In C++ one can create an array of predefined size, such as 20, with int myarray[20]. However, the online documentation on vectors doesn't show an alike way of initialising vectors: Instead, a vector should be initialised with, for example, std::vector<int> myvector (4, 100);. This gives a vector of size 4 with all elements being the value 100.
How can a vector be initialised with only a predefined size and no predefined value, like with arrays?
With the constructor:
// create a vector with 20 integer elements
std::vector<int> arr(20);
for(int x = 0; x < 20; ++x)
arr[x] = x;
std::vector<int> arr = std::vector<int> (20); worked fine for me, specifically stating that I want to use the constructor for vector.
arr.at(x)=x to check for out_of_bounds errors. arr[x]=x will fail silently if some mistake occurs.
at() does the checking and will throw an exception for an out of bounds error, while [] does no checking.
std::vector<type> v_name = std::vector<type>(size) Or alternatively define std::vector<type> v_name and initialise the size in the constructors by just using class() : v_name(size) {}.So sometimes you'd want to do this to preallocate a static buffer, and for some reason this question is the first Google result for that task... So
static std::vector<int> buffer = [](){ std::vector<int> buffer; buffer.reserve(1000); return buffer; }();
So in this case the items in the vector will be in an unitialized state but you won't pay the direct memory allocation cost of growing the vector until you hit 1000 elements
reserve only changes the capacity of the vector, no item is added: godbolt.org/z/GxhqoahsG