for_each算法和transform算法的區別

• for_each:  用指定函數依次對指定範圍內所有元素進行迭代訪問，可對容器的元素進行修改。
• transform:   與for_each類似，遍歷所有元素，可對容器的元素進行修改。

class PrintElem
{
public:
	void operator()(int &e)
	{
		m_n++;
		cout << e << " ";
	}

 	void printN()
	{
		cout << m_n << endl;
	}

private:
	int m_n;

};

void PrintV(int &n)
{
	cout << n << " ";
}

template <typename T>
void PrintElement(vector<T> &v)
{
	for (vector<int>::iterator it = v.begin(); it != v.end(); it++)
	{
		cout << *it << " ";
	}
}

int Increase(int &i)
{
	return i + 99;
}

void Fun1()
{
	vector<int> v1;

	v1.push_back(1);
	v1.push_back(2);
	v1.push_back(3);

	//普通函數 & 回調函數的入口地址
	for_each(v1.begin(), v1.end(), PrintV);

	//函數對象
	for_each(v1.begin(), v1.end(), PrintElem());

	//PrintElem p = for_each(v1.begin(), v1.end(), PrintElem());//初始化
	PrintElem p;
	p = for_each(v1.begin(), v1.end(), PrintElem());//賦值
	p.printN();



	vector<int> v2;
	for (int i = 0; i < 10; i++)
	{
		v2.push_back(i + 1);
	}
	cout << endl;
	//回調函數 
	transform(v2.begin(), v2.end(), v2.begin(), Increase);
	PrintElement(v2);
	cout << endl;

	//預定義函數
	transform(v2.begin(), v2.end(), v2.begin(), negate<int>());
	PrintElement(v2);
	cout << endl;

	//適配器
	list<int> l1;
	l1.resize(v2.size());

	transform(v2.begin(), v2.end(), l1.begin(), bind2nd(multiplies<int>(), 10));
	for (list<int>::iterator it = l1.begin();it != l1.end();it++)
	{
		cout << *it << " ";
	}
	cout << endl;

	//結合輸出迭代器
	transform(l1.begin(), l1.end(), ostream_iterator<int>(cout, " "), negate<int>());
	cout << endl;
}

它們的不同在於函數對象的返回值上，一般情況下for_each所使用的函數對象，參數是引用，沒有返回值。transform所使用的函數對象，參數一般不使用引用，而是還有返回值

for_each源碼

template<class _InIt,
	class _Fn> inline
	_Fn for_each(_InIt _First, _InIt _Last, _Fn _Func)
	{	// perform function for each element [_First, _Last)
	_Adl_verify_range(_First, _Last);
	auto _UFirst = _Get_unwrapped(_First);
	const auto _ULast = _Get_unwrapped(_Last);
	for (; _UFirst != _ULast; ++_UFirst)
		{
		_Func(*_UFirst);//並沒有返回值
		}

	return (_Func);//返回函數對象本身
	}

transform源碼

template<class _InIt,
	class _OutIt,
	class _Fn> inline
	_OutIt transform(const _InIt _First, const _InIt _Last, _OutIt _Dest, _Fn _Func)
	{	// transform [_First, _Last) with _Func
	_Adl_verify_range(_First, _Last);
	auto _UFirst = _Get_unwrapped(_First);
	const auto _ULast = _Get_unwrapped(_Last);
	auto _UDest = _Get_unwrapped_n(_Dest, _Idl_distance<_InIt>(_UFirst, _ULast));
	for (; _UFirst != _ULast; ++_UFirst, (void)++_UDest)
		{
		*_UDest = _Func(*_UFirst);//有返回值
		}

	_Seek_wrapped(_Dest, _UDest);
	return (_Dest);
	}