1 //===--- SlicingCheck.cpp - clang-tidy-------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #include "SlicingCheck.h"
10 #include "clang/AST/ASTContext.h"
11 #include "clang/AST/RecordLayout.h"
12 #include "clang/ASTMatchers/ASTMatchFinder.h"
13 #include "clang/ASTMatchers/ASTMatchers.h"
14
15 using namespace clang::ast_matchers;
16
17 namespace clang {
18 namespace tidy {
19 namespace cppcoreguidelines {
20
registerMatchers(MatchFinder * Finder)21 void SlicingCheck::registerMatchers(MatchFinder *Finder) {
22 // When we see:
23 // class B : public A { ... };
24 // A a;
25 // B b;
26 // a = b;
27 // The assignment is OK if:
28 // - the assignment operator is defined as taking a B as second parameter,
29 // or
30 // - B does not define any additional members (either variables or
31 // overrides) wrt A.
32 //
33 // The same holds for copy ctor calls. This also captures stuff like:
34 // void f(A a);
35 // f(b);
36
37 // Helpers.
38 const auto OfBaseClass = ofClass(cxxRecordDecl().bind("BaseDecl"));
39 const auto IsDerivedFromBaseDecl =
40 cxxRecordDecl(isDerivedFrom(equalsBoundNode("BaseDecl")))
41 .bind("DerivedDecl");
42 const auto HasTypeDerivedFromBaseDecl =
43 anyOf(hasType(IsDerivedFromBaseDecl),
44 hasType(references(IsDerivedFromBaseDecl)));
45 const auto IsWithinDerivedCtor =
46 hasParent(cxxConstructorDecl(ofClass(equalsBoundNode("DerivedDecl"))));
47
48 // Assignment slicing: "a = b;" and "a = std::move(b);" variants.
49 const auto SlicesObjectInAssignment =
50 callExpr(callee(cxxMethodDecl(anyOf(isCopyAssignmentOperator(),
51 isMoveAssignmentOperator()),
52 OfBaseClass)),
53 hasArgument(1, HasTypeDerivedFromBaseDecl));
54
55 // Construction slicing: "A a{b};" and "f(b);" variants. Note that in case of
56 // slicing the letter will create a temporary and therefore call a ctor.
57 const auto SlicesObjectInCtor = cxxConstructExpr(
58 hasDeclaration(cxxConstructorDecl(
59 anyOf(isCopyConstructor(), isMoveConstructor()), OfBaseClass)),
60 hasArgument(0, HasTypeDerivedFromBaseDecl),
61 // We need to disable matching on the call to the base copy/move
62 // constructor in DerivedDecl's constructors.
63 unless(IsWithinDerivedCtor));
64
65 Finder->addMatcher(traverse(TK_AsIs, expr(anyOf(SlicesObjectInAssignment,
66 SlicesObjectInCtor))
67 .bind("Call")),
68 this);
69 }
70
71 /// Warns on methods overridden in DerivedDecl with respect to BaseDecl.
72 /// FIXME: this warns on all overrides outside of the sliced path in case of
73 /// multiple inheritance.
diagnoseSlicedOverriddenMethods(const Expr & Call,const CXXRecordDecl & DerivedDecl,const CXXRecordDecl & BaseDecl)74 void SlicingCheck::diagnoseSlicedOverriddenMethods(
75 const Expr &Call, const CXXRecordDecl &DerivedDecl,
76 const CXXRecordDecl &BaseDecl) {
77 if (DerivedDecl.getCanonicalDecl() == BaseDecl.getCanonicalDecl())
78 return;
79 for (const auto *Method : DerivedDecl.methods()) {
80 // Virtual destructors are OK. We're ignoring constructors since they are
81 // tagged as overrides.
82 if (isa<CXXConstructorDecl>(Method) || isa<CXXDestructorDecl>(Method))
83 continue;
84 if (Method->size_overridden_methods() > 0) {
85 diag(Call.getExprLoc(),
86 "slicing object from type %0 to %1 discards override %2")
87 << &DerivedDecl << &BaseDecl << Method;
88 }
89 }
90 // Recursively process bases.
91 for (const auto &Base : DerivedDecl.bases()) {
92 if (const auto *BaseRecordType = Base.getType()->getAs<RecordType>()) {
93 if (const auto *BaseRecord = cast_or_null<CXXRecordDecl>(
94 BaseRecordType->getDecl()->getDefinition()))
95 diagnoseSlicedOverriddenMethods(Call, *BaseRecord, BaseDecl);
96 }
97 }
98 }
99
check(const MatchFinder::MatchResult & Result)100 void SlicingCheck::check(const MatchFinder::MatchResult &Result) {
101 const auto *BaseDecl = Result.Nodes.getNodeAs<CXXRecordDecl>("BaseDecl");
102 const auto *DerivedDecl =
103 Result.Nodes.getNodeAs<CXXRecordDecl>("DerivedDecl");
104 const auto *Call = Result.Nodes.getNodeAs<Expr>("Call");
105 assert(BaseDecl != nullptr);
106 assert(DerivedDecl != nullptr);
107 assert(Call != nullptr);
108
109 // Warn when slicing the vtable.
110 // We're looking through all the methods in the derived class and see if they
111 // override some methods in the base class.
112 // It's not enough to just test whether the class is polymorphic because we
113 // would be fine slicing B to A if no method in B (or its bases) overrides
114 // anything in A:
115 // class A { virtual void f(); };
116 // class B : public A {};
117 // because in that case calling A::f is the same as calling B::f.
118 diagnoseSlicedOverriddenMethods(*Call, *DerivedDecl, *BaseDecl);
119
120 // Warn when slicing member variables.
121 const auto &BaseLayout =
122 BaseDecl->getASTContext().getASTRecordLayout(BaseDecl);
123 const auto &DerivedLayout =
124 DerivedDecl->getASTContext().getASTRecordLayout(DerivedDecl);
125 const CharUnits StateSize =
126 DerivedLayout.getDataSize() - BaseLayout.getDataSize();
127 if (StateSize.isPositive()) {
128 diag(Call->getExprLoc(), "slicing object from type %0 to %1 discards "
129 "%2 bytes of state")
130 << DerivedDecl << BaseDecl << static_cast<int>(StateSize.getQuantity());
131 }
132 }
133
134 } // namespace cppcoreguidelines
135 } // namespace tidy
136 } // namespace clang
137