You might think that a std::string (and all of its friends in the std::basic_string family) are basically a vector of characters internally. But strings are organized differently due to specific optimizations permitted for strings but not for vectors.

Inside STL: The string

By Raymond Chen

From the article:

The starting point for a std::basic_string is this:¹

template<typename T>
struct basic_string
{
    T* ptr;
    size_t size;
    size_t capacity;
};

The ptr is a pointer to the beginning of the string contents.

The size is the number of characters in the string, not including the null terminator.

The capacity is the string capacity, not including the null terminator.

The picture for this simplified version is as follows: