Question

why >?= and <?= don’t work in VC++?
but they work fine in gcc/g++
like: a>?=b;
are they right usages?

Answer 1

Because those are the old GC++-specific extensions for minimum and maximum.

From 6. Extensions to the C++ Language:

The GNU compiler provides these extensions to the C++ language (and you can also use most of the C language extensions in your C++ programs). If you want to write code that checks whether these features are available, you can test for the GNU compiler the same way as for C programs: check for a predefined macro __GNUC__. You can also use __GNUG__ to test specifically for GNU C++ (see section ‘Predefined Macros’ in The GNU C Preprocessor).

• 6.1 Minimum and Maximum Operators in C++ C++ Minimum and maximum operators.
• …

Instead, you should use std::min and std::max.