Add C.13 to ensure data member lifetimes nest when needed... see #2316 by hsutter · Pull Request #2321 · isocpp/CppCoreGuidelines

hsutter · 2026-03-05T21:28:45Z

beinhaerter · 2026-03-06T18:29:37Z

CppCoreGuidelines.md

+
+##### Example; bad
+
+This can also come up with concurrency. Ensure that an async operation that access a value is joined before the value it accesses is destroyed.


Suggested change

This can also come up with concurrency. Ensure that an async operation that access a value is joined before the value it accesses is destroyed.

This can also come up with concurrency. Ensure that an async operation that accesses a value is joined before the value it accesses is destroyed.

beinhaerter · 2026-03-06T18:31:17Z

CppCoreGuidelines.md

+    class X {
+        struct B {
+            string* p;
+            explicit B(string& a) : p(&a) {}


Suggested change

explicit B(string& a) : p(&a) {}

explicit B(string& a) : p{&a} {}

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es23-prefer-the--initializer-syntax

beinhaerter · 2026-03-06T18:32:11Z

CppCoreGuidelines.md

+        string a = "some heap allocated string value"; // constructed after b; destroyed before b
+
+    public:
+        X() : b(a) {}   // uses a before it is constructed -> use-before-alloc UB


Suggested change

X() : b(a) {} // uses a before it is constructed -> use-before-alloc UB

X() : b{a} {} // uses a before it is constructed -> use-before-alloc UB

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es23-prefer-the--initializer-syntax

beinhaerter · 2026-03-06T18:32:26Z

CppCoreGuidelines.md

+    class X {
+        struct B {
+            string* p;
+            explicit B(string& a) : p(&a) {}


Suggested change

explicit B(string& a) : p(&a) {}

explicit B(string& a) : p{&a} {}

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es23-prefer-the--initializer-syntax

beinhaerter · 2026-03-06T18:32:44Z

CppCoreGuidelines.md

+        B      b;                                      // constructed second
+
+    public:
+        X() : b(a) {}   // ok


Suggested change

X() : b(a) {} // ok

X() : b{a} {} // ok

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es23-prefer-the--initializer-syntax

beinhaerter · 2026-03-06T18:33:12Z

CppCoreGuidelines.md

+    class X {
+    public:
+        X()
+            : a(std::make_unique<int>(12)) 


Suggested change

: a(std::make_unique<int>(12))

: a{std::make_unique<int>(12)}

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es23-prefer-the--initializer-syntax

beinhaerter · 2026-03-06T18:33:46Z

CppCoreGuidelines.md

+
+##### Example; good
+
+This can also come up with concurrency. Ensure that an async operation that access a value is joined before the value it accesses is destroyed.


Suggested change

This can also come up with concurrency. Ensure that an async operation that access a value is joined before the value it accesses is destroyed.

This can also come up with concurrency. Ensure that an async operation that accesses a value is joined before the value it accesses is destroyed.

beinhaerter · 2026-03-06T18:34:07Z

CppCoreGuidelines.md

+    class X {
+    public:
+        X()
+            : a(std::make_unique<int>(12)) 


Suggested change

: a(std::make_unique<int>(12))

: a{std::make_unique<int>(12)}

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es23-prefer-the--initializer-syntax

beinhaerter · 2026-03-06T18:39:42Z

CppCoreGuidelines.md

 Flag a data member that is `const`, `&`, or `&&` in a type that has any copy or move operation.


+### <a name="rc-lifetime"></a>C.13: If one data member uses another, declare it before the other


My reading of the title is (italic text is mine):

If one data member B uses another data member A, declare it (B) before the other (A)

This is in contrast to what the Discussion section says:

If data member B uses another data member A, then A must be declared before B so that A outlives B

I am not a native English speaker. Is my reading of the title wrong?

I agree with your interpretation of the title and that they contradict.

Maybe title could be 'If one data member uses another, declare it after the used member' ?

I think maybe it could rephrase to 'Declare a data member before members that depend on it' — it should convey roughly the same meaning?

bgloyer · 2026-03-10T04:28:03Z

CppCoreGuidelines.md

+
+    // Bad: b uses a, but a is declared after b.
+    //      Construction order is b then a; destruction order is a then b.
+    //      So b touches a outside a's lifetime.


looks like the build failed because a's isn't in the dictionary

I think it could also rephrase this line to "So b touches a outside the lifetime of a.", which conveys the same meaning.

I think this phrasing might also express the intent more clearly, I'm not entirely sure

Add C.13 to ensure data member lifetimes nest when needed... see #2316

063ee83

Closes #2316

beinhaerter reviewed Mar 6, 2026

View reviewed changes

bgloyer reviewed Mar 10, 2026

View reviewed changes


		##### Example; bad

		This can also come up with concurrency. Ensure that an async operation that access a value is joined before the value it accesses is destroyed.

	explicit B(string& a) : p(&a) {}
	explicit B(string& a) : p{&a} {}

	X() : b(a) {} // uses a before it is constructed -> use-before-alloc UB
	X() : b{a} {} // uses a before it is constructed -> use-before-alloc UB

	: a(std::make_unique<int>(12))
	: a{std::make_unique<int>(12)}


		##### Example; good

		This can also come up with concurrency. Ensure that an async operation that access a value is joined before the value it accesses is destroyed.

		Flag a data member that is `const`, `&`, or `&&` in a type that has any copy or move operation.


		### <a name="rc-lifetime"></a>C.13: If one data member uses another, declare it before the other

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hsutter commented Mar 5, 2026

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rainy-juzixiao Mar 13, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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bgloyer Mar 10, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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rainy-juzixiao Mar 13, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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rainy-juzixiao Mar 13, 2026 •

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