072e67b8d4
"C++" spelled as-is can result in unexpected rendering, such as
C++20 was released before C++17
renders as
C20 was released before C17
Make consistent use of `[source,cpp]`.
57 lines
1.9 KiB
Plaintext
57 lines
1.9 KiB
Plaintext
== Why is this an issue?
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Coroutines, introduced in {cpp}20, are functions in which execution can be suspended and resumed.
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When a coroutine resumes, it takes over where it left thanks to the coroutine state.
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A _coroutine state_ is an object which contains all the information a coroutine needs to resume its execution correctly:
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local variables, copy of the parameters...
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This means that if a coroutine has a parameter that is a reference to an object, this object must exist as long as the coroutine is not destroyed.
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Otherwise, the reference stored in the _coroutine state_ will become a dangling reference and will lead to undefined behavior when the coroutine resumes.
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The issue is raised for all coroutine parameters with reference-to-const semantics
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(such as a `const` reference, a `std::string_view`, or a `std::span` with `const` elements)
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that might be used after the coroutine is suspended.
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To fix the issue, you can either pass the parameter by value,
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or not use the parameter after the first suspension point (`co_await`, `co_yield`, or `initial_suspend`).
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=== Noncompliant code example
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[source,cpp]
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----
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generator<char> spell(const std::string& m) { // Noncompliant
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for (char letter : m) {
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co_yield letter;
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}
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}
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void print() {
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for (char letter : spell("hello world")) { // Here the parameter "m" binds to a temporary
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std::cout << letter << '\n'; // and becomes dangling on the next iteration
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}
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}
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----
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=== Compliant solution
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[source,cpp]
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----
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generator<char> spell(const std::string m) { // Compliant: take the argument by copy
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for (char letter : m) {
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co_yield letter;
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}
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}
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void print() {
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for (char letter : spell("hello world")) {
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std::cout << letter << '\n';
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}
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}
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----
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=== Exceptions
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This rule does not raise an issue for `std::reference_wrapper` parameters
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taking it as a witness of the care taken to prevent the reference to become dangling.
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