Why does this boolean function return true (74) and the program display 74?-CodePudding

I am just messing around with the function and somehow palindromes returns 74. I am using Visual studio 2022. Was it supposed to not return anything and catch compiler error since false is never returned in the case below?

bool palindromes(string str) {
    if (str.length() == 0 || str.length() == 1) return true;

    if (str[0] == str[str.length() - 1])
        palindromes(str.substr(1, str.length() - 2));
    else
        return false;
}


int main()
{
    cout << palindromes("lol");
}

CodePudding user response：

palindromes(str.substr(1, str.length() - 2)); is a typical mistake with recursion. I suppose you want to do something with the value returned from that call, but you ignore it. Recursive functions work very much the same as non-recursive functions, there is no implicit return.

Very much related, the function does not return something when this condition str.length() == 0 || str.length() == 1 is false but this condition str[0] == str[str.length() - 1] is true.

This line

palindromes(str.substr(1, str.length() - 2));

Does nothing. The function gets a parameter passed by value and the returned value is ignored.

Not returning something from a function that is declared to return something is undefined behavior.

Fix:

return palindromes(str.substr(1, str.length() - 2));

There might be other issues though.

CodePudding user response：

Seems that you ignored the warning:

warning C4715: 'palindromes': not all control paths return a value

This is a sin.

CodePudding user response：

Don't use recursion for what is a linear problem. Your multiple string copies add unnecessary overhead.

An alternative:

return std::equal(str.begin(), str.begin() str.size() / 2, str.rbegin());

This is readable and therefore less prone to bugs - your function not returning a value on all control paths is a bug: str.length() - 2 is vulnerable to unsigned arithmetic.