why ++++i regular but i++++ not regular in C++?

Question

When I use i++++ give compile error :

for (int i=1;i<=10;i++++) {} //a.cpp:63: error: lvalue required as increment operand

or

int i = 0;
i++++; // a.cpp:65: error: lvalue required as increment operand

but when I use ++++i is working. Anybody explain me why ++++i regular but i++++ not regular?

Thanks.

Answer 1

Since type of x is built-in primitive type, both expressions invoke undefined behaviour, as both attempt to modify the same object twice between two sequence points.

Don't do either of them.

Read this FAQ :

Undefined behavior and sequence points

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However if the type of x is a user-defined type and you've overloaded operator++ for both expressions, then both would be well-defined.

For this, see this topic to know the explanation and details:

Undefined behavior and sequence points reloaded