why can't I return local variable? - function

Question

I don't know why c++'s rule says that I can't return local variable?
this simple "max" function totally has no problem;
int max(int a,int b)
{
int maxnum;
if(a>b)maxnum= a;
else maxnum= b;
return maxnum;
}
int main( int argc, char** argv )
{
cout<<max(10,30);
system("PAUSE");
return 0;
}
but, when I want to return local variable as before in opencv,it turns out wrong data
why can't I return local variable here?
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include <sstream>
using namespace cv;
using namespace std;
Mat GetIntensity(Mat *pic,int y,int x)
{
float Array[6];
for(int i=0;i<6;i++)
{
Array[i]=pic[i].at<uchar>(y,x);
}
Mat Intensity(6,1,CV_32FC1,&Array);
cout<<Intensity;
return Intensity;
}
int main( int argc, char** argv )
{
stringstream ss;
string pic_name;
Mat pic[6];
for(int i=0;i<6;i++)
{
ss.clear();
ss.str(std::string());
ss<<"pic"<<i<<".bmp"<<endl;
ss>>pic_name;
pic[i] = imread(pic_name, CV_LOAD_IMAGE_GRAYSCALE);
if(! pic[i].data )
{// Check for invalid input
cout << "Could not open or find the image" << endl ;
system("pause");
return -1;
}
//namedWindow( pic_name, CV_WINDOW_AUTOSIZE );
// Create a window for display.
//imshow(pic_name, pic[i] );
}
cout<<=GetIntensity(pic,60,60);
system("PAUSE");
//waitKey(0);
return 0;
}

Answer 1

your construct here uses a local array to hold the values. this will get invalid, once your Mat leaves the scope:
Mat GetIntensity(Mat *pic,int y,int x)
{
float Array[6]; //problem
for(int i=0;i<6;i++)
{
Array[i]=pic[i].at<uchar>(y,x);
}
Mat Intensity(6,1,CV_32FC1,&Array); // problem
cout<<Intensity;
return Intensity;
}
better create a Mat with it's own data:
Mat GetIntensity(Mat *pic,int y,int x)
{
Mat Intensity(6,1,CV_32FC1); // no problem
for(int i=0;i<6;i++)
{
Intensity.at<float>(i) = pic[i].at<uchar>(y,x);
}
cout<<Intensity;
return Intensity;
}