Mechanism to check if a C++ member is private

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.

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.

To append cigien's correct answer as I am testing a non-constant object, i.e., T is altered during construction of the object, and thus will test at runtime rather than compile time. This just involves removing the const keywords:

// Check that color is a public member
template<typename T>
auto has_public_color(T &t) -> decltype(t.color, true)
{
    // Returns true if T has a public member named color
    return true;
}
auto has_public_color(...)
{
    return false;
}

Then I just insert into my unit testing framework (Boost unit testing) as such:

BOOST_AUTO_TEST_CASE(test_cell_no_public_color)
{
    BOOST_TEST_MESSAGE("Testing that Cell has no public member of color");
    // Check that Cell has no public color
    BOOST_CHECK(not has_public_color(Cell{}));
}