Capturing Local Variables in C++ Lambdas

Lambdas in C++ can capture variables from the enclosing scope to use within the lambda body. There are two primary ways to capture:

1. Capture by value - Capture a copy of the variable. Denoted by [var] or [=] to capture all local variables by value.
2. Capture by reference - Capture a reference to the variable. Denoted by [&var] or [&] to capture all local variables by reference.

Here's an example demonstrating both:

1#include <iostream>
2
3int main() {
4  int a{1};
5  int b{2};
6
7  auto by_val = [a]() { return a; };
8  auto by_ref = [&b]() { b++; };
9
10  std::cout << by_val() << ' ' << a << '\n';
11  by_ref();
12  std::cout << b << '\n';
13}

11 1
23

Capture by value is used when you want the lambda to have its own copy of the data that won't be modified externally. Capture by reference is used when you want to modify external data or avoid copying.

Some additional notes:

• Capturing by reference can lead to dangling references if the referenced data goes out of scope, so extra care must be taken.
• You can mix captures, like [a, &b], capturing some variables by value and others by reference.
• Capturing this in a class method will give access to class members.
• Avoid capturing by reference if the lambda will outlive the current scope.