The function glMultiDrawElements does not work correctly - opengl

Question

I want to render a simple cube using the function 'glMultiDrawElements' (even if there is just one object!) but there is a bad display! However if I use the function 'glDrawElements' everything works! I want to precise 'glewInit' has been correctly initialized.
Here's the code using 'glDrawElements' :
[...]
std::vector<std::vector<int32_t>> indexArray = pRenderBatch->GetIndexAttribArray();
std::vector<int32_t> countArray = pRenderBatch->GetCountElementArray(indexArray);
// indexArray[0] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 0, 2, 19, 3, 5, 20, 21, 22, 10, 23, 11, 13, 24, 14, 25, 15, 17}
//Size(indexArray) = 1, Size(indexArray[0])=36
// countArray = {36} //Size(countArray)=1
glDrawElements(GL_TRIANGLES, countArray[0], GL_UNSIGNED_INT, (const GLvoid *)&indexArray[0][0]);
[...]
Now, here's the code using 'glMultiDrawElements' :
[...]
std::vector<std::vector<int32_t>> indexArray = pRenderBatch->GetIndexAttribArray();
std::vector<int32_t> countArray = pRenderBatch->GetCountElementArray(indexArray);
glMultiDrawElements(GL_TRIANGLES, &countArray[0], GL_UNSIGNED_INT, (const GLvoid **)&indexArray[0], countArray.size());
[...]
Here's the display:
However according the OpenGL documentation (http://www.opengl.org/sdk/docs/man3/xhtml/glMultiDrawElements.xml), the function 'glMultiDrawElements' has the following signature:
void glMultiDrawElements(GLenum mode, const GLsizei * count, GLenum type, const GLvoid ** indices, GLsizei primcount);
I don't understand why the display is not correct. Does anyone can help me?

Answer 1

std::vector<std::vector<int32_t>> indexArray
glMultiDrawElements() is expecting a pointer to an array of indexes. It doesn't know anything about the layout of std::vector<int32_t>.
Try this instead:
std::vector< int32_t* > indexArray2
You can construct it like this:
std::vector< int32_t* > indexArray2( indexArray.size() );
for( size_t i = 0; i < indexArray.size(); ++i )
{
indexArray2[i] = &indexArray[i][0];
}
EDIT: Complete example:
#include <GL/glew.h>
#include <GL/glut.h>
#include <vector>
using namespace std;
void display()
{
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
glOrtho( -2, 2, -2, 2, -1, 1 );
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
GLfloat verts[] =
{
0, 0,
1, 0,
1, 1,
0, 0,
-1, 0,
-1, -1,
};
vector< GLsizei > counts;
vector< vector< unsigned int > > indexes;
{
vector< unsigned int > index;
index.push_back( 0 );
index.push_back( 1 );
index.push_back( 2 );
counts.push_back( index.size() );
indexes.push_back( index );
}
{
vector< unsigned int > index;
index.push_back( 3 );
index.push_back( 4 );
index.push_back( 5 );
counts.push_back( index.size() );
indexes.push_back( index );
}
vector< unsigned int* > indexes2( indexes.size() );
for( size_t i = 0; i < indexes.size(); ++i )
{
indexes2[i] = &indexes[i][0];
}
glEnableClientState( GL_VERTEX_ARRAY );
glVertexPointer( 2, GL_FLOAT, 0, verts );
glMultiDrawElements
(
GL_TRIANGLES,
&counts[0],
GL_UNSIGNED_INT,
(const GLvoid **)&indexes2[0],
counts.size()
);
glutSwapBuffers();
}
int main( int argc, char **argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
glutInitWindowSize( 600, 600 );
glutCreateWindow( "GLUT" );
glewInit();
glutDisplayFunc( display );
glutMainLoop();
return 0;
}