Accessor

Header only, fully template based library which enables accessing private data members. Techniques used by this library to achieve its purpose are fully legal and allowed by the standard. The library is based on Explicit instantiation of class template instantion.

Explicit instantiation definitions ignore member access specifiers: parameter types and return types may be private.

source: https://en.cppreference.com/w/cpp/language/class_template

The article describing implementation of this technique can be find Here

First reference to this technique is in Johannes Schaub - litb blog

Herb Sutter GotW blog post why in general you should not access private members. Be wise and don't try to break things if you don't have to!

Motivation

This library is not meant to break any C++ design rules, even though it's fully legal from standard point of view. The purpose of this library is to use it when necessary within tests. Code shall be written in the way that it can be later tested, but as we all know in most cases and it especially applies to legacy code, there are situations where one need to access private member and no mocking can be applied anymore.

In situations like that provided technique is far better than common #define private public, using friend keyword or even flag no-access-control for gcc.

Installation

Installation is done using CMake

mkdir build
cd build
cmake ..
make && make install

Importing Accessor as a target

Installed Accessor library can be easily imported by any CMake based project

find_package(accessor REQUIRED)
# ...
target_link_libraries(${exec_name} accessor)

Building examples

There is set of mini examples, which shows how various data members and methods can be accessed. To build them, the additional CMake flag shall be passed.

cmake -DEXAMPLES=1 ..
make

Running tests

There is set of tests using only CMake CTest.

make test

Usage

Full set of examples is avaiable here. Below there is only quick getting started section

There are two ways of using this library, first is using straight library API and the second is by calling Macro which calls library API. Both ways will be presented, choose which one is better for you, yet both will use the same test class to access its data.

class Test
{
  void foo() { std::cout << "private method: Foo" << '\n'; }

  int getSum(int first, int second ) const { return first + second; }

  template<typename T>
  T max(T& lhs, T& rhs) { return (lhs > rhs) ? lhs : rhs; }
  
  int mFooBar {1};
};

Macroless

The first step for accessing private data, is to create own type or an alias which will be further passed for accessing functions.

struct TestFoo : ::accessor::FunctionWrapper<Test, void> {};
struct TestFooBar : ::accessor::MemberWrapper<Test, int> {};

using TestFoo = ::accessor::FunctionWrapper<Test, void>;
using TestFooBar = ::accessor::MemberWrapper<Test, int>;

Accessing multiple members with the same type is only possible with the definition of a struct. FunctionWrapper takes types <BaseClass, FunctionReturnType, FunctionArgsTypes...> MemberWrapper works similar to FunctionWrapper it takes types <BaseClass, MemberType>

Then explicit template instantantion of created type TestFoo and TestFooBar shall be used

using TestFoo = ::accessor::FunctionWrapper<Test, void>;
template class ::accessor::MakeProxy<TestFoo, &Test::foo>;

using TestFooBar = ::accessor::MemberWrapper<Test, int>;
template class ::accessor::MakeProxy<TestFooBar, &Test::mFooBar>;

MakeProxy takes as parameters <CreatedType, MemberAddress>

After those steps we can simply call private foo method of class Test

Test t;
::accessor::callFunction<TestFoo>(t);
auto ref = ::accessor::accessMember<TestFooBar>(t); \\ return std::ref(Test::mFooBar)

callFunction takes any Test object which has Test::foo method to call, and variadic arguments needed for that method. accessMember works as callFunction yet it return std::ref to private data member of class Test

Examples of instantion and calling for rest of Test class methods.

using TestGetSum = ::accessor::ConstFunctionWrapper<Test, int, int, int>;
template class ::accessor::MakeProxy<TestGetSum, &Test::getSum>;

template<typename T>
using TestMax = ::accessor::FunctionWrapper<Test, T, T&, T&>;
template class ::accessor::MakeProxy<TestMax<int>, &Test::max>;
template class ::accessor::MakeProxy<TestMax<uint32_t>, &Test::max>;

....

int main()
{
  int result = ::accessor::callFunction<TestGetSum>(t, 1, 1);
  result = ::accessor::callFunction<TestMax<int>>(t, 10, 20);
  uint32_t res = ::accessor::callFunction<TestMax<uint32_t>>(t, 100u, 200u);
  return 0;
}

With Macros

If you are not happy with the need of explicitly calling template instantiation, then there are also macros which does it for you

FUNCTION_ACCESSOR(TestFoo, Test, foo, void) // Test::foo
CONST_FUNCTION_ACCESSOR(TestSum, Test, getSum, int, int, int) //Test::getSum
FUNCTION_ACCESSOR(TestMaxInt, Test, max, int, int&, int&) //Test::max<int>
FUNCTION_ACCESSOR(TestMaxUInt, Test, max, uint32_t, uint32_t&, uint32_t&) //Test::max<uint32_t>
MEMBER_ACCESSOR(TestFooBar, Test, mFooBar, int) // Test::mFooBar

Below is what those Macros shall be satisfied with

FUNCTION_ACCESSOR(TypeForAccessing, BaseClass, nameOfMemberToAccess, returnType, functionArgumentTypes...)
CONST_FUNCTION_ACCESSOR(TypeForAccessing, BaseClass, nameOfMemberToAccess, returnType, functionArgumentTypes...)
MEMBER_ACCESSOR(TypeForAccessing, BaseClass, nameOfMemberToAccess, MemberType)

The rest is done the very same way as it was done without macros, using callFunction and accessMember

Full code

FUNCTION_ACCESSOR(TestFoo, Test, foo, void) // Test::foo
CONST_FUNCTION_ACCESSOR(TestSum, Test, getSum, int, int, int) //Test::getSum
FUNCTION_ACCESSOR(TestMaxInt, Test, max, int, int&, int&) //Test::max<int>
FUNCTION_ACCESSOR(TestMaxUInt, Test, max, uint32_t, uint32_t&, uint32_t&) //Test::max<uint32_t>
MEMBER_ACCESSOR(TestFooBar, Test, mFooBar, int) // Test::mFooBar

int main()
{
  accessor::callFunction<TestFoo>(t); //Test::foo
  int result = ::accessor::callFunction<TestGetSum>(t, 1, 1); //Test::getSum
  result = ::accessor::callFunction<TestMax<int>>(t, 10, 20); //Test::max<int>
  uint32_t res = ::accessor::callFunction<TestMax<uint32_t>>(t, 100u, 200u); //Test::max<uint32_t>
  
  auto ref = accessor::accessMember<TestFooBar>(t); //Test::mFoobar
  ref.get() = 200; //change mFooBar value;
  return 0;
}

License

MIT License

hliberacki/cpp-member-accessor