Main.cc: In function ‘int main()’: Main.cc:7:23: warning: left operand of comma operator has no effect [-Wunused-value] printf<<("%7d%7d%7d",a,b,c); ^ Main.cc:7:25: warning: right operand of comma operator has no effect [-Wunused-value] printf<<("%7d%7d%7d",a,b,c); ^ Main.cc:7:27: warning: right operand of comma operator has no effect [-Wunused-value] printf<<("%7d%7d%7d",a,b,c); ^ Main.cc:7:8: error: invalid operands of types ‘int(const char*, ...)’ and ‘int’ to binary ‘operator<<’ printf<<("%7d%7d%7d",a,b,c); ~~~~~~^~~~~~~~~~~~~~~~~~~~~