pre/post incrementing operators

Question

a=4
b=  ++a  +  ++a  +    ++a;
In C++   b is 21
In C#  b is 18.
Why such a radical difference in operators implementation?

Vulpes · Answer

The fact that the order of evaluation of sub-expressions is unspecified in C++ means that the compiler can rearrange the evaluation order to optimize code.
This also means that the evaluation of an expression such as the one you posted may depend on which compiler you're using and which optimizations (if any) are enabled.
For example, when I executed that line using GCC 4.7, the result was 19 rather than 21.

Abhay Shanker · Answer

In C++, the order of evaluation of subexpressions is unspecified whereas C# evaluate expressions from left to right, and the side-effects are visible immediately