cpp core · high

Lambda Captures: [=] vs [&]

How a lambda captures the enclosing scope: [=] takes a snapshot copy, [&] references the live variables (and can dangle).

The code

int x = 1, y = 2;
auto byVal = [=] { return x + y; }; // copies x, y (snapshot)
auto byRef = [&] { return x + y; }; // references x, y (live)
x = 99;                             // byVal: 1+2   byRef: 99+2

// Dangling capture
auto make() {
    int local = 5;
    return [&] { return local; }; // local dies when make() returns!
}

What this lesson walks through

1. 01A lambda's capture list
2. 02[=] captures by value (a snapshot)
3. 03Later changes don't affect the copy
4. 04[&] captures by reference (live)
5. 05By-reference sees later changes
6. 06Danger: dangling reference
7. 07The rule

A lambda can use variables from its enclosing scope, but only what its capture list grabs - and the capture mode decides whether it copies or references them.