利用http协议异步下载图片的c++类。类里面包含了一些其他的逻辑，但有良好的下载图片的函数。downloadPicExAsy。经测试，类下载图片安全稳定-The use of asynchronous http protocol to download pictures c++ classes. Class which contains some other logic, but there are a good function to download images. downloadPic