equal 源码剖析

一：功能解析

函数原型：

equality (1)    
template
  bool equal (InputIterator1 first1, InputIterator1 last1,
              InputIterator2 first2);
predicate (2)   
template
  bool equal (InputIterator1 first1, InputIterator1 last1,
              InputIterator2 first2, BinaryPredicate pred);

功能：

区间[ first1 , last1 )与以first2开始的区间逐一比较，如果与[ first1 , last1 )全部相等（或者满足pred），返回布尔值真，否则返回假。

例子：

// equal algorithm example
#include      // std::cout
#include     // std::equal
#include        // std::vector

bool mypredicate (int i, int j) {
  return (i==j);
}

int main () {
  int myints[] = {20,40,60,80,100};               //   myints: 20 40 60 80 100
  std::vectormyvector (myints,myints+5);     // myvector: 20 40 60 80 100

  // using default comparison:
  if ( std::equal (myvector.begin(), myvector.end(), myints) )
    std::cout << "The contents of both sequences are equal.\n";
  else
    std::cout << "The contents of both sequences differ.\n";

  myvector[3]=81;                                 // myvector: 20 40 60 81 100

  // using predicate comparison:
  if ( std::equal (myvector.begin(), myvector.end(), myints, mypredicate) )
    std::cout << "The contents of both sequences are equal.\n";
  else
    std::cout << "The contents of both sequences differ.\n";

  return 0;
}

运行如下：

The contents of both sequences are equal.
The contents of both sequence differ.

二：源码剖析

// TEMPLATE FUNCTION equal WITH PRED
template inline
    bool _Equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _Pr _Pred)
    {   // compare [_First1, _Last1) to [_First2, ...) using _Pred
    for (; _First1 != _Last1; ++_First1, (void)++_First2)
        if (!_Pred(*_First1, *_First2))
            return (false);
    return (true);
    }

inline bool _Equal(const char *_First1, const char *_Last1,
    const char *_First2, equal_to<>)
    {   // compare [_First1, _Last1) to [_First2, ...), for chars
    return (_CSTD memcmp(_First1, _First2, _Last1 - _First1) == 0);
    }

inline bool _Equal(const signed char *_First1, const signed char *_Last1,
    const signed char *_First2, equal_to<>)
    {   // compare [_First1, _Last1) to [_First2, ...), for signed chars
    return (_CSTD memcmp(_First1, _First2, _Last1 - _First1) == 0);
    }

inline bool _Equal(const unsigned char *_First1, const unsigned char *_Last1,
    const unsigned char *_First2, equal_to<>)
    {   // compare [_First1, _Last1) to [_First2, ...), for unsigned chars
    return (_CSTD memcmp(_First1, _First2, _Last1 - _First1) == 0);
    }

inline bool _Equal(const char *_First1, const char *_Last1,
    const char *_First2, _Char_traits_eq>)
    {   // compare [_First1, _Last1) to [_First2, ...), for chars
    return (_CSTD memcmp(_First1, _First2, _Last1 - _First1) == 0);
    }

 #if _ITERATOR_DEBUG_LEVEL == 0
template inline
    bool equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _Pr _Pred)
    {   // compare [_First1, _Last1) to [_First2, ...) using _Pred
    return (_Equal(_Unchecked(_First1), _Unchecked(_Last1),
        _Unchecked(_First2), _Pred));
    }

 #else /* _ITERATOR_DEBUG_LEVEL == 0 */
template inline
    bool _Equal2(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _Pr _Pred, true_type)
    {   // compare [_First1, _Last1) to [_First2, ...) using _Pred, checked
    return (_Equal(_First1, _Last1,
        _First2, _Pred));
    }

template inline
_SCL_INSECURE_DEPRECATE
    bool _Equal2(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _Pr _Pred, false_type)
    {   // compare [_First1, _Last1) to [_First2, ...) using _Pred, unchecked
    return (_Equal(_First1, _Last1,
        _First2, _Pred));
    }

template inline
    bool equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _Pr _Pred)
    {   // compare [_First1, _Last1) to [_First2, ...) using _Pred
    _DEBUG_RANGE_PTR(_First1, _Last1, _First2);
    _DEBUG_POINTER_IF(_First1 != _Last1, _Pred);
    return (_Equal2(_Unchecked(_First1), _Unchecked(_Last1),
        _First2, _Pred, _Is_checked(_First2)));
    }

 #if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template inline
    bool equal(_InIt1 _First1, _InIt1 _Last1,
        _InTy (&_First2)[_InSize], _Pr _Pred)
    {   // compare [_First1, _Last1) to [_First2, ...) using _Pred
    return (_STD equal(_First1, _Last1,
        _Array_iterator<_InTy, _InSize>(_First2), _Pred));
    }
 #endif /* _ITERATOR_DEBUG_ARRAY_OVERLOADS */
 #endif /* _ITERATOR_DEBUG_LEVEL == 0 */

        // TEMPLATE FUNCTION equal
template inline
    bool equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2)
    {   // compare [_First1, _Last1) to [_First2, ...)
    return (_STD equal(_First1, _Last1, _First2,
        equal_to<>()));
    }

 #if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template inline
    bool equal(_InIt1 _First1, _InIt1 _Last1,
        _InTy (&_First2)[_InSize])
    {   // compare [_First1, _Last1) to [_First2, ...)
    return (_STD equal(_First1, _Last1, _First2,
        equal_to<>()));
    }
 #endif /* _ITERATOR_DEBUG_ARRAY_OVERLOADS */

        // TEMPLATE FUNCTION equal WITH TWO RANGES, PRED
template inline
    bool _Equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _InIt2 _Last2, _Pr _Pred,
            input_iterator_tag, input_iterator_tag)
    {   // compare [_First1, _Last1) to [_First2, _Last2)
        // using _Pred, arbitrary iterators
    _DEBUG_POINTER_IF(_First1 != _Last1 && _First2 != _Last2, _Pred);
    for (; _First1 != _Last1 && _First2 != _Last2; ++_First1, (void)++_First2)
        if (!_Pred(*_First1, *_First2))
            return (false);
    return (_First1 == _Last1 && _First2 == _Last2);
    }

template inline
    bool _Equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _InIt2 _Last2, _Pr _Pred,
            random_access_iterator_tag, random_access_iterator_tag)
    {   // compare [_First1, _Last1) to [_First2, _Last2)
        // using _Pred, random-access iterators
    if (_Last1 - _First1 != _Last2 - _First2)
        return (false);
    _DEBUG_POINTER_IF(_First1 != _Last1, _Pred);
    return (_Equal(_First1, _Last1, _First2, _Pred));
    }

template inline
    bool equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _InIt2 _Last2, _Pr _Pred)
    {   // compare [_First1, _Last1) to [_First2, _Last2) using _Pred
    _DEBUG_RANGE(_First1, _Last1);
    _DEBUG_RANGE(_First2, _Last2);
    return (_Equal(_Unchecked(_First1), _Unchecked(_Last1),
        _Unchecked(_First2), _Unchecked(_Last2), _Pred,
            _Iter_cat(_First1), _Iter_cat(_First2)));
    }

        // TEMPLATE FUNCTION equal WITH TWO RANGES
template inline
    bool equal(_InIt1 _First1, _InIt1 _Last1,
        _InIt2 _First2, _InIt2 _Last2)
    {   // compare [_First1, _Last1) to [_First2, _Last2)
    return (_STD equal(_First1, _Last1, _First2, _Last2,
        equal_to<>()));
    }

源码摘抄自Visual Studio 2015安装目录algorithm文件中。

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