CODEWITHSUNDEEP
c++boostundefined-behavior
SoapBox
SoapBox

Reputation: 20609

Parameter evaluation and undefined behavior

Given this example (which is a simplication of an example in the documentation for boost::multi_index_container about update rollbacks):

#include <iostream>

struct change
{
        int val_;
        change(int const &val) : val_(val)
        {
        }

        void operator() (int &v) const
        {
                v = val_;
        }
};

void do_it(int &v, change const &c1, change const &c2)
{
        c1(v);
        c2(v);
}

main()
{
        int i = 17;
        int orig = i;
        do_it(i, change(11), change(orig));
        std::cout << "i = " << i << std::endl;
}

Is the explicit copy of i (into orig) needed here? Or could this be more simply written as:

        int i = 17;
        do_it(i, change(11), change(i));

In that case, is the value of i after the call to do_it still guaranteed to be 17? Both change parameters would need to be constructed before the body of the function is executed.

Does the language guarantee that all of the parameters be constructed before the function begins? (If there is no guarantee, then the compiler could delay the construction of c2 until after c1's operator() has run and thus the final value would be 11)

Upvotes: 2

Views: 93

Answers (1)

Mark B
Mark B

Reputation: 96281

All the parameters to a function call are evaluated before the function is called. The order in which the parameters are evaluated is unspecified.

In your case i would still be 17.

Upvotes: 4

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