Initializing a Vector of Vectors Having a Fixed Size with Boost Assign

Initializing a vector of vectors of ints in C++17

You can use squiggly brackets to let the compiler know you're trying to call a constructor:

std::vector<std::vector<int>> sudoku_field{9, std::vector<int>(9, 0)};

Alternatively, you could do this work in the initialization list of your default constructor:

SudokuSolver() : sudoku_field(9, std::vector<int>(9, 0)) {}

And then run your default constructor from every new constructor you make to ensure that gets set:

SudokuSolver(int thing) : SudokuSolver() { }

How to initialize a vector of vectors on a struct?

You use new to perform dynamic allocation. It returns a pointer that points to the dynamically allocated object.

You have no reason to use new, since A is an automatic variable. You can simply initialise A using its constructor:

vector<vector<int> > A(dimension, vector<int>(dimension));

What is the easiest way to initialize a std::vector with hardcoded elements?

One method would be to use the array to initialize the vector

static const int arr[] = {16,2,77,29};
vector<int> vec (arr, arr + sizeof(arr) / sizeof(arr[0]) );

Creating a vector of fixed sized queue (boost circular queue)

You want a vector of queues:

#include <boost/circular_buffer.hpp>

struct pkt { int data; };

int main() {
    // Boost Circular Queue -- This works fine
    typedef boost::circular_buffer<pkt> pkt_queue;

    pkt_queue a_queue(3);

    // Vector of queues - This has errors, i also wish to initialize the vector
    std::vector<pkt_queue> per_port_pkt_queue;

    per_port_pkt_queue.emplace_back(3);
    per_port_pkt_queue.emplace_back(3);
    per_port_pkt_queue.emplace_back(3);

    // or
    per_port_pkt_queue.assign(20, pkt_queue(3)); // twenty 3-element queues
}

See it Live On Coliru

Constant-sized vector

The std::vector can always grow dynamically, but there are two ways you can allocate an initial size:

This allocates initial size and fills the elements with zeroes:

std::vector<int> v(10);
v.size(); //returns 10

This allocates an initial size but does not populate the array with zeroes:

std::vector<int> v;
v.reserve(10);
v.size(); //returns 0

syntax failure with vector of vectors

I will guess here that you are forgetting to tell us that you are trying to declare a data member for a class. So you are really trying to compile something like:

struct A {
    vector <vector <int> > v(10, vector <int>(10,1));       
};

and

struct A {
    vector <vector<int> > v = vector <vector<int> >(1, vector<int>(1, 0));
};

A data member can only be initialized in a class definition with an equal sign or with braces. The initialization with parentheses is not allowed.

Try:

struct A {
    vector<vector<int>> v{10, vector<int>(10,1)};        
};

In any case you need at least C++11 for the brace-initialization, as well as non-static member initialization in the declaration (your second error where you use equals instead of parentheses, but with older C++ standard).

Initialize multidimensional vector with fixed value

You were close:

std::vector<std::vector<int>> vec(10, std::vector<int>(15, fixed_value));
//                                                         ^^^^^^^^^^^

Live demo

initialize 3D std::vector with size and value

It's the same thing as a 1D vector, but you have to change the parameters a bit:

vector<vector<vector<int>>> vsfl(200, vector<vector<int>>(200, vector<int>(200, 1)));

Because the std::vector constructor takes T as the second argument and it's a 3D vector, so you have to call Ts constructor if you want to have it initialized.

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