C++ Coding Reference: Copy N values by using std::fill_n()

  • Time：2020-09-17 11:25:43
  • Class：Weblog
  • Read：253

We talked about std::fill() that we can use to copy over a single value to a range [First, Last). If you know the number of elements you want to copy into, you can use the std::fill_n() instead. In the header xutility, the std::fill_n() using generic template definition is:

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template<class _OutIt,
    class _Diff,
    class _Ty> inline
    _OutIt _Fill_n_unchecked2(_OutIt _Dest, _Diff _Count, const _Ty& _Val, true_type)
    {   // copy _Val _Count times through [_Dest, ...), memset optimization
    _CSTD memset(_Dest, static_cast<unsigned char>(_Val), static_cast<size_t>(_Count));
    return (_Dest + _Count);
    }
 
template<class _OutIt,
    class _Diff,
    class _Ty> inline
    _OutIt fill_n(_OutIt _Dest, _Diff _Count_raw, const _Ty& _Val)
    {   // copy _Val _Count times through [_Dest, ...)
    const _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count)
        {
        const auto _UDest = _Get_unwrapped_n(_Dest, _Count);
        _Seek_wrapped(_Dest,
            _Fill_n_unchecked2(_UDest, _Count, _Val, _Fill_memset_is_safe(_UDest, _Val)));
        }
 
    return (_Dest);
    }
template<class _OutIt,
	class _Diff,
	class _Ty> inline
	_OutIt _Fill_n_unchecked2(_OutIt _Dest, _Diff _Count, const _Ty& _Val, true_type)
	{	// copy _Val _Count times through [_Dest, ...), memset optimization
	_CSTD memset(_Dest, static_cast<unsigned char>(_Val), static_cast<size_t>(_Count));
	return (_Dest + _Count);
	}

template<class _OutIt,
	class _Diff,
	class _Ty> inline
	_OutIt fill_n(_OutIt _Dest, _Diff _Count_raw, const _Ty& _Val)
	{	// copy _Val _Count times through [_Dest, ...)
	const _Algorithm_int_t<_Diff> _Count = _Count_raw;
	if (0 < _Count)
		{
		const auto _UDest = _Get_unwrapped_n(_Dest, _Count);
		_Seek_wrapped(_Dest,
			_Fill_n_unchecked2(_UDest, _Count, _Val, _Fill_memset_is_safe(_UDest, _Val)));
		}

	return (_Dest);
	}

As we can see, it is based on memset.

Example Usages of std::fill_n()

The std::fill_n() takes three parameters: the destination, the count and the value to copy over, namely:

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std::fill_n(destination, count, value);
std::fill_n(destination, count, value);

For example, using std::fill_n() to copy value of 3 into a static array (of integer) for the first 4 elements:

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int nums[10];
std::fill_n(begin(nums), 4, 3); // nums is now [3, 3, 3, 3, .., .., ..]
int nums[10];
std::fill_n(begin(nums), 4, 3); // nums is now [3, 3, 3, 3, .., .., ..]

std::fill_n() takes generic types, thus, you can use it on array of chars:

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char buf[6];
std::fill_n(begin(buf), 3, 'a'); // buf is ['a', 'a', 'a', .., ..]
char buf[6];
std::fill_n(begin(buf), 3, 'a'); // buf is ['a', 'a', 'a', .., ..]

You can also use std::fill_n() to copy over custom data type, for example, the following copies over 3 structs:

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typedef struct {
    int x, y;
} data;
vector<data> arr(10);
data x = { 1, 2 };
std::fill_n(begin(arr), 3, x);
typedef struct {
	int x, y;
} data;
vector<data> arr(10);
data x = { 1, 2 };
std::fill_n(begin(arr), 3, x);

Similar to std::fill(), you would need to make sure the destination memory locations can be written to otherwise a runtime memory Access Violation could be expected.

–EOF (The Ultimate Computing & Technology Blog) —

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