View Compilation Error

0_0_4869903_22068.cpp: In function `mat operator*(const mat&, const mat&)':
0_0_4869903_22068.cpp:67: error: `size' undeclared (first use this function)
0_0_4869903_22068.cpp:67: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_4869903_22068.cpp:67: error: expected `;' at end of input
0_0_4869903_22068.cpp:67: error: expected `}' at end of input