C++: Problems with a vector of vectors of structs

Question

I need a variable size 2-D matrix where each element is a struct. The number of rows is known at compile time and never changes. The number of columns is different for each row. I implemented this with a vector of vectors, but I don't know if I did it right:

struct MyStruct {
    int a, b;
    MyStruct(int arg_a, int arg_b) { a = arg_a; b = arg_b; };
};

int main(void) {
    vector < vector< MyStruct > > myVVOfStructs;
    myVVOfStructs.reserve(10);    // Intended to reserve 10 rows of variable size vectors

    vector< MyStruct > tmpVector = myVVOfStructs[5];
    tmpVector.push_back(MyStruct(10, 20));

}

Is this the right way to do it? The above snippet works when I put it in a C++ file by itself. However, in my much larger C++ program, the above fails with this message

terminate called after throwing an instance of 'std::bad_alloc'

So, there must be a problem with the way I'm doing it, but it's complicated since it works in a simple C++ source file, but not in a larger more complicated program. Any ideas? Is there a better way to do it?

LATER EDIT: There's an answer to this question in another question titled "When should we use reserve() of vector?". The problem is that in order to find that answer, I'd already have to know that reserve() was the cause of the problem. As it stands, I never would have read the page with that title since I did not know reserve() was the cause of my problems. If I already knew that, then I would have known enough to figure the problem out by myself.

By itself it doesn't make your program run out of memory. So I assume something else in you larger program is very resource hungry. — StoryTeller - Unslander Monica, Aug 29 '16 at 13:12
My c++ knowledge is a little rusty, but how do you call myVVOfStructs.reserve(10) when you haven't instantiated the vector, but only declared it? — Kokozaurus, Aug 29 '16 at 13:14
Also, you can use uniform initialization syntax like `std::vector> vv = { { {10, 20}, {30, 40}, }, };`, if you use c++11 — Semyon Burov, Aug 29 '16 at 13:36

score 6 · Accepted Answer · answered Aug 29 '16 at 13:14

6

The method:

myVVOfStructs.reserve(10);

does not change the size of the vector itself. Reserve - Vector:

This function has no effect on the vector size and cannot alter its elements.

So, when you're trying to access the 6-th element with:

vector< MyStruct > tmpVector = myVVOfStructs[5];

That will produce an UB, because the myVVOfStructs[5] does not exists yet.

In order to change the size of the vector you should use the method:

mmVVOfStruct.resize(10);

answered Aug 29 '16 at 13:14

BiagioF

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Yup. That's it. I should have done resize(), not reserve(). Bonehead move on my part. Thanks! – JB_User Aug 29 '16 at 13:17
@JB_User also beware that `tmpVector` is a copy of `myVVOfStructs[5]`. You would want to use a reference instead if you intend for that `push_back` to modify `myVVOfStructs`. – crashmstr Aug 29 '16 at 13:19

score -1 · Answer 2 · answered Aug 29 '16 at 13:13

Request a change in capacity Requests that the vector capacity be at least enough to contain n elements.

If n is greater than the current vector capacity, the function causes the container to reallocate its storage increasing its capacity to n (or greater).

In all other cases, the function call does not cause a reallocation and the vector capacity is not affected.

This function has no effect on the vector size and cannot alter its elements.

You'd use std::vector::resize

Consider rewording this reply if you can, it's hard to read. — David, Aug 29 '16 at 13:20

C++: Problems with a vector of vectors of structs

2 Answers2