Mechanism to check if a C++ member is private

Question

I am writing unit tests for undergraduate students and want to enforce certain members as public or private. I am aware of methods to actually test private members, e.g., #define private public or using a friend class, but have not seen anything that would allow me to check exactly if a member is private or not.

A brute force method would be trying a compile and parsing the output error, e.g., look for something like error: 'foo' is a private member of 'Bar', but I am hoping someone will have a better trick!

Answer 1

If you want to assert that a type Bar doesn't have a public member named foo, you can write the following test:

template<typename T> constexpr auto has_public_foo(T const &t) -> decltype(t.foo, true)  {     return true; }  constexpr auto has_public_foo(...)  {     return false; }  static_assert(not has_public_foo(Bar{}), "Public members are bad practice"); 

Here's a demo.

Answer 2

You can use the AST output of the Clang compiler to validate that a certain member is private or public. For example, for the following code:

class test { public:     int pub; private:     int prv; }; 

Running this command: clang -Xclang -ast-dump -fsyntax-only t.cpp

gives the AST dump:

TranslationUnitDecl 0x55f6f550e3f8 <<invalid sloc>> <invalid sloc> |-TypedefDecl 0x55f6f550e9b0 <<invalid sloc>> <invalid sloc> implicit __int128_t '__int128' | `-BuiltinType 0x55f6f550e690 '__int128' |-TypedefDecl 0x55f6f550ea20 <<invalid sloc>> <invalid sloc> implicit __uint128_t 'unsigned __int128' | `-BuiltinType 0x55f6f550e6b0 'unsigned __int128' |-TypedefDecl 0x55f6f550ed68 <<invalid sloc>> <invalid sloc> implicit __NSConstantString '__NSConstantString_tag' | `-RecordType 0x55f6f550eb10 '__NSConstantString_tag' |   `-CXXRecord 0x55f6f550ea78 '__NSConstantString_tag' |-TypedefDecl 0x55f6f550ee00 <<invalid sloc>> <invalid sloc> implicit __builtin_ms_va_list 'char *' | `-PointerType 0x55f6f550edc0 'char *' |   `-BuiltinType 0x55f6f550e490 'char' |-TypedefDecl 0x55f6f5545bf8 <<invalid sloc>> <invalid sloc> implicit __builtin_va_list '__va_list_tag [1]' | `-ConstantArrayType 0x55f6f5545ba0 '__va_list_tag [1]' 1 |   `-RecordType 0x55f6f550eef0 '__va_list_tag' |     `-CXXRecord 0x55f6f550ee58 '__va_list_tag' `-CXXRecordDecl 0x55f6f5545c50 <t.cpp:1:1, line:6:1> line:1:7 class test definition   |-DefinitionData pass_in_registers trivially_copyable trivial literal   | |-DefaultConstructor exists trivial needs_implicit   | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param   | |-MoveConstructor exists simple trivial needs_implicit   | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param   | |-MoveAssignment exists simple trivial needs_implicit   | `-Destructor simple irrelevant trivial needs_implicit   |-CXXRecordDecl 0x55f6f5545d78 <col:1, col:7> col:7 implicit class test   |-AccessSpecDecl 0x55f6f5545e10 <line:2:1, col:7> col:1 public   |-FieldDecl 0x55f6f5545e50 <line:3:5, col:9> col:9 pub 'int'   |-AccessSpecDecl 0x55f6f5545e98 <line:4:1, col:8> col:1 private   `-FieldDecl 0x55f6f5545ed8 <line:5:5, col:9> col:9 prv 'int' 

which is quite simple to parse by a script. Or you can use the Clang AST library to create LibASTMatcher to validate using data itself as described in the documentation.