C++利用随机数生成验证码

#include 
using namespace std;
#include 
#include 
#include 

void myrand(string &str,int leng)
{
	srand(int(time(0)));//产生随机种子，保证每次的数据都不重复
	char temp= '0';
	for (int i = 0; i < leng; i++)
	{
		switch (rand() % 3)
		{
		case 0:
			temp = rand() % 10 + '0';
			break;

		case 1:
			temp = rand() % 26 + 'a';
			break;

		case 2:
			temp = rand() % 26 + 'A';
			break;
		}
		str += temp;
		
	}


}

int main()
{
	string str;
	myrand(str,4);
	cout <