overload operator for vector, C++ (vc++)

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#include <iostream>
#include <vector>

struct base
{
    struct vec2d // wrapper class to hold the vector
    {
        std::vector< std::vector<int> > data ;

// this operator can be found only via ADL (good idea)
        friend std::ostream& operator<< ( std::ostream& stm, const vec2d& v2d )
        {
            for( const auto& row : v2d.data )
            {
                for( int v : row ) stm << v << ' ' ;
                stm << '\n' ;
            }
            return stm << "\n\n" ;
        }
    };

// Guideline #1: Prefer to make interfaces non-virtual
    // see: http://www.gotw.ca/publications/mill18.htm
    vec2d some_func() const { return some_func_impl() ; }
    void print_some_func_result() const { std::cout << some_func() << '\n' ; }

// ...

// Guideline #2: Prefer to make virtual functions private
    // see: http://www.gotw.ca/publications/mill18.htm
    private: virtual vec2d some_func_impl() const
    { return { { {1,2,3}, {4,5,6}, {7,8,9} } } ; }
};

struct derived : base
{
    private: virtual vec2d some_func_impl() const override
    { return { { {1,2,3,4}, {5,6,7,8}, {2,3,4,5}, {6,7,8,9} } } ; }

// ...
};

int main()
{
    const base bb ;
    const derived dd ;

const base& b1 = bb ;
    const base& b2 = dd ;

b1.print_some_func_result() ;
    b2.print_some_func_result() ;
 }

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Compilation time: 1,77 sec, absolute running time: 0,33 sec, absolute service time: 2,11 sec