[1]. Type conversions. http://www.cplusplus.com/doc/tutorial/typecasting/
[2]. C++标准转换运算符. http://www.cnblogs.com/ider/archive/2011/07/22/cpp_cast_operator_part2.html
C++标准转换运算符
原文：http://www.cnblogs.com/yaozhongxiao/p/4421944.html
