string标准库的重载>>是怎么实现的(当输入的数据量很大时)？
我这种显然不行，求高人指点

C/C++ code

std::istream& operator>>(std::istream &in,string &str){    char temp[80];    in>>temp;    str = string(temp);    return in;}

[解决办法]
不是很明白你的意思，你是要把所有的输入搞成一个字符串吗？
你的代码可以简化如下：

C/C++ code

std::istream& operator>>(std::istream &in,string &str){    in>>str;    return in;}
[解决办法]
建议楼主去看stl中string的源代码。
[解决办法]
C/C++ code
template<class _FwdIterator>static _CharT*    _S_construct(_FwdIterator __beg, _FwdIterator __end, const _Alloc& __a, forward_iterator_tag){if (__beg == __end && __a == _Alloc())return _S_empty_rep()._M_refdata(); // NB: Not required, but considered best practice.if (__builtin_expect(__is_null_pointer(__beg), 0))__throw_logic_error(__N("basic_string::_S_construct NULL not valid")); const size_type __dnew = static_cast<size_type>(std::distance(__beg,__end));// Check for out_of_range and length_error exceptions.//直接根据已知数据长度分配字符串的空间_Rep* __r = _Rep::_S_create(__dnew, size_type(0), __a);try{_S_copy_chars(__r->_M_refdata(), __beg, __end);}catch(...){__r->_M_destroy(__a);__throw_exception_again;}__r->_M_length = __dnew;__r->_M_refdata()[__dnew] = _Rep::_S_terminal;  // grrr.return __r->_M_refdata();}
[解决办法]
string是basic_string模板的特化，头文件中有他的重载实现
C/C++ code
template<class _Elem,    class _Traits,    class _Alloc> inline    basic_istream<_Elem, _Traits>& operator>>(        basic_istream<_Elem, _Traits> && _Istr,        basic_string<_Elem, _Traits, _Alloc>& _Str)    {    // extract a string    typedef ctype<_Elem> _Ctype;    typedef basic_istream<_Elem, _Traits> _Myis;    typedef basic_string<_Elem, _Traits, _Alloc> _Mystr;    typedef typename _Mystr::size_type _Mysizt;    ios_base::iostate _State = ios_base::goodbit;    bool _Changed = false;    const typename _Myis::sentry _Ok(_Istr);    if (_Ok)        {    // state okay, extract characters        const _Ctype& _Ctype_fac = _USE(_Istr.getloc(), _Ctype);        _Str.erase();        _TRY_IO_BEGIN        _Mysizt _Size = 0 < _Istr.width()            && (_Mysizt)_Istr.width() < _Str.max_size()                ? (_Mysizt)_Istr.width() : _Str.max_size();        typename _Traits::int_type _Meta = _Istr.rdbuf()->sgetc();        for (; 0 < _Size; --_Size, _Meta = _Istr.rdbuf()->snextc())            if(_Traits::eq_int_type(_Traits::eof(), _Meta))                {    // end of file, quit                _State |= ios_base::eofbit;                break;                }            else if (_Ctype_fac.is(_Ctype::space,                _Traits::to_char_type(_Meta)))                break;    // whitespace, quit            else                {    // add character to string                _Str.append(1, _Traits::to_char_type(_Meta));                _Changed = true;                }        _CATCH_IO_(_Istr)        }    _Istr.width(0);    if (!_Changed)        _State |= ios_base::failbit;    _Istr.setstate(_State);    return (_Istr);    }