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C++: Do not use [=] or [=,this] to capture 'this'

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Summary: C++20 deprecates [=] capturing 'this'
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0806r2.html)
but the recommended alternative [=,this] is not standards-legal prior to
C++20 (though it is widely accepted *with warning*).

For portability (and readability), style should forbid using
[=] or [=,this] to capture 'this'. Any by-value captures must be
explicitly listed if also capturing 'this', though 'this' can still
be implicitly default-captured by reference, as in
[&, some_var_to_copy].

Test Plan: documentation only
This commit is contained in:
Peter Dillinger 2020-04-14 10:41:30 -07:00
parent 305ac8725a
commit bacabed1ad
1 changed files with 14 additions and 7 deletions
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cppguide.html
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@ -3479,7 +3479,8 @@ array(T, U...) -> std::array<T, 1 + sizeof...(U)>;
<h3 id="Lambda_expressions">Lambda expressions</h3> <h3 id="Lambda_expressions">Lambda expressions</h3>
<p>Use lambda expressions where appropriate. Prefer explicit captures <p>Use lambda expressions where appropriate. Prefer explicit captures
when the lambda will escape the current scope.</p> when the lambda will escape the current scope. Do not use <code>[=]</code>
or <code>[=, this]</code> to capture <code>this</code>.</p>
<p class="definition"></p> <p class="definition"></p>
<p> Lambda expressions are a concise way of creating anonymous <p> Lambda expressions are a concise way of creating anonymous
@ -3555,13 +3556,19 @@ std::sort(indices.begin(), indices.end(), [&amp;](int a, int b) {
<p class="cons"></p> <p class="cons"></p>
<ul> <ul>
<li>Variable capture in lambdas can be a source of dangling-pointer <li>Variable capture in lambdas can be a source of dangling-pointer
bugs, particularly if a lambda escapes the current scope.</li> bugs, particularly if a lambda escapes the current scope. Default capture
by reference (<code>[&amp;]</code>) makes such bugs easier to introduce.</li>
<li>Default captures by value can be misleading because they do not prevent <li>Default capture by value (<code>[=]</code>) can be misleading because
dangling-pointer bugs. Capturing a pointer by value doesn't cause a deep it does not prevent dangling-pointer bugs. Capturing a pointer by value
copy, so it often has the same lifetime issues as capture by reference. doesn't cause a deep copy, so it has essentially the same lifetime issues
This is especially confusing when capturing 'this' by value, since the use as capture by reference.</li>
of 'this' is often implicit.</li>
<li>Capturing <code>this</code> along with default capture by value is
especially confusing, and strict support among C++ versions is incompatible.
If capturing <code>this</code> (either explicitly or with by-reference
capture default), list any by-value captures explicitly. Keep in mind that
uses of captured <code>this</code> are often implicit.</li>
<li>Captures actually declare new variables (whether or not the captures have <li>Captures actually declare new variables (whether or not the captures have
initializers), but they look nothing like any other variable declaration initializers), but they look nothing like any other variable declaration