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c++vectormacrostypedef

What does this declared vector in C++ mean?

I'm fairly new to C++, so I'm having a hard time understanding some complex terms such as Macros and Preprocessors. The code that I can't understand is attached below:

#define NP 2
#define MAX 2000

typedef pair<int, double> edge;

vector<edge> vec[NP][MAX];

What I can't understand here is, what does the vector really store and what does [NP][MAX] determine?

Solution

  • There is not "a vector" — there are many vectors. Many!

    Just as int foo[5] creates an array of five ints, and int foo[5][10] creates a two-dimensional array of fifty ints total…

    … vector<edge> vec[5][10] creates an array of fifty vector<edge>s.

    Now add on the macros: that's just text substitution (more or less), so the array dimensions are being provided via a constant name rather than right there as numbers in the declaration.

    So it's a vector<edge> vec[2][2000], or four thousand vector<edge>s.

    The type edge is an alias for pair<int, double> (thanks, typedef!), so it's really:

    vector<pair<int, double>> vec[2][2000].

    This is probably not what was actually intended. Though there is nothing illegal about arrays of vectors, they're a bit of a "double-positive". The vector is already an array-like thing, and it seems likely that the author wanted to create a sort of "two-dimensional vector", but did it wrong. We cannot know this for sure, though, without asking them.