c++ - about the const in an operator overloading definition -

for following definition of

const vector3f operator*(const vector3f &v, float s); 

there 2 const, respective usages?

the const-reference in argument means don't change v, can pass constant vectors (and temporaries!) function. that's thing.

the constant by-value return sort of gimmick. prevents writing things this:

 vector3f v = get_vector();  vector3f w = v;   (v * 1.5) = w; // outch! cannot assign constant, though, we're good. 

returning by-value constant problematic, though, since interferes c++11's rvalue references , move semantics:

 move_me(v * 1.5);  // cannot bind `vector3f &&` :-( 

because of that, , because abuse 1 showed above unlikely happen accident, it's best return value non-constant.