Question

I have a following code:

#include <iostream>
int main() {
    int i = 3;
    do {
        (i == 3) ? (std::cout << "Is 3.\n") : ++i;
        ++i;

    } while ( i < 4 );
    return 0;
}

And got a following error in response:

ternary.cc: In function ‘int main()’:
ternary.cc:5:43: error: invalid conversion from ‘void*’ to ‘int’ [-fpermissive]

What is wrong with my code?

Answer 1

You’re abusing the ternary operator a bit, for any given a ? b : c I would expect the result to be stored somewhere and I wouldn’t recommend b or c having side effects.

The root of your problem is that the ternary operator requires b and c to be the same/equivalent type. Andrew’s explanation that they need to be able to “resolve to the same … type” is probably more accurate.

For what it’s worth, you can use a sleight-of-hand (or even more abuse, depending on your perspective) to make the code work:

#include <iostream>
int main() {
    int i = 3;
    do {
        (i == 3) ? (std::cout << "Is 3.\n", 0) : ++i;
        ++i;

    } while ( i < 4 );
    return 0;
}

Or, more explicitly, make sure both are the same type:

#include <iostream>
int main() {
    int i = 3;
    do {
        (i == 3) ? (std::cout << "Is 3.\n") : (void*)++i;
        ++i;

    } while ( i < 4 );
    return 0;
}