[Solved] Why can not use “return lhs.size() == rhs.size()” in std::sort() compare function?

Because comp1(a,b) and comp1(b,a) can both return true, which isn’t allowed. std::sort states that:

comp – comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

And following the link to Compare we also see:

Establishes strict weak ordering relation with the following properties
* For all a, comp(a,a)==false
* If comp(a,b)==true then comp(b,a)==false
* if comp(a,b)==true and comp(b,c)==true then comp(a,c)==true

Your first function, comp, satisfies those requirements: no two objects could return true. If comp(a,b) returns true, then comp(b,a) returns false; however, this is not the case in your second function, comp1. comp1(a,b) can return true, as can comp(b,a), so sort will never finish and therefore it won’t compile.

You state that

i want to put those same size strings together

But just using operator< will achieve those results: any string the same size will be grouped together.