Is it okay to use constructors to initialize a 2D Vector as a one-liner in C++?

Question

Is it okay to initialize a 2D vector like this (here all values in a 5x4 2D vectro are initialized to 3)?

std::vector<std::vector<int> > foo(5, std::vector<int>(4, 3));

This seems to behave okay, but everywhere I look on the web people seem to recommend initializing such a vector with for loops and push_back(). I was initially afraid that all rows here would point to the same vector, but that doesn't seem to be the case. Am I missing something?

Answer 1

This is perfectly valid - You'll get a 2D vector ([5, 4] elements) with every element initialized to 3.

For most other cases (where you eg want different values in different elements) you cannot use any one-liner - and therefore need loops.

Answer 2

Well, the code is valid and it indeed does what you want it to do (assuming I understood your intent correctly).

However, doing it that way is generally inefficient (at least in the current version of the language/library). The above initialization creates a temporary vector and then initializes individual sub-vectors by copying the original. That can be rather inefficient. For this reason in many cases it is preferrable to construct the top-level vector by itself

std::vector<std::vector<int> > foo(5);

and then iterate over it and build its individual sub-vectors in-place by doing something like

foo[i].resize(4, 3);