Question

I’m trying to write a simple function to extract the next number from an expression in a string and my problem is that the reference parameter ioIsValidExpression is not being updated after the function exits. It looks like I am actually referencing the correct variable (and not a temporary) since the addresses of test3 (as it is called before calling getNextNumber(…)) and ioIsValidExpression (its name when inside the function) are the same.

I expect this to be an embarassing mistake somewhere but I’m getting nowhere trying to figure this one out; I’ve included my int main and int getNextNumber with a comment line where I return a value from the function.

int getNextNumber(std::string& iExpression, int& ioLoopShift, bool& ioIsValidExpression)
{
    //spaces are ignored, a minus is a unary minus (so must be seated next to a number (no spaces between '-' and number))
    int length = iExpression.size(); //evaluate only once
    bool unaryMinusFound = false;
    bool numberFound = false;

    int oSubExpression = 0;
    for(int i =0; i < length; i++)
    {
        if((int)iExpression[i] == 32)
        {
            if(!numberFound)
            {
                if(!unaryMinusFound)
                {
                    continue;
                }
                else
                {
                    ioIsValidExpression = false;
                    td::cout << "address of ioIsValidExpression: " << &ioIsValidExpression << ", value: " << ioIsValidExpression << "\n";
                    return 0;
                }
            }
            else
            {
                return unaryMinusFound? -1 * oSubExpression: oSubExpression;
            }
        }
        if((int)iExpression[i] == 45 && !unaryMinusFound)
        {
            unaryMinusFound = true;
            continue;
        }
        if((int)iExpression[i] >= 48 && (int)iExpression[i] <= 57)
        {
            numberFound = true;
            oSubExpression = (int)(iExpression[i]-48) + 10*oSubExpression;
            ioLoopShift++;
            continue;
        }
        else
        {
            //garbage characters
            ioIsValidExpression = false;
            return 0;
        }
    }
    return unaryMinusFound? -1 * oSubExpression: oSubExpression;
}

int main:

int main(int argc, char* argv[])
{
    std::string test = " - 45 & ";
    int test2 = 0;
    bool test3 = true;
    std::cout << getNextNumber(test,test2,test3) << ", address of test3: " << &test3 << ", value: " << test3 << "\n";

    return 0;
}

output from running the program:

address of ioIsValidExpression: 0xbf86e58f, value: 0
0, address of test3: 0xbf86e58f, value: 1

Answer 1

The order in which function arguments (and your output statement is a chain of functions) are evaluated is unspecified. Consider how it works correctly upon holding the result of the function in a variable before outputting the whole thing: test

int temp = getNextNumber(test, test2, test3);
std::cout << temp << ", address of test3: " << &test3 << ", value: " << test3 << "\n";

Output:

address of ioIsValidExpression: 0xbfaa85cf, value: 0
0, address of test3: 0xbfaa85cf, value: 0