Converting from std::vector<> to a double pointer?

Question

I was wondering out of curiosity if it is possible to cast a std::vector<> to a double pointer.

I've never had an issue passing a std::vector as a pointer in this fashion:

std::vector<char> myCharVector;
myCharVector.push_back('a');
myCharVector.push_back('b');
myCharVector.push_back('c');
char *myCharPointer = &myCharVector[0];

So I was curious if it was possible to assign the address of the pointer in a similar way to this:

char *myPointer = "abc";
char **myDoublePointer = &myPointer; 

I've tried:

char **myDoublePointer = (char**)&myCharVector;

But it doesn't work. Is there any way of achieving this?

Answer 1

You already know that &myCharVector[0] is a pointer to char. So if you store it in a variable:

char *cstr = &myCharVector[0];

then you can take the address of that variable:

char **ptrToCstr = &cstr;

But simply dereferencing twice like this:

char **ptrToCstr = &(&myCharVector[0])

is invalid because the value (&myCharVector[0]) isn't stored in memory anywhere yet.

Answer 2

In C++11, you can do:

char *myCharPointer = myCharVector.data();

But you cannot take the address of the return value of data() because it does not return a reference to the underlying storage, just the pointer value.

If the purpose is to be able to change what the pointer is pointing to, then you may really want a pointer to a vector, rather than a pointer to a pointer to a char. But, the STL doesn't let you change the underlying pointer within the vector itself without going through the regular vector APIs (like resize or swap).