std::for_each 相较于for而言，它可以帮助在STL容器中的每个元素都执行一次fn()函数。如果你用过 find_first_of 或者 find_last_of，那么现在你要替换所有符合判断表达式的，可以考虑使用for_each了！

来自 https://en.cppreference.com/w/cpp/algorithm/for_each 的demo

#include <vector>
#include <algorithm>
#include <iostream>
 
struct Sum
{
    void operator()(int n) { sum += n; }
    int sum{0};
};
 
int main()
{
    std::vector<int> nums{3, 4, 2, 8, 15, 267};
 
    auto print = [](const int& n) { std::cout << " " << n; };
 
    std::cout << "before:";
    std::for_each(nums.cbegin(), nums.cend(), print);
    std::cout << '\n';
 
    std::for_each(nums.begin(), nums.end(), [](int &n){ n++; });
 
    // calls Sum::operator() for each number
    Sum s = std::for_each(nums.begin(), nums.end(), Sum());
 
    std::cout << "after: ";
    std::for_each(nums.cbegin(), nums.cend(), print);
    std::cout << '\n';
    std::cout << "sum: " << s.sum << '\n';
}

示例代码<1> 计算从1加到100的和（1+2+3+…+100=？）

#include <iostream>
#include <vector>
#include <algorithm>


// reference: http://en.cppreference.com/w/cpp/algorithm/for_each
struct Sum 
{
	Sum() : sum{ 0 } { }
	void operator()(int n) { sum += n; }
	int sum;
};

int main()
{
	//计算从1至100的和
	std::vector<int> intArray = {};
	for (size_t i = 1; i <= 100; i++)
	{
		intArray.push_back(i);
	}

	//calls Sum::operator() for each number
	Sum total = std::for_each(intArray.begin(), intArray.end(), Sum());
	std::cout << "s=" << total.sum << '\n';

    return 0;
}

示例代码<2> 打印每科的分数，计算总分数和平均分

#include <iostream>
#include <vector>
#include <algorithm>

using kv = std::pair<std::string, int>;

void printScore(kv score)
{
	std::cout << score.first << "=" << score.second << std::endl;
}

// The function object to determine the average
class Average 
{
private:
	long num;      // The number of elements
	long sum;      // The sum of the elements
public:
	// Constructor initializes the value to multiply by
	Average() : num(0), sum(0)
	{
	}

	// The function call to process the next element
	void operator ( ) (int elem)
	{
		num++;      // Increment the element count
		sum += elem;   // Add the value to the partial sum
	}

	// return Average
	operator double()
	{
		return  static_cast<double>(sum) / static_cast<double>(num);
	}
};


int main()
{
	std::unordered_map<std::string, int> scoreMap =
	{
		{"语文", 95},
		{"数学", 97},
		{"英语", 99},
	};
	//打印每科分数
	std::for_each(scoreMap.begin(), scoreMap.end(), printScore);

	//计算总分数
	int sum = 0;
	std::for_each(scoreMap.begin(), scoreMap.end(), [&](kv score)
		{ 
			sum += score.second;
		});
	std::cout << "总分数=" << sum << std::endl;
	
	//
	std::vector<int> scores = { 95, 97, 99 };
	//
	double avg = std::for_each(scores.begin(), scores.end(), Average());
	std::cout << "平均分=" << avg << std::endl;

    return 0;
}

示例代码<3> 替换字符操作

考虑到 for_each() 可以针对容器里面每个元素做操作。那么假设抖音直播时，要屏蔽“围观群众”发送的不文明用语，那么就可以尝试做关键字屏蔽

#include <iostream>
#include <vector>
#include <unordered_map>
#include <algorithm>

void replaceChar(char& c)
{
	if (c == 'f' || c == 'u' || c == 'c' || c == 'k')
		c = '*';
}

int main()
{
	std::string str = "Fuck, Guangzhou is so hot lately!";
	std::cout << "raw string is:" << str << std::endl;
	//
	std::for_each(str.begin(), str.end(), replaceChar);
	std::cout << "after update is:" << str << std::endl;

	return 0;
}