rspec/rules/S6191/cfamily/rule.adoc

The behavior of volatile types in compound expressions that both read and write the same variable (for instance ``a{plus}{plus}`` or ``++a+=2++``, where ``++a++`` is a ``++volatile int++``) is not well defined, and is deprecated since {cpp}20.


Such expressions should be replaced by explicit operations that only read or write the value.


== Noncompliant Code Example

[source,cpp]
----
void f1(int volatile* p) {
  ++(*p); // Noncompliant
}

void f2(volatile int& in) {
  in += 2; // Noncompliant
}

void f3(volatile int& in) {
  int i = in = 2; // Noncompliant
}
----


== Compliant Solution

[source,cpp]
----
void f1(int volatile* p) {
  auto val = *p;  // One access to read the register
  *p = val + 1; // One access to write to it (and potentially overwrite another change)
}

void f2(volatile int& in) {
  auto val = in;
  in = val + 2;
}

void f3(volatile int& in) {
  in = 2;
  int i = in;
}
----