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//===--- ThrowByValueCatchByReferenceCheck.cpp - clang-tidy----------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "ThrowByValueCatchByReferenceCheck.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/OperationKinds.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
using namespace clang::ast_matchers;
namespace clang {
namespace tidy {
namespace misc {
StringRef Name, ClangTidyContext *Context)
: ClangTidyCheck(Name, Context),
CheckAnonymousTemporaries(Options.get("CheckThrowTemporaries", true)),
WarnOnLargeObject(Options.get("WarnOnLargeObject", false)),
// Cannot access `ASTContext` from here so set it to an extremal value.
MaxSize(Options.get("MaxSize", std::numeric_limits<uint64_t>::max())) {}
void ThrowByValueCatchByReferenceCheck::registerMatchers(MatchFinder *Finder) {
// This is a C++ only check thus we register the matchers only for C++
if (!getLangOpts().CPlusPlus)
Finder->addMatcher(cxxThrowExpr().bind("throw"), this);
Finder->addMatcher(cxxCatchStmt().bind("catch"), this);
void ThrowByValueCatchByReferenceCheck::storeOptions(
ClangTidyOptions::OptionMap &Opts) {, "CheckThrowTemporaries", true);
void ThrowByValueCatchByReferenceCheck::check(
const MatchFinder::MatchResult &Result) {
bool ThrowByValueCatchByReferenceCheck::isFunctionParameter(
const DeclRefExpr *declRefExpr) {
return isa<ParmVarDecl>(declRefExpr->getDecl());
bool ThrowByValueCatchByReferenceCheck::isCatchVariable(
const DeclRefExpr *declRefExpr) {
auto *valueDecl = declRefExpr->getDecl();
if (auto *varDecl = dyn_cast<VarDecl>(valueDecl))
return varDecl->isExceptionVariable();
return false;
bool ThrowByValueCatchByReferenceCheck::isFunctionOrCatchVar(
const DeclRefExpr *declRefExpr) {
return isFunctionParameter(declRefExpr) || isCatchVariable(declRefExpr);
void ThrowByValueCatchByReferenceCheck::diagnoseThrowLocations(
const CXXThrowExpr *throwExpr) {
if (!throwExpr)
auto *subExpr = throwExpr->getSubExpr();
if (!subExpr)
auto qualType = subExpr->getType();
if (qualType->isPointerType()) {
// The code is throwing a pointer.
// In case it is strng literal, it is safe and we return.
auto *inner = subExpr->IgnoreParenImpCasts();
if (isa<StringLiteral>(inner))
// If it's a variable from a catch statement, we return as well.
auto *declRef = dyn_cast<DeclRefExpr>(inner);
if (declRef && isCatchVariable(declRef)) {
diag(subExpr->getBeginLoc(), "throw expression throws a pointer; it should "
"throw a non-pointer value instead");
// If the throw statement does not throw by pointer then it throws by value
// which is ok.
// There are addition checks that emit diagnosis messages if the thrown value
// is not an RValue. See:
// This behavior can be influenced by an option.
// If we encounter a CXXThrowExpr, we move through all casts until you either
// encounter a DeclRefExpr or a CXXConstructExpr.
// If it's a DeclRefExpr, we emit a message if the referenced variable is not
// a catch variable or function parameter.
// When encountering a CopyOrMoveConstructor: emit message if after casts,
// the expression is a LValue
if (CheckAnonymousTemporaries) {
bool emit = false;
auto *currentSubExpr = subExpr->IgnoreImpCasts();
const auto *variableReference = dyn_cast<DeclRefExpr>(currentSubExpr);
const auto *constructorCall = dyn_cast<CXXConstructExpr>(currentSubExpr);
// If we have a DeclRefExpr, we flag for emitting a diagnosis message in
// case the referenced variable is neither a function parameter nor a
// variable declared in the catch statement.
if (variableReference)
emit = !isFunctionOrCatchVar(variableReference);
else if (constructorCall &&
constructorCall->getConstructor()->isCopyOrMoveConstructor()) {
// If we have a copy / move construction, we emit a diagnosis message if
// the object that we copy construct from is neither a function parameter
// nor a variable declared in a catch statement
auto argIter =
->arg_begin(); // there's only one for copy constructors
auto *currentSubExpr = (*argIter)->IgnoreImpCasts();
if (currentSubExpr->isLValue()) {
if (auto *tmp = dyn_cast<DeclRefExpr>(currentSubExpr))
emit = !isFunctionOrCatchVar(tmp);
else if (isa<CallExpr>(currentSubExpr))
emit = true;
if (emit)
"throw expression should throw anonymous temporary values instead");
void ThrowByValueCatchByReferenceCheck::diagnoseCatchLocations(
const CXXCatchStmt *catchStmt, ASTContext &context) {
if (!catchStmt)
auto caughtType = catchStmt->getCaughtType();
if (caughtType.isNull())
auto *varDecl = catchStmt->getExceptionDecl();
if (const auto *PT = caughtType.getCanonicalType()->getAs<PointerType>()) {
const char *diagMsgCatchReference = "catch handler catches a pointer value; "
"should throw a non-pointer value and "
"catch by reference instead";
// We do not diagnose when catching pointer to strings since we also allow
// throwing string literals.
if (!PT->getPointeeType()->isAnyCharacterType())
diag(varDecl->getBeginLoc(), diagMsgCatchReference);
} else if (!caughtType->isReferenceType()) {
const char *diagMsgCatchReference = "catch handler catches by value; "
"should catch by reference instead";
// If it's not a pointer and not a reference then it must be caught "by
// value". In this case we should emit a diagnosis message unless the type
// is trivial.
if (!caughtType.isTrivialType(context)) {
diag(varDecl->getBeginLoc(), diagMsgCatchReference);
} else if (WarnOnLargeObject) {
// If the type is trivial, then catching it by reference is not dangerous.
// However, catching large objects by value decreases the performance.
// We can now access `ASTContext` so if `MaxSize` is an extremal value
// then set it to the size of `size_t`.
if (MaxSize == std::numeric_limits<uint64_t>::max())
MaxSize = context.getTypeSize(context.getSizeType());
if (context.getTypeSize(caughtType) > MaxSize)
diag(varDecl->getBeginLoc(), diagMsgCatchReference);
} // namespace misc
} // namespace tidy
} // namespace clang