist across calls to that lambda instance but don't affect the originals).
int n = 0;
auto counter = [n]() mutable { return ++n; };
counter(); counter();  // returns 1, then 2
Real-world example A mutable lambda maintains an internal counter across calls without an external variable.

Common follow-ups: Why is operator() const by default? | Do mutable changes affect the original?

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All 12 Lambda Expressions questions