github twitter linkedin instagram
How to access private C++ members.
Dec 12, 2013
One minute read

So, I was just thinking about how C++ is really stored in memory, and realized that private class variables are not private at all, in reality.

When a class is stored in memory, all of the member variables are simply stored in a big blob of data. So in theory, if you simply typecast a pointer to a class to another type, pointer arithmetic could be done to access any member variable.

Here is a simple example:


class SeeminglyImmutableInteger {
	int value;
	SeeminglyImmutableInteger(int value) {
		this->value = value;

int main() {
	SeeminglyImmutableInteger* notSoImmutableInteger = new SeeminglyImmutableInteger(10);
	int* privateInt = (int*)notSoImmutableInteger;

	std::cout << *privateInt << std::endl;
	(*privateInt) *= 42;
	std::cout << *privateInt << std::endl;

And as you can see, the so called ‘immutable’ integer technically isn’t immutable.

% g++ single.cpp && ./a.out

Another way to do the same thing is to create a struct with the same internal data types. For instance:

struct MutableInteger {
	int value;

Then, all you have to do is typecast the SeemingleImmutableInteger into a MutableInteger.

Of course this shouldn’t ever be used in practice, but I thought it was interesting.

Back to posts