cv::Mat assignment or copy with guarantee that there is no reallocation

edit

use clone method.

what do you mean by "my case that would be a programming error"?

clone is reallocation and not copy, so it doesn't apply.

I actually had a bug in the example code. I fixed and edited to make my intent more clear.

An example can be reproduced. i cannot with your example. I understand your problem like this :

    Mat a(6, 6,CV_8UC1,Scalar(1));
    Mat b(7, 7, CV_32FC1,Scalar(3.5));
    cout << a << "\n";
    cout << b<< "\n";
    Rect r1(1, 1, 4, 4);
    Rect r2(1, 1, 4, 4);
    try {
        cout << "before copy "<<int64(a.data) << "\n";
    b(r1).copyTo(a(r2));
    cout << "after copy " << int64(a.data) << "\n";
    cout << a << "\n";
    cout << b << "\n";

    }
    catch (cv::Exception &e)
    {
        cout << "sizes are not equal\n";
    }

No exception. Of course if b and rect size are different there is an exception

Indeed.

Is the documentation wrong? It says (cv::Mat::copyTo):

Before copying the data, the method invokes :
m.create(this->size(), this->type()); 
so that the destination matrix is reallocated if needed.

and (cv::Mat::create):

1. If the current array shape and the type match the new ones, return immediately. Otherwise, de-reference the previous data by calling Mat::release.
2. Initialize the new header.
3. Allocate the new data of total()*elemSize() bytes.

or am I misunderstanding something?

From what i see in the source code, (copy.cpp function Mat::copyTo and mat.inl.hpp function _OutputArray::_OutputArray), it indeed does not behave as documented.

Output array is initialised with flags called FIXED_TYPE + FIXED_SIZE. copyTo checks for fixedType, but not for FIXED_SIZE. The code path using the create function is still there, but it is not taken. instead there is said exception (plus "OpenCV Error: Assertion failed" warnings in 3.2, depending on what mismatches, but not always).

you can test the code below ( see memory usage )

#include "opencv2/core.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;

Mat getROI(const Mat image, Rect rect)
{
    return image(rect);
}

int main()
{
    Mat image(20000, 20000, CV_8UC3);
    Mat roi = getROI(image, Rect(0, 0, 19000, 19000));
    imshow("image", image);
    imshow("roi", roi);
    waitKey();
}

there is a reallocation when it is mat object and not ROI. In a roi there is no real data opencv plays with rows and column and stride.

In b.copyTo(a) a is reallocated.

I don't think so... If it were reallocated, you wouldn't see it change the original. But the copyTo still affects the original image. So b.copyTo(a) is still using the original memory.

a is not using original if a is smaller than b :

    Mat a(6, 6,CV_8UC1,Scalar(1));
    Mat b(7, 7, CV_32FC1,Scalar(3.5));
    try {
        cout << "before copy "<<int64(a.data) << "\n";
    b.copyTo(a);
    cout << "after copy " << int64(a.data) << "\n";

Ah, I think I see the confusion. Yes, that will reallocate. No there is not a built-in copy function that will throw, but you can build a simple check that will.

For example:

if(a.rows != b.rows || a.cols != b.cols || a.type() != b.type())
    throw;