Lambda Functions and Captures

intermediate30 min readLesson 100 of 204

The four-part syntax, value vs reference captures, lifetime dangers, and mutable state.

A lambda is an unnamed function object written where it is used. The syntax has four parts: captures, parameters, return type (usually deduced), body.

auto square = [](int x) { return x * x; };          // no capture
int factor = 3;
auto scale = [factor](int x) { return x * factor; }; // capture by value
auto push  = [&out](int x) { out.push_back(x); };    // capture by reference
auto all   = [=](int x) { return x * factor; };      // (C++20: explicit)

Capture semantics are the interview question and the bug source:

[factor] copies at lambda-creation time — later changes to factor do not affect the lambda.
[&factor] stores a reference — the lambda must not outlive factor. Returning a by-reference-capturing lambda from a function hands back a dangling reference.
[this] captures the enclosing object's pointer; [this, factor] is the common correct combination.
Mutable state needs mutable: [n]() mutable { return ++n; }.

Default capture-by-value ([=]) is deprecated in C++20 for this scenarios; prefer listing what you capture — the list documents the lambda's dependencies.

Now practice

Lambda practiceFilter and rank students with copy_if plus a comparator.1 challenge · · ~25 min