求教这段程序哪错误

#include <iostream>
#include <memory>
#include <string>
#include <vector>

class Strblob
{
public:
    typedef std::vector<std::string>::size_type size_type;
    Strblob();
    Strblob( const std::vector<std::string>& a );

private:
    std::shared_ptr<std::vector<std::string>> data_;

    friend std::ostream& operator<<( std::ostream& os, const Strblob& sb );
};

using namespace std;

Strblob::Strblob() : data_( std::make_shared<std::vector<std::string>>() )
{
}
Strblob::Strblob( const std::vector<std::string>& a ) : data_( std::make_shared<std::vector<std::string>>(a) )
{
}
std::ostream& operator<<( std::ostream& os, const Strblob& sb )
{
    for( auto a : *sb.data_ )
        os << a << '\n';
    return os;
}

int main( void )
{
    Strblob a = { { "a", "b", "c" } };
    Strblob b = a;
    cout << b << endl;
}