Access a vector inside a struct that is inside a class

Question

I'm getting a runtime error when trying that scenario, and I get vector subscript out of range

struct Mesh
{
  std::vector<Vertex> mVertices;
}

class Grid
{

public:
Mesh mMesh;
}

Grid g_Grid;

g_Grid.mMesh.mVertices.reserve(40);
g_Grid.mMesh.mVertices[0] = 10; // allocation error

what do you mean I didn't put anything? I have reserves a 40 spaces and and I need to put element 0 to be 10 — Ahmed Saleh, Aug 16 '14 at 20:40
@andre let me reiterate, `reserve()` is not `resize()`. You need to **read the documentation** for those two methods before complaining. — The Paramagnetic Croissant, Aug 16 '14 at 20:41
No need to negate my question!. It was just a typo, I know the difference :S — Ahmed Saleh, Aug 16 '14 at 20:42

Vlad from Moscow · Accepted Answer · 2014-08-16T20:49:57.717

Method reserve does not create elements of a vector. it only reserves memory for elements that can be added to the vector in future with for example method push_back

So write instead

g_Grid.mMesh.mVertices.reserve(40);
g_Grid.mMesh.mVertices.push_back( 10 );

Or if you want that the elements would already exist then write

g_Grid.mMesh.mVertices.resize(40);
g_Grid.mMesh.mVertices[0] = 10;

If you want to add at once several elements to the vector that does not yet contain any elements then you can write

g_Grid.mMesh.mVertices.reserve(40);
g_Grid.mMesh.mVertices = { 10, 20, 30 };

Access a vector inside a struct that is inside a class

1 Answers1