Compilare: main.cpp: In function 'matrix multiply(const matrix&, const matrix&)': main.cpp:28:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<long long int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare] 28 | for (int i = 0; i < a.size(); ++i) { | ~~^~~~~~~~~~ main.cpp:29:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 29 | for (int j = 0; j < b[0].size(); ++j) { | ~~^~~~~~~~~~~~~ main.cpp:30:31: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 30 | for (int k = 0; k < a[0].size(); ++k) { | ~~^~~~~~~~~~~~~