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Respect initializer side effects in unused-variable queries #22938

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Merged
jketema merged 5 commits into from
Oct 30, 2026
Merged

Respect initializer side effects in unused-variable queries #22938

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f985aee
Test side effects in unused-variable queries
jketema Oct 29, 2026
8a8c4f3
Respect initializer side effects in unused-variable queries
jketema Oct 29, 2026
24f492b
Document unused-variable initializer side-effect fix
jketema Oct 29, 2026
0ecfb5e
Mark side-effecting iterator dereference as a good case
jketema Oct 29, 2026
b9d9c24
Update cpp/ql/src/change-notes/2026-10-29-unused-variable-initializer…
jketema Oct 29, 2026
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2 changes: 2 additions & 0 deletions cpp/ql/src/Best Practices/Unused Entities/UnusedLocals.ql
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Original file line number Diff line number Diff line change
Expand Up @@ -46,6 +46,8 @@ predicate declarationHasSideEffects(Variable v) {
)
or
v.getType() instanceof TemplateDependentType // may have a constructor/destructor
or
v.getInitializer().getExpr().mayBeImpure()
}

from LocalVariable v, Function f
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2 changes: 2 additions & 0 deletions cpp/ql/src/Best Practices/Unused Entities/UnusedStaticVariables.ql
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Expand Up @@ -15,6 +15,8 @@ import cpp

predicate declarationHasSideEffects(Variable v) {
exists(Class c | c = v.getUnspecifiedType() | c.hasConstructor() or c.hasDestructor())
or
v.getInitializer().getExpr().mayBeImpure()
}

from Variable v
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4 changes: 4 additions & 0 deletions cpp/ql/src/change-notes/2026-10-29-unused-variable-initializers.md
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@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* The `cpp/unused-local-variable` and `cpp/unused-static-variable` queries no longer report variables whose initializers may have side effects.
1 change: 0 additions & 1 deletion cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/UnusedLocals.expected
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Expand Up @@ -4,7 +4,6 @@
| code2.cpp:26:16:26:17 | v1 | Variable v1 is not used. |
| code2.cpp:42:11:42:16 | myVar1 | Variable myVar1 is not used. |
| code2.cpp:64:7:64:8 | v3 | Variable v3 is not used. |
| code2.cpp:108:11:108:12 | v2 | Variable v2 is not used. |
| code2.cpp:128:9:128:9 | b | Variable b is not used. |
| code.c:10:18:10:18 | y | Variable y is not used. |
| code.c:11:18:11:18 | z | Variable z is not used. |
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14 changes: 13 additions & 1 deletion cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/code2.cpp
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Expand Up @@ -105,7 +105,7 @@ void test_range_based_for()
output(v1);
}

for (int v2 : myContainer) // $ Alert // BAD: v2 is not used
for (int v2 : myContainer) // GOOD: iterator dereference may have side effects

@jketema jketema Oct 29, 2026

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Note that the relevant member functions in MyContainer are missing function bodies, so we don't known if there may be side-effects.

@geoffw0 geoffw0 Oct 30, 2026

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I would hope that iterating through a container doesn't have side-effects in the vast majority of cases. But this is C++, people are bound to have come up with containers where they do.

{
}
}
Expand Down Expand Up @@ -181,3 +181,15 @@ void test_maybe_unused()
{
[[maybe_unused]] int unused; // GOOD: explicitly marked as maybe_unused
}

int globalVar;

int initialize()
{
return ++globalVar;
}

void test_initializer_side_effect()
{
int initialized = initialize(); // GOOD (initializer has a side effect)
}
20 changes: 20 additions & 0 deletions cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp
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Expand Up @@ -29,3 +29,23 @@ class MyTemplateClass // never instantiated
// ...
}
};

int initialize()
{
return ++globalVar;
}

static int initialized = initialize(); // GOOD (initializer has a side effect)

struct WithConstructor
{
WithConstructor() { ++globalVar; }
};

struct WithDestructor
{
~WithDestructor() { ++globalVar; }
};

static WithConstructor constructed; // GOOD (constructor has a side effect)
static WithDestructor destructed; // GOOD (destructor has a side effect)
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