Seemingly incorrect conversion from int to char* - how to see an issue

Question

To my understanding this code snippet has a problem, since std::string will be removed after to_string and port should have an incorrect value.

#include <iostream>
#include <stdio.h>

int main() {
        int a = 123;
        const char* port = std::to_string(a).c_str();
        printf("val = %s", port);

        return 0;
}

However, program works just fine and prints "123" (with g++ compiler). Valgrind does not spot a problem as well.

Is there any way to demonstrate (maybe some tool or compiler option) that code above indeed has a problem?

https://stackoverflow.com/questions/10006891/stdstringc-str-and-temporaries — Cory Kramer, Dec 14 '15 at 12:48
Working fine (or at least *seemingly* working fine) is unfortunately one of the possibilities of UB. — Some programmer dude, Dec 14 '15 at 12:49
That's why that conversion is explicit and has that **ugly** name `c_str()`. — Pete Becker, Dec 14 '15 at 14:22
Unfortunately, this does not access after a free due to the small string optimization. If you change the code to `unsigned long long a = 1234567891343433ull;`, you will get complaints from `valgrind`. — David Schwartz, Sep 26 '17 at 20:14

Lightness Races in Orbit · Accepted Answer · 2015-12-14T12:53:57.707

2

No, you cannot "detect" undefined behaviour, in general. Electric Fence may help.

You should catch these kinds of problems during peer review.

It is also possible to almost eradicate the risk of creating them, by avoiding const char* entirely.

edited Dec 14 '15 at 12:53

answered Dec 14 '15 at 12:48

Lightness Races in Orbit

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Seemingly incorrect conversion from int to char* - how to see an issue

1 Answers1