How much bad can be done using register variables in C++

Question

I just came to know that we can use registers, explicitly in C++ programs. I wonder what if i declare and use all available registers in a single C++ program and run it for cons

Answer 1

The register keyword is only a polite suggestion to the compiler that you think this variable will be heavily used and could it pretty-please just keep it in a register. The compiler is free to ignore this suggestion and, in fact, will usually do so in a modern environment.

register is basically a vestigial remnant of the old, grossly-inefficient C compilers that were available way back when. (The same compilers that led to things like the execrable Duff's Device and other monstrosities, in fact.) Modern compilers are far more capable than you are of keeping track of which variables should be placed into which registers at which points in execution. They will, thus, politely ignore you without saying a word.