CODEWITHSUNDEEP
c++stdvector
jsj
jsj

Reputation: 9391

Can I have a variably modified vector in c++?

Is there any way to do this:

std::vector<char[foo]> bar;

I need ONE contiguous array of char[foo] but I don't know the length until runtime. I need to be able to add blocks of char[foo] dynamically.

I need the inner array to be exactly the right size, so if foo was 3:

[ [1, 2, 3] [4, 5, 6] ... [x, y, z] ]

I could implement it myself but don't want to reinvent the wheel.

Upvotes: 3

Views: 151

Answers (4)

egur
egur

Reputation: 7970

After the question was reworded, you're probably looking for std::vector< std::array<char, 3> >

Memory will be contiguous for all elements. Assuming the last dimentsion is indeed a build time const.

Example:

std::vector< std::array<char, 3> > bar;
std::array<char, 3> foo = { 'a', 'b', 'c' };
bar.insert(bar.end(), 10, foo);

Upvotes: 0

MSalters
MSalters

Reputation: 179917

I'd create a container class myself, use std::vector<char> internally, and create a wrapper slice class which holds a pointer to the container class and an index into the vector. The container class would hold the length foo. The container would have an iterator returning slices, each slice would have an iterator returning char.

[edit] This solution also allocates contiguous memory.

Upvotes: 3

Kiril Kirov
Kiril Kirov

Reputation: 38173

I'd suggest using 

std::vector< std::vector< char > >

std::vector's internal memory is continuous.

Well, after a while, it appeared, that the whole memory must be continuous, which changes everything..

For this case, you can use directly std::vector< char >::insert( pos, begin, end ) to insert whole "chunks".

Something like: (I didn't test it, but you'll get the idea)

std::vector< char > bar;
// ..
{
    std::vector< char > buff( foo );
    // fill buff
    bar.insert( bar.end(), buff.begin(), buff.end() );
}

Only keep in mind, that this may/will cause reallocation of the internal memory and several copies of the elements, but we're talking about char, so it's not a big deal.

You can avoid these reallocations using std::vector::reserve, if you have idea about the final size of bar.

Upvotes: 5

Yaspoon
Yaspoon

Reputation: 1

std::vector<char> bar(foo)

Constructs foo chars with in the vector, but because char is a built in type the constructor won't be called so it will be garbage values until initialised.

Instead use std::vector<char> bar(foo, char()) to fill it with initialised chars.

Upvotes: -3

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