Lambda captures are the mechanism by which lambdas access variables from their enclosing scope. Capture by value [x] copies the variable; capture by reference [&x] refers to the original. Default capture [=] or [&] captures all variables in value or reference form; [=, &x] captures most by value but x by reference.

Be careful with lambda captures: prefer explicit captures; avoid capturing by reference if the lambda outlives the variables' scope.

This example shows lambda capture by value and reference with different scopes.

// compile: g++ -std=c++17 -o lambda lambda.cpp
// run: ./lambda
// description: lambda capture mechanics and lifetime rules
 
#include <iostream>
#include <vector>
#include <functional>
 
int main() {
    int x = 42;
 
    auto by_value = [x]() { std::cout << "Value: " << x << "\n"; };
    auto by_ref = [&x]() { std::cout << "Ref: " << x << "\n"; };
 
    x = 100;
 
    by_value();  // prints 42 (captured value)
    by_ref();    // prints 100 (refers to x)
 
    // Safe: capture by value if lambda outlives scope
    std::vector<std::function<void()>> funcs;
    {
        int y = 10;
        funcs.push_back([y]() { std::cout << "Captured: " << y << "\n"; });
        // funcs.push_back([&y]() {...});  // UNSAFE: y destroyed
    }
 
    funcs[0]();  // safe: y was captured by value
 
    return 0;
}