C ++向量 <vector<vector<char> &gt;&gt; myVector-如何初始化大小？

Question

With a 1D vector, I can use something like this:

vector<char> letters(5).

How can I accomplish the same thing while initializing a 3D vector?

Here's what I've tried:

vector<vector<vector<char> row(farms[0][0].size())> farm(farms[0].size())> 
   path(farms.size());

That's a bit tricky to parse, but it's just the first example nested thrice and using the same dimensions as the farms 3D vector. It produces these errors:

p1.cpp:109: error: template argument 1 is invalid p1.cpp:109: error: template argument 2 is invalid p1.cpp:109: error: template argument 1 is invalid p1.cpp:109: error: template argument 2 is invalid p1.cpp:109: error: invalid type in declaration before '(' token

Answer 1

vector<vector<vector<char> row(farms[0][0].size())> is clearly not a type.

The type you're after is the type you wrote in the title:

vector<vector<vector<char> > >

Now, if you want to pre-fill each dimension so that it's a 5×5×5 vector from the outset:

vector<vector<vector<char> > > letters(
  5,
  vector<vector<char> >(
    5,
    vector<char>(
      5,
      '\0'
    )
  )
);

At each nested level, the first argument is the number of desired elements, and the second is the value with which to fill each of those elements.

I think it's clear that this is not good code. Ask yourself again whether you really need three dimensions and, even if you do, consider a 1D vector of size 5×5×5 , with 3D indexing simulated over the top with a wrapper type. This is only inappropriate if your matrix won't always be square, but otherwise has — among other benefits — the property of vastly reduced dynamic allocations and locality-busting indirections.

Answer 2

I guess, if that is necessary, you'll have to initialize it by iterating over the dimensions, ie

typedef std::vector<std::vector<std::vector<char > > > vector3d;

vector3d letters(5);

for(int i=0; i<letters.size(); ++i) {
    letters[i].resize(5);
    for(int j=0; j<letters[i].size(); ++j) {
        letters[i][j].resize(5);
    }
}

something like that. Please note, that did not test that code, so there might be small errors.

Answer 3

typedef std::vector<std::vector<std::vector<char>>> threeVec;
static constexpr unsigned int SIZE = 5;

...

threeVec tv;
tv.reserve(SIZE);
for (auto & i : tv) {
    i.reserve(SIZE);
    for (auto & j : i) {
        j.reserve(SIZE);
    }
}