Как объявить и инициализировать двумерный массив

std::vector<int> vec {{1,2,3,4},
                      {2,3,4,5},
                      {5,6,7,8}};

std::vector< std::vector<int> > m_vector

#include <vector>
 
int main()
{
    std::vector<std::vector<int>> v = {{1,2},{3,4}};
    return 0;
}

struct complex {
    complex();
    complex(double);
    complex(double,double);
};
complex sqrt(complex,complex);
complex a(1); // initialize by a call of complex(double)
complex b = a; // initialize by a copy of a
complex c = complex(1,2); // construct complex(1,2) using complex(double,double) copy/move it into c
complex d = sqrt(b,c); // call sqrt(complex,complex) and copy/move the result into d
complex e; // initialize by a call of complex()
complex f = 3; // construct complex(3) using complex(double) copy/move it into f
complex g = { 1, 2 }; // initialize by a call of complex(double, double)

    // c++11 initializer list syntax:
    std::vector<std::string> words1 {"the", "frogurt", "is", "also", "cursed"};

std::vector<std::string> words1 {"the", "frogurt", "is", "also", "cursed"};

vector<string>vecStr1{{'a','b','c','d'},
                      {'e','f','g','h'},
                      {'i','j','k','l'}};

std::vector<int> vec {{1,2,3,4},
                      {2,3,4,5},
                      {5,6,7,8}};