How do I access the internal contiguous buffer of a std::vector and can I use it with memcpy, etc?

Question

How can I access the contiguous memory buffer used within a std::vector so I can perform direct memory operations on it (e.g. memcpy)? Also, it is safe to perform operations like memcpy on that buffer?

I've read that the standard guarantees that a vector uses a contiguous memory buffer internally, but that it is not necessarily implemented as a dynamic array. I figure given that it is definitely contiguous, I should be able to use it as such - but I wasn't sure if the vector implementation stored book-keeping data as part of that buffer. If it did, then something like memcpying the vector buffer would destroy its internal state.

Answer 1

In practice, virtually all compilers implement vector as an array under the hood. You can get a pointer to this array by doing &somevector[0]. If the contents of the vector are POD ('plain-old-data') types, doing memcpy should be safe - however if they're C++ classes with complex initialization logic, you'd be safer using std::copy.

Answer 2

Simply do

&vec[0];

// or Goz's suggestion:
&vec.front();

// or
&*vec.begin();
// but I don't know why you'd want to do that

This returns the address of the first element in the vector (assuming vec has more than 0 elements), which is the address of the array it uses. vector storage is guaranteed by the standard to be contiguous, so this is a safe way to use a vector with functions that expect arrays.

Be aware that if you add, or remove elements from the vector, or [potentially] modify the vector in any way (such as calling reserve), this pointer could become invalid and point to a deallocated area of memory.