How can I resize a 2D C++ vector?

    const size_t ROW = 10;
    const size_t COL = 20;
    std::vector<std::vector<char>> v;

    v.resize( ROW );

    std::for_each( v.begin(), v.end(), 
                   std::bind2nd( std::mem_fun_ref( &std::vector<char>::resize ), COL ) );

    std::cout << "size = " << v.size() << std::endl;
    for ( const std::vector<char> &v1 : v ) std::cout << v1.size() << ' ';
    std::cout << std::endl;

Given the vector is empty, you can simply resize the outer vector with preallocated inner vectors without the need of a loop:

matrix.resize(COL, vector<char>(ROW));

Alternatively, when initializing or if you want to reset a non-empty vector, you can use the constructor overload taking a size and initial value to initialize all the inner vectors:

matrix = vector<vector<char> >(COL, vector<char>(ROW));

Depending on whether your matrix is column- or row-major, you need to swap the arguments ROW and COL. The first one (the first parameter on the outer vector) is your first dimension to access the matrix, i.e. I assumed you access it with matrix[col][row].