Detect if a CXXConstructExpr is implicit

Clang Frontend
1

In the following code:

classA{
public:
A();
A(A const&a);
A(inti);
};

voidf(A a);

voidg() {
A a;
f(a); // 1
f(2); // 2
}

The generated AST contains nodes about object construction that are implicit (the copy constructor for 1, the conversion then the copy constructor for 2).

 >\-CallExpr <line:12:5, col:8> 'void'
 > >\-ImplicitCastExpr <col:5> 'void \(\*\)\(A\)' <FunctionToPointerDecay>
 > > \`\-DeclRefExpr <col:5> 'void \(A\)' lvalue Function 0x55c42589b208 'f' 'void \(A\)'
 > \`\-CXXConstructExpr <col:7> 'A' 'void \(const A &\)'
 >   \`\-ImplicitCastExpr <col:7> 'const A' lvalue <NoOp>
 >     \`\-DeclRefExpr <col:7> 'A' lvalue Var 0x55c42589b3c8 'a' 'A'
 \`\-ExprWithCleanups <line:13:5, col:8> 'void'
   \`\-CallExpr <col:5, col:8> 'void'
     >\-ImplicitCastExpr <col:5> 'void \(\*\)\(A\)' <FunctionToPointerDecay>
     > \`\-DeclRefExpr <col:5> 'void \(A\)' lvalue Function 0x55c42589b208 'f' 'void \(A\)'
     \`\-CXXConstructExpr <col:7> 'A' 'void \(const A &\)' elidable
       \`\-MaterializeTemporaryExpr <col:7> 'const A' lvalue
         \`\-ImplicitCastExpr <col:7> 'const A' <NoOp>
           \`\-ImplicitCastExpr <col:7> 'A' <ConstructorConversion>
             \`\-CXXConstructExpr <col:7> 'A' 'void \(int\)'
               \`\-IntegerLiteral <col:7> 'int' 2

Do you know if there is a way to detect that these constructions are implicit (as opposed to constructions written as the type name followed by initialization?)

Thank you!

2

If you look at the AST for the record decl, there is a bunch of flags in the DefinitionData of interest:

-CXXRecordDecl <line:1:1, line:6:1> line:1:7 referenced class A
definition
>-DefinitionData empty standard_layout has_user_declared_ctor
>can_const_default_init
> >-DefaultConstructor exists non_trivial user_provided
> >defaulted_is_constexpr -CopyConstructor non_trivial user_declared
> >has_const_param implicit_has_const_param -MoveConstructor
> >-CopyAssignment trivial has_const_param needs_implicit
> >implicit_has_const_param -MoveAssignment

See "user_provided" and "user_declared". I'm not sure what the difference between those two is however.

3

Unless I'm mistaken, those notions apply to CXXContructorDecl, not to CXXConstructExpr. A user declared copy constructor may or may not be called implicitely.

4

Ah, I see. I got lost in the pronoun game :slight_smile:

None that I'm aware of, and I don't see anything promising here: https://clang.llvm.org/doxygen/classclang_1_1CXXConstructExpr.html

Perhaps someone with more knowledge of this can elaborate better.

5

If it's useful, I have plans to upstream changes such that you can print and AST-match on code like this while eliding invisible AST nodes. See:

  http://ec2-18-191-7-3.us-east-2.compute.amazonaws.com:10240/z/PlPx1d

Thanks,

Stephen.