Resizing array of vector

Question

class graph
{
    vector<int > a[];
    int nodes,edges;

public:
    graph(int n,int m)
    {
        nodes=n;edges=m;
        a = new vector<int> [n];
    }
};

This is a snippet from my code. How do I resize the array of vector<int>? I tried to dynamically assign the size. But its giving error.

@Cheersandhth.-Alf Actually it is in some context, but in this case it declares a variable of "an incomplete object type". — Holt
– Holt, Commented Jul 1, 2016 at 6:33
@Holt: It's good to be aware of contexts, but in your answer you write "you cannot declare object of incomplete type", which isn't sufficiently context-qualified for the most pedantic, I think. :) — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented Jul 1, 2016 at 6:52
@Cheersandhth.-Alf I am not aware of context where it is allowed to define object with incomplete type - Are there any? The goal of my note was to point out that this is not a valid declaration in this context (there may be context where it is, but I am not going to list them, it would be useless here... ) and to explain why it compiles on gcc (and thus why it raises an error when assigning from new ...). — Holt
– Holt, Commented Jul 1, 2016 at 6:59

Holt · Accepted Answer · 2016-07-01 06:41:06Z

Use std::vector<std::vector<...>> instead of a "raw" array of std::vector<...> and then use .resize():

std::vector<std::vector<int>> a;
a.resize(n);

This will save you from having to write a lots of boilerplate code (custom destructor, copy-constructor, ...), and will be much less error prone than your actual code.

The actual problem in your code is that vector<int> a[] should not be valid in this context: it declares a variable a of "derived-declarator-type-list array of unknown bound of int”, which is "an incomplete object type", and you cannot declare object of incomplete type. On gcc, if you add -pedantic, you will get a warning:

warning: ISO C++ forbids zero-size array 'a' [-Wpedantic]

But without -pedantic, it declares a std::vector<int> a[0], which cannot be assigned a std::vector<int> * and it is why you get an error when assigning new std::vector<int> [n].

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