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& x) {
    return [&]{ std::cout <p>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?</p>
<p><strong>Standard-Based Answer</strong></p>
<p>The code is guaranteed to output 5, without invoking undefined behavior.</p>
<p><strong>Explanation</strong></p>
<p>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.</p>
<p>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.</p></void></iostream></functional></code>

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