Resizing a C++ std::vector without initializing data [duplicate]

Answer 1

vector<char> buf;
buf.reserve(N);
int M = read(fd, &buf[0], N);

This code fragment invokes undefined behavior. You can't write beyond than size() elements, even if you have reserved the space.

The correct code is like:

vector<char> buf;
buf.resize(N);
int M = read(fd, &buf[0], N);
buf.resize(M);

---

PS. Your statement "With vectors, one can assume that elements are stored contiguously in memory, allowing the range [&vec[0], &vec[vec.capacity()) to be used as a normal array" isn't true. The allowable range is [&vec[0], &vec[vec.size()).

Answer 2

Another, newer, question, a duplicate of this one, has an answer, which looks like exactly what is asked here. Here's its copy (of v3) for quick reference:

It is a known issue that initialization can not be turned off even explicitly for std::vector.

People normally implement their own pod_vector<> that does not do any initialization of the elements.

Another way is to create a type which is layout-compatible with char, whose constructor does nothing:

struct NoInitChar
{
    char value;
    NoInitChar() {
        // do nothing
        static_assert(sizeof *this == sizeof value, "invalid size");
        static_assert(__alignof *this == __alignof value, "invalid alignment");
    }
};

int main() {
    std::vector<NoInitChar> v;
    v.resize(10); // calls NoInitChar() which does not initialize

    // Look ma, no reinterpret_cast<>!
    char* beg = &v.front().value;
    char* end = beg + v.size();
}

Answer 3

Your program fragment has entered the realm of undefined behavior.

when buf.empty() is true, buf[0] has undefined behavior, and therefore &buf[0] is also undefined.

This fragment probably does what you want.

vector<char> buf;
buf.resize(N); // preallocate space
int M = read(fd, &buf[0], N);
buf.resize(M); // disallow access to the remainder

Answer 4

Writing into and after the size()th element is an undefined behavior.

Next example copies whole file into a vector in a c++ way (no need to know the file's size and no need to preallocate the memory in the vector):

#include <algorithm>
#include <fstream>
#include <iterator>
#include <vector>

int main()
{
    typedef std::istream_iterator<char> istream_iterator;
    std::ifstream file("example.txt");
    std::vector<char> input;

    file >> std::noskipws;
    std::copy( istream_iterator(file), 
               istream_iterator(),
               std::back_inserter(input));
}

Answer 5

It looks like you can do what you want in C++11 (though I haven't tried this myself). You'll have to define a custom allocator for the vector, then use emplace_back().

First, define

struct do_not_initialize_tag {};

Then define your allocator with this member function:

class my_allocator {
    void construct(char* c, do_not_initialize_tag) const {
        // do nothing
    }

    // details omitted
    // ...
}

Now you can add elements to your array without initializing them:

std::vector<char, my_allocator> buf;
buf.reserve(N);
for (int i = 0; i != N; ++i)
    buf.emplace_back(do_not_initialize_tag());
int M = read(fd, buf.data(), N);
buf.resize(M);

The efficiency of this depends on the compiler's optimizer. For instance, the loop may increment the size member variable N times.