Is Capturing a Reference by Reference in a C 11 Lambda Safe?

Capturing a Reference by Reference in a C 11 Lambda

In C 11, we often use lambdas to capture variables from the surrounding context. But what happens when we capture a reference by reference? Is it safe to do so?

Problem Details

Consider the following code:

<code class="cpp">#include <functional>
#include <iostream>

std::function<void()> make_function(int&amp; x) {
    return [&amp;]{ std::cout << x << std::endl; };
}

int main() {
    int i = 3;
    auto f = make_function(i);
    i = 5;
    f();
}</code>

In this code, we have a lambda that captures a reference to the integer variable i. We then modify i in the main function and call the lambda. The question is, will the lambda output 3 or 5?

Standard-Based Answer

The code is guaranteed to output 5, without invoking undefined behavior.

Explanation

According to the C 11 standard ([expr.prim.lambda]/17), only id-expressions referring to entities captured by copy are transformed into a member access on the lambda closure type. Id-expressions referring to entities captured by reference are left alone and still denote the same entity they would have denoted in the enclosing scope.

This means that the reference x in the lambda is not changed. It still refers to the same integer variable i in the main function. Therefore, when we modify i in the main function, the lambda will still see the modified value.

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