错误信息：
undefined reference to`_imp___ZN10QTcpSocketD1Ev'

undefined reference to `_imp___ZN10QTcpSocketC1EP7QObject'
undefined reference to `_imp___ZN10QTcpSocketD1Ev'
undefined reference to `_imp___ZN10QTcpSocketD1Ev'
undefined reference to `_imp___ZN10QTcpSocketD1Ev'
undefined reference to `_imp___ZN10QTcpSocketD1Ev'
undefined reference to `_imp___ZN10QTcpSocketD1Ev'
undefined reference to`_imp___ZN15QAbstractSocket13connectToHostERK7QStringt6QFlagsIN9QIODevice12OpenModeFlagEE'
undefined reference to `_imp___ZN15QAbstractSocket5closeEv'

collect2: ld returned 1 exit status

解决方案：

链接库问题，在工程文件（.pro）文件中

QT += core gui network

加上network选项