1 //===- RedundantStringCStrCheck.cpp - Check for redundant c_str calls -----===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 //  This file implements a check for redundant calls of c_str() on strings.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "RedundantStringCStrCheck.h"
15 #include "clang/Lex/Lexer.h"
16 
17 namespace clang {
18 
19 using namespace ast_matchers;
20 
21 namespace {
22 
23 template <typename T>
24 StringRef getText(const ast_matchers::MatchFinder::MatchResult &Result,
25                   T const &Node) {
26   return Lexer::getSourceText(
27       CharSourceRange::getTokenRange(Node.getSourceRange()),
28       *Result.SourceManager, Result.Context->getLangOpts());
29 }
30 
31 // Return true if expr needs to be put in parens when it is an argument of a
32 // prefix unary operator, e.g. when it is a binary or ternary operator
33 // syntactically.
34 bool needParensAfterUnaryOperator(const Expr &ExprNode) {
35   if (isa<clang::BinaryOperator>(&ExprNode) ||
36       isa<clang::ConditionalOperator>(&ExprNode)) {
37     return true;
38   }
39   if (const auto *Op = dyn_cast<CXXOperatorCallExpr>(&ExprNode)) {
40     return Op->getNumArgs() == 2 && Op->getOperator() != OO_PlusPlus &&
41            Op->getOperator() != OO_MinusMinus && Op->getOperator() != OO_Call &&
42            Op->getOperator() != OO_Subscript;
43   }
44   return false;
45 }
46 
47 // Format a pointer to an expression: prefix with '*' but simplify
48 // when it already begins with '&'.  Return empty string on failure.
49 std::string
50 formatDereference(const ast_matchers::MatchFinder::MatchResult &Result,
51                   const Expr &ExprNode) {
52   if (const auto *Op = dyn_cast<clang::UnaryOperator>(&ExprNode)) {
53     if (Op->getOpcode() == UO_AddrOf) {
54       // Strip leading '&'.
55       return getText(Result, *Op->getSubExpr()->IgnoreParens());
56     }
57   }
58   StringRef Text = getText(Result, ExprNode);
59   if (Text.empty())
60     return std::string();
61   // Add leading '*'.
62   if (needParensAfterUnaryOperator(ExprNode)) {
63     return (llvm::Twine("*(") + Text + ")").str();
64   }
65   return (llvm::Twine("*") + Text).str();
66 }
67 
68 const char StringConstructor[] =
69     "::std::basic_string<char, std::char_traits<char>, std::allocator<char> >"
70     "::basic_string";
71 
72 const char StringCStrMethod[] =
73     "::std::basic_string<char, std::char_traits<char>, std::allocator<char> >"
74     "::c_str";
75 
76 } // end namespace
77 
78 namespace tidy {
79 namespace readability {
80 
81 void RedundantStringCStrCheck::registerMatchers(
82     ast_matchers::MatchFinder *Finder) {
83   // Only register the matchers for C++; the functionality currently does not
84   // provide any benefit to other languages, despite being benign.
85   if (!getLangOpts().CPlusPlus)
86     return;
87 
88   Finder->addMatcher(
89       cxxConstructExpr(
90           hasDeclaration(cxxMethodDecl(hasName(StringConstructor))),
91           argumentCountIs(2),
92           // The first argument must have the form x.c_str() or p->c_str()
93           // where the method is string::c_str().  We can use the copy
94           // constructor of string instead (or the compiler might share
95           // the string object).
96           hasArgument(0, cxxMemberCallExpr(
97                              callee(memberExpr().bind("member")),
98                              callee(cxxMethodDecl(hasName(StringCStrMethod))),
99                              on(expr().bind("arg")))
100                              .bind("call")),
101           // The second argument is the alloc object which must not be
102           // present explicitly.
103           hasArgument(1, cxxDefaultArgExpr())),
104       this);
105   Finder->addMatcher(
106       cxxConstructExpr(
107           // Implicit constructors of these classes are overloaded
108           // wrt. string types and they internally make a StringRef
109           // referring to the argument.  Passing a string directly to
110           // them is preferred to passing a char pointer.
111           hasDeclaration(
112               cxxMethodDecl(anyOf(hasName("::llvm::StringRef::StringRef"),
113                                   hasName("::llvm::Twine::Twine")))),
114           argumentCountIs(1),
115           // The only argument must have the form x.c_str() or p->c_str()
116           // where the method is string::c_str().  StringRef also has
117           // a constructor from string which is more efficient (avoids
118           // strlen), so we can construct StringRef from the string
119           // directly.
120           hasArgument(0, cxxMemberCallExpr(
121                              callee(memberExpr().bind("member")),
122                              callee(cxxMethodDecl(hasName(StringCStrMethod))),
123                              on(expr().bind("arg")))
124                              .bind("call"))),
125       this);
126 }
127 
128 void RedundantStringCStrCheck::check(const MatchFinder::MatchResult &Result) {
129   const auto *Call = Result.Nodes.getStmtAs<CallExpr>("call");
130   const auto *Arg = Result.Nodes.getStmtAs<Expr>("arg");
131   bool Arrow = Result.Nodes.getStmtAs<MemberExpr>("member")->isArrow();
132   // Replace the "call" node with the "arg" node, prefixed with '*'
133   // if the call was using '->' rather than '.'.
134   std::string ArgText =
135       Arrow ? formatDereference(Result, *Arg) : getText(Result, *Arg).str();
136   if (ArgText.empty())
137     return;
138 
139   diag(Call->getLocStart(), "redundant call to `c_str()`")
140       << FixItHint::CreateReplacement(Call->getSourceRange(), ArgText);
141 }
142 
143 } // namespace readability
144 } // namespace tidy
145 } // namespace clang
146