格式化输出与宏定义的一个例子

#include<fstream> #include<iostream> using namespace std; #define D(A) T<<#A<<endl; A int main() { ofstream T("format.out"); D(int i=47;) D(float f=2300114.232343;) const char *s="Is there any more?"; D(T.setf(ios::unitbuf);) D(T.setf(ios::showbase);) D(T.setf(ios::uppercase|ios::showpos);) D(T<<i<<endl;) D(T.unsetf(ios::showbase);) D(T.setf(ios::dec,ios::basefield);) D(T.setf(ios::left,ios::adjustfield);) D(T.fill('0');) D(T<<"fill char "<<T.fill()<<endl;) D(T.width(10);) T<<i<<endl; D(T<<i<<endl;) D(T.unsetf(ios::showpos);) D(T.setf(ios::showpoint);) D(T<<"prec ="<<T.precision()<<endl;) D(T.setf(ios::scientific,ios::floatfield);) D(T<<endl<<f<<endl;) D(T.setf(ios::fixed,ios::floatfield);) D(T<<f<<endl;) D(T.precision(20);) D(T<<"prec ="<<T.precision()<<endl;) D(T<<endl<<f<<endl;) D(T.setf(ios::fixed,ios::floatfield);) D(T<<f<<endl;) D(T.width(10);) T<<s<<endl; D(T.width(40);) T<<s<<endl; D(T.setf(ios::left,ios::adjustfield);) D(T.width(40);) T<<s<<endl; return 0; }

这个例子中用到了一个技巧来创建一个跟踪文件，以监视程序执行时发生了什么事。宏定义D(A)用预处理器把A转化为字符串并输出。然后对A进行重复迭代，所以语句顺序执行。宏把所以信息输出到跟踪文件T.

文件"format.out"中的内容如下：

int i=47; float f=2300114.232343; T.setf(ios::unitbuf); T.setf(ios::showbase); T.setf(ios::uppercase|ios::showpos); T<<i<<endl; +47 T.unsetf(ios::showbase); T.setf(ios::dec,ios::basefield); T.setf(ios::left,ios::adjustfield); T.fill('0'); T<<"fill char "<<T.fill()<<endl; fill char 0 T.width(10); +470000000 T<<i<<endl; +47 T.unsetf(ios::showpos); T.setf(ios::showpoint); T<<"prec ="<<T.precision()<<endl; prec =6 T.setf(ios::scientific,ios::floatfield); T<<endl<<f<<endl; 2.300114E+006 T.setf(ios::fixed,ios::floatfield); T<<f<<endl; 2300114.250000 T.precision(20); T<<"prec ="<<T.precision()<<endl; prec =20 T<<endl<<f<<endl; 2300114.25000000000000000000 T.setf(ios::fixed,ios::floatfield); T<<f<<endl; 2300114.25000000000000000000 T.width(10); Is there any more? T.width(40); Is there any more?0000000000000000000000 T.setf(ios::left,ios::adjustfield); T.width(40); Is there any more?0000000000000000000000

——摘自Thinking in C++ Volume Two