Using std transform on a C++ string - c++

Question

I am trying to get a bit of practice with std::transform by using it to decrypt a simple Caesar cypher. But apparentrly my function definition is wrong, since the compiler considers the unary function argument invalid. Here is my code:
char CryptoModule::UndoCaesar(char& letter)
{
return (letter - atoi(key_.c_str()) % 128);
}
void CryptoModule::DecryptCaesar()
{
std::transform(data_.begin(), data_.end(), data_.begin(), UndoCaesar);
}
Could anyone please tell me where the error lies ? In addition, what would I need to modify in order to use for_each ? I believe it would involve changing the return value to a void and storing the result in letter.
Thanks and have a nice day
Edit: Tried adding bind, but still not working. Current call is :
std::transform(data_.begin(), data_.end(), data_.begin(), bind(&UndoCaesar, this, std::placeholders::_1));

Answer 1

Your given member function has a hidden this parameter, so you need to bind that in order to pass it:
std::transform(data_.begin(), data_.end(), data_.begin(),
std::bind(&CryptoModule::UndoCaesar, this, std::placeholders::_1)
);
Note that std::bind is in <functional>. All this does is basically cause std::transform to call whatever function-like object, f, it takes like f(currentElement) and have that in turn call UndoCaesar(this, currentElement) with this being part of its state because you bound it.
Alternatively, you can wrap the call in a lambda that captures this so that it can be used for the member function call (implicitly, like normal):
std::transform(data_.begin(), data_.end(), data_.begin(),
[this](char c) {return UndoCaesar(c);}
);