c++ - Why does this OpenCV code segfault? -

i'm preparing parallelize of c++ opencv code. i've come across segfault, , can't understand what's happening.

here code:

class exhaustivecomparisonmt {     vector<mat> *m_sigs;     mat *m_dist; public:     exhaustivecomparisonmt(vector<mat> sigs, mat dist)     {         m_sigs = &sigs;         m_dist = &dist; // gdb breakpoint 1 here     }      void operator() (size_t last_row, size_t last_col) const     {         mat diff = (*m_sigs)[0].clone(); // segfault happens here, gdb breakpoint 2 here         (size_t = 0; != last_row; ++i)         (size_t j = 0; j != last_row; ++j)         {             cv::absdiff((*m_sigs)[i], (*m_sigs)[j], diff);             m_dist->at<double>(i, j) = cv::sum(diff).val[0];         }     } };  void exhaustive_comparison(vector<mat> sigs, mat dist) {     size_t width = sigs.size();     exhaustivecomparisonmt ecmt(sigs, dist);     ecmt(width, width); } 

basically, vector of matrices being passed constructor. pointer vector kept member variable, vector can accessed again in exhaustive_comparison. however, function trips trying access first element of vector.

i've tried diagnose problem gdb putting 2 breakpoints (see code). @ breakpoint 1:

(gdb) p (*m_sigs)[0] $1 = (cv::mat &) @0x7fffee77d010: {flags = 1124024325, dims = 2, rows = 1, cols = 712, data = 0x624ec0 "", refcount = 0x0, datastart = 0x624ec0 "", dataend = 0x6259e0 "",    datalimit = 0x6259e0 "", allocator = 0x0, size = {p = 0x7fffee77d018}, step = {p = 0x7fffee77d060, buf = {2848, 4}}} 

therefore, first element being accessed correctly. now, go breakpoint 2 , try same thing:

(gdb) p (*m_sigs)[0] $2 = (cv::mat &) @0x7fffee77d010: <error reading variable> 

the first element appears no longer accessible! address same (0x7fffee77d010). what's going on here?

finally, if step forward, get:

program received signal sigsegv, segmentation fault. 0x00007ffff7a0dd50 in cv::mat::copyto(cv::_outputarray const&) const () /usr/local/lib/libopencv_core.so.2.4 

opencv tries access first element clone it, , fails.

why first element accessible in constructor, not accessible in exhaustive_comparison member function?

exhaustivecomparisonmt(vector<mat> sigs, mat dist) {         m_sigs = &sigs;         m_dist = &dist; // gdb breakpoint 1 here } 

you passing variables sigs , dist copy - meaning, lifetime limited function.

if try address , use afterwards, going point unknown data - because upon leaving function, these objects destroyed , addresses used else later on.

if want have address of objects passed parameters, should use references - way, retrieve address of original objects. work assuming, of course, you're passing lvalues functions , not destroyed in meantime.

exhaustivecomparisonmt(vector<mat> &sigs, mat &dist) {         m_sigs = &sigs;         m_dist = &dist; // gdb breakpoint 1 here }