How To Use Vectors in OpenGL VBO [duplicate] - c++

Question

I'm trying to pass a vector into the last argument of glDrawElements(). If I used array, it worked fine. However, when I switched to a vector, it only rendered a portion of the object.
This worked just fine:
//TA_CartesianSys.h
class TA_CartesianSys
{
private:
int drawOrder[86];
//the rest of the class
}
//---------------------------------
//TA_CartesianSys.cpp
TA_CartesianSys::TA_CartesianSys()
{
GLfloat CartesianVertices[] = { ... };
int tempOrder[] = { ... };
for(int i = 0; i < sizeof(tempOrder) / sizeof(int); i++)
{
drawOrder[i] = tempOrder[i];
}
//Code to generate and bind vertex array and buffer
glDrawElements(GL_LINES, sizeof(drawOrder)/sizeof(int), GL_UNSIGNED_INT, drawOrder);
}
It worked as expected, and this was how it looked like:
Now, I decided to use vector instead of array for drawOrder[]. This is the new code:
//TA_CartesianSys.h
class TA_CartesianSys
{
private:
std::vector<int> drawOrder; //***NOTE: This is the change
//the rest of the class
}
//---------------------------------
//TA_CartesianSys.cpp
TA_CartesianSys::TA_CartesianSys()
{
GLfloat CartesianVertices[] = { ... };
int tempOrder[] = { ... };
drawOrder.resize(sizeof(tempOrder) / sizeof(int));
for(int i = 0; i < (sizeof(tempOrder) / sizeof(int)); i++)
{
drawOrder[i] = tempOrder[i];
}
//Code to generate and bind vertex array and buffer - Same as above
glDrawElements(GL_LINES, sizeof(drawOrder)/sizeof(int), GL_UNSIGNED_INT, &drawOrder[0]);
}
And this was what I got when I ran the program:
NOTE: the square in the middle was not part of this object. It belonged to a totally different class.
So, basically, when I changed the drawOrder[] to vector instead of array, only a small part of my object was rendered (the 2 lines). The rest were no seen.
I put a break point right at the draw() function, and it showed that the drawOrder vector was initialized properly, with the exact same value as its array counter part.
So, why am I only getting 2 lines instead of the whole grids? What am I missing?

Answer 1

sizeof on a vector object just tells you the size of the vector class instance, not the number of elements. What you want is the size member fucction of std::vector, which tells you the number of elements (i.e. you must not divide the number returned by std::vector::size by the element sizeof).
glDrawElements(
GL_LINES,
drawOrder.size(), // <<<<<
GL_UNSIGNED_INT,
&drawOrder[0] );