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//===--- SlicingCheck.cpp - clang-tidy-------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "SlicingCheck.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/RecordLayout.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang/ASTMatchers/ASTMatchers.h"
using namespace clang::ast_matchers;
namespace clang {
namespace tidy {
namespace cppcoreguidelines {
void SlicingCheck::registerMatchers(MatchFinder *Finder) {
// When we see:
// class B : public A { ... };
// A a;
// B b;
// a = b;
// The assignment is OK if:
// - the assignment operator is defined as taking a B as second parameter,
// or
// - B does not define any additional members (either variables or
// overrides) wrt A.
//
// The same holds for copy ctor calls. This also captures stuff like:
// void f(A a);
// f(b);
// Helpers.
const auto OfBaseClass = ofClass(cxxRecordDecl().bind("BaseDecl"));
const auto IsDerivedFromBaseDecl =
cxxRecordDecl(isDerivedFrom(equalsBoundNode("BaseDecl")))
.bind("DerivedDecl");
const auto HasTypeDerivedFromBaseDecl =
anyOf(hasType(IsDerivedFromBaseDecl),
hasType(references(IsDerivedFromBaseDecl)));
const auto IsWithinDerivedCtor =
hasParent(cxxConstructorDecl(ofClass(equalsBoundNode("DerivedDecl"))));
// Assignement slicing: "a = b;" and "a = std::move(b);" variants.
const auto SlicesObjectInAssignment =
callExpr(callee(cxxMethodDecl(anyOf(isCopyAssignmentOperator(),
isMoveAssignmentOperator()),
OfBaseClass)),
hasArgument(1, HasTypeDerivedFromBaseDecl));
// Construction slicing: "A a{b};" and "f(b);" variants. Note that in case of
// slicing the letter will create a temporary and therefore call a ctor.
const auto SlicesObjectInCtor = cxxConstructExpr(
hasDeclaration(cxxConstructorDecl(
anyOf(isCopyConstructor(), isMoveConstructor()), OfBaseClass)),
hasArgument(0, HasTypeDerivedFromBaseDecl),
// We need to disable matching on the call to the base copy/move
// constructor in DerivedDecl's constructors.
unless(IsWithinDerivedCtor));
Finder->addMatcher(
expr(anyOf(SlicesObjectInAssignment, SlicesObjectInCtor)).bind("Call"),
this);
}
/// Warns on methods overridden in DerivedDecl with respect to BaseDecl.
/// FIXME: this warns on all overrides outside of the sliced path in case of
/// multiple inheritance.
void SlicingCheck::DiagnoseSlicedOverriddenMethods(
const Expr &Call, const CXXRecordDecl &DerivedDecl,
const CXXRecordDecl &BaseDecl) {
if (DerivedDecl.getCanonicalDecl() == BaseDecl.getCanonicalDecl())
return;
for (const auto &Method : DerivedDecl.methods()) {
// Virtual destructors are OK. We're ignoring constructors since they are
// tagged as overrides.
if (isa<CXXConstructorDecl>(Method) || isa<CXXDestructorDecl>(Method))
continue;
if (Method->size_overridden_methods() > 0) {
diag(Call.getExprLoc(),
"slicing object from type %0 to %1 discards override %2")
<< &DerivedDecl << &BaseDecl << Method;
}
}
// Recursively process bases.
for (const auto &Base : DerivedDecl.bases()) {
if (const auto *BaseRecordType = Base.getType()->getAs<RecordType>()) {
if (const auto *BaseRecord = cast_or_null<CXXRecordDecl>(
BaseRecordType->getDecl()->getDefinition()))
DiagnoseSlicedOverriddenMethods(Call, *BaseRecord, BaseDecl);
}
}
}
void SlicingCheck::check(const MatchFinder::MatchResult &Result) {
const auto *BaseDecl = Result.Nodes.getNodeAs<CXXRecordDecl>("BaseDecl");
const auto *DerivedDecl =
Result.Nodes.getNodeAs<CXXRecordDecl>("DerivedDecl");
const auto *Call = Result.Nodes.getNodeAs<Expr>("Call");
assert(BaseDecl != nullptr);
assert(DerivedDecl != nullptr);
assert(Call != nullptr);
// Warn when slicing the vtable.
// We're looking through all the methods in the derived class and see if they
// override some methods in the base class.
// It's not enough to just test whether the class is polymorphic because we
// would be fine slicing B to A if no method in B (or its bases) overrides
// anything in A:
// class A { virtual void f(); };
// class B : public A {};
// because in that case calling A::f is the same as calling B::f.
DiagnoseSlicedOverriddenMethods(*Call, *DerivedDecl, *BaseDecl);
// Warn when slicing member variables.
const auto &BaseLayout =
BaseDecl->getASTContext().getASTRecordLayout(BaseDecl);
const auto &DerivedLayout =
DerivedDecl->getASTContext().getASTRecordLayout(DerivedDecl);
const CharUnits StateSize =
DerivedLayout.getDataSize() - BaseLayout.getDataSize();
if (StateSize.isPositive()) {
diag(Call->getExprLoc(), "slicing object from type %0 to %1 discards "
"%2 bytes of state")
<< DerivedDecl << BaseDecl << static_cast<int>(StateSize.getQuantity());
}
}
} // namespace cppcoreguidelines
} // namespace tidy
} // namespace clang
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