Vector is not a member of std with everything included

Question

I'm getting a very strange error and something I've never experienced before, I'm trying to initialize a vector at declaration like so:

  vector <int> myVector (5,4,3,4); 

It gives me an error saying that it cannot find a call matching that function, however, if I plug in only 2 numbers it doesn't give me an error.

Upon further investigation, even this piece of code throws that vector is not a member of std or that myVector is not a type when I try to call

myVector.push_back(4);

Now here is the code that gives me the vector is not a member of std, not matching function found, of that nature...

#include <vector>

using std::vector;

const std::vector<int> newvector;
std::newvector.push_back(5);

int main()
{
}

Error given: newvector in namespace std does not name a type

Solution 1

Regarding the final code example,

you can't have this statement outside a function body:

std::newvector.push_back(5);

Place it in main's function body, and remove the std:: prefix.

Also note that for this vector modification to be possible, the vector can't be const.

---

Initialization with round parentheses provides arguments to a constructor for the class. There must be a corresponding constructor. And there is no std::vector constructor that corresponds to the four arguments in

vector <int> myVector (5,4,3,4); 

Instead do

vector <int> myVector = {5,4,3,4};

If your compiler supports this C++11 syntax, that is.

If not, then the C++03 way would be

static int const data[] = {5,4,3,4};
static int const n_items = sizeof(data)/sizeof(data[0]);

vector <int> myVector( data, data + n_items );

Solution 2

std::newvector.push_back(5);

since newvector is a variable, it is not in namespace std.

For this case:

vector <int> myVector (5,4,3,4); 

If you only use two values (5,4) in this case, it will create a vector with size 5 and each of the elements initialized to 4 (one form of vector construction). If you need to initialize a vector with a list of values, you may try uniform initialization since C++11 as follows:

std::vector <int> myVector{5,4,3,4};

Some example of uniform initialization can be found here: uniform initialization and other C++11 features

Solution 3

There are two issues with this code.

1. using std::vector; brings a vector from namespace std but you have to use this as follows:

   std::vector<int> newvector;
   newvector.push_back(5);
   

2. You can notice that const dissapeared. This is because you want to change vector at last, so it cannot be const, it would be compile error to call a push_back on it otherwise.

Finally, this should be:

#include <vector>
using std::vector;

int main() {
  std::vector<int> newvector;
  newvector.push_back(5);
  std::vector<int> newvector;
  return 0;
}

Solution 4

The second error, you cannot initialize a vector with its elements in the constructor, you must use its initializer list (C++11 only)

vector <int> myVector = {5,4,3,4};  

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