A conditional statement that I can't understand

user_17541457 0
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I saw a few lines of code in a C++ tutorial video

void copy_string(char* from, char* to)
{
    while ((*to = *from) != '\0')
    {
        to++;
        from++;
    }
}

The function is used to copy the string "from" into the string "to", but I can't understand the conditional statement inside the while loop. Why is it able to assign and compare at the same time?

conditional-statements c++ string
2 Answers

It's both an assignment *to = *from as well as a comparison

(*to = *from) != 0

It first assigns the dereferenced pointer from "from" to "to" then checks if the assigned value was equal to 0 which is a null terminator. If so, then exit the loop and the copying is done.

user_83506470

Why is it able to assign and compare at the same time?

The same expression both assigns and compares because it contains a separate operator for each purpose.

= is an assignment operator. It assigns the value of the right-hand operand to the left-hand operand (this is technically a side effect) and it evaluates to the value that was assigned. That it evaluates to a value may be the point you are missing.

!= is a comparison operator. It determines whether the values of its left-hand and right-hand operands are unequal. If so, it evaluates to 1, otherwise, it evaluates to 0.

There is also ==, a comparison operator that evaluates whether the values of its left-hand and right-hand operands are equal (opposite of !=) without modifying the value of either one.

Do note, however, that although the overall expression performs both assignment and comparison, those don't necessarily happen at the same time.

user_24022720
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