C++ string替换单个指定字符为其它字符问题

#include <algorithm>
int main()
{
	string str = "[1,2,3,4,5]";
	cout << str << endl;
	replace(str.begin(), str.end(), ',', ' ');//将逗号替换为空格
	cout << str << endl;
}

std::replace(str.begin(), str.end(), '\"', '@');//单字符替换-----将双引号换成@符

/// <summary>
/// 字符串批量替换
/// </summary>
/// <param name="str">输入的文本</param>
/// <param name="a">目标文本</param>
/// <param name="b">替换内容</param>
/// <returns>替换好的文本</returns>
std::string spp(std::string str, std::string a, std::string b)
{
	int oldPos = 0;
	while (str.find(a, oldPos) != -1)//在未被替换的文本中寻找目标文本
	{
		int start = str.find(a, oldPos);//找到目标文本的起始下标
		str.replace(start, a.size(), b);
		//从str[start]开始到str[a.size()]替换为b
		//str[start]到str[a.size()]也就是a所在得片段
		oldPos = start + b.size();//记录未替换文本的起始下标
	}
	return str;
}

int main()
{
    std::string str = "{\"QQ1\":123,\"QQ2\":123,\"QQ3\":123}";
    str = spp(str, "123", "321");
    std::cout << str << std::endl;
    str = spp(str, "\"", "\\\"");
    std::cout << str << std::endl;
}