Precedence of post and pre incrementation operators

There is code:

#include <iostream>

class Int {
public:
    Int() : x(0) {}
    Int(int x_) : x(x_) {}
    Int& operator=(const Int& b) {
        std::cout << "= from " << x << " = " << b.x << std::endl;
        x = b.x;
    }
    Int& operator+=(const Int& b) {
        std::cout << "+= from " << x << " + " << b.x << std::endl;
        x += b.x;
        return *this;
    }
    Int& operator++() {
        std::cout << "++ prefix " << x << std::endl;
        ++x;
        return *this;
    }
    Int operator++(int) {
        std::cout << "++ postfix " << x << std::endl;
        Int result(*this);
        ++x;
        return result;
    }
private:
    int x;

};

Int operator+(const Int& a, const Int& b) {
    std::cout << "operator+" << std::endl;
    Int result(a);
    result += b;
    return result;
}

int main() {
    Int a(2), b(3), c(4), d;
    d = ++a + b++ + ++c;
    return 0;
}

Result:

++ prefix 4
++ postfix 3
++ prefix 2
operator+
+= from 3 + 3
operator+
+= from 6 + 5
= from 0 = 11

Why postfix operator isn't execute开发者_Go百科d before prefix operator (++ prefix 4) altough priority of postfix operator is higher than prefix operator?

This was compiled by g++.

The order of evaluation of the different operands is unspecified which means that the compiler is free to reorder the evaluation of the ++a, b++ and ++c subexpressions as it pleases. The precedence of the operators does not really have any impact in that example.

It does have an effect if you try to write ++i++ (where i is an int) which will be grouped as ++(i++) and it will fail to compile as the subexpression i++ is an rvalue and prefix increment requires an lvalue. If the precedence was reversed, then that expression would compile (and cause Undefined Behavior)

Postfix ++ has the highest precedence in the expression ++a + b++ + ++c, but + has the lowest precedence and is left associative. This expression can be equivalently written as (++a) + (b++) + (++c) (each ++ is part of a different subexpressions) which explains why ++a is evaluated first. Consider traversing/evaluating the corresponding parse tree and it becomes obvious what the order of evaluation:

           E
         / | \
        /  |  E
       /   |  | \
      E    +  ++  c
    / | \
   /  |  \
  E   +   E
 / \     / \
++  a   b  ++