What is the difference between operators associativity and order of evaluation?
I have expected that operator associativity is a precedence of operators which in the same group and have the same precedence, but I cannot understand the difference between operators associativity and order of evaluation
Associativity informs the order of evaluation of the components of an expression, but it does not entirely define it.
Consider this expression: a + b + c. Associativity of + is left-to-right, so we know that this is conceptually equivalent to ((a + b) + c). What this means is that the expression a + b gets evaluated before the addition of that result to c. That informs the order of evaluation.
But this does not entirely define ordering. That it, it does not mean that a or b gets evaluated before c. It is entirely possible for a compiler to evaluate c first, then a and then b, then +ing a and b, and finally +ing that to the result of c. Indeed, in C++14, it is possible for a compiler to partially evaluate c, then evaluate part of a, then some of b, then some of c etc. That all depends on how complex a, b, and c are.
This is particularly important if one of these is a function call: a(x + y, z) + b + c. The compiler can evaluate x, then c, then z, then y, then b, then x + y, then evaluate a, then call the result of that evaluation, then adding that to b, then adding that to c.