Ошибка c2106 левый операнд должен быть левосторонним значением

Okay, so, ignoring my lazy coding (this is just to get the program to work, I’ll clean it up after I get it working). I’ve set up a couple of if statements that will throw exceptions if I don’t get the input I’d like.

#include<string>
#include<iostream>
using namespace std;

int main()
{

    bool flag = false;
    int month, day, year;
    void header();

    class monthClassException
    {
    public:
        monthClassException()
        {
            message = "Invalid Month"; 
        }
        monthClassException(string str)
        {
            message = str;
        }
        string what()
        {
            return message;
        }

    private:
        string message;
    };
    class dayClassException
    {
    };
    class yearClassException
    {
    };


    header();

    do 
    {
        try
        {
            cout << "Please enter your date of birth (MM-DD-YYYY): " << endl;
            cin >> month;

            cin.ignore(10,'-');
            cin >> day;

            cin.ignore(10,'-');
            cin >> year;


            if (month > 12 || month < 1)
                throw monthClassException("Invalid Month Entry");

            if( ((month == 4 || month == 6 || month == 9 || month == 11) && day > 30) || day < 1)
                throw dayClassException();

            else if ( ((month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12 ) && day > 31) || day < 1)
                throw dayClassException();

            else if (month == 2 && year % 4 != 0 && day > 28)
                throw dayClassException();

            else if((month == 2 && year % 4 = 0) && day > 29)
                throw dayClassException();
        }
        catch(monthClassException mCEO)
        {
            cout << mCEO.what() << endl;
            system("pause");
        }
        catch(dayClassException)
        {
            cout << "Invalid Day Entered for Selected Month" << endl;
            system("pause");
        }
        catch(yearClassException yCEO)
        {
        }
    }while(!flag);


    return 0;
}

I’m getting my error at that very last exception:

            else if((month == 2 && year % 4 = 0) && day > 29)
                throw dayClassException();

it’s saying that month is an invalid l-value (Why now? At the very end after I’ve already used it -catastrophically, I will admit.) It could be something really obvious that I fail to see because I’m the one who coded it, or it could be because my really really crazy if statements messed something up somewhere.

Any ideas?

asked Mar 23, 2014 at 18:23

= operator as in

year % 4 = 0

means assignment, not comparison. Hence your error.
Fix it to

year % 4 == 0

answered Mar 23, 2014 at 18:26

You have year % 4 = 0.
I think you have a typo: you may want year % 4 == 0.

In addition, I prefer using parentheses to make the code clearer:

...
else if ((month == 2) && (year % 4 == 0) && (day > 29)) {
    throw dayClassException();
}

answered Mar 23, 2014 at 18:27

You have a the assignment operator = in your condition instead of the comparison operator ==.

That’s pretty clearly a logic error. However, why is it a compiler error? After all, C++ allows assignment inside a condition, and that is something you might legitimately do.

In your case, month == 2 && year % 4 = 0 is processed as ((month == 2) && (year % 4)) = 0 (see C++ Operator Precedence). That expression in the parens evaluates to a temporary. But the left side of an assignment operator must refer to a storage address you can write to (an l-value). So your code is invalid for the same reason that 3 = 3 is invalid. Visual Studio calls this error C2106.

A suggestion in this regard, is to always put the constant on the left hand side of a comparison statement. It helps prevent logical errors. As an example consider the code

    if year == 0 

and mistakenly you had written:

    if year = 0 

the result would have been a logical error.
Instead putting the constant 0 on the left side, such that

     if 0 = year

would generate a syntax error at compilation, hence preventing you from committing a logical error (that can be harder to debug)

answered Mar 23, 2014 at 18:37

I am just starting to teach myself C++ and i have no coding experience so please bear with me on this if I ask you to explain your reasoning behind your response.

My code is:

// operating with varables
#include <iostream>
using namespace std;

//declarations
char STAR;
int PRIME = 71;

//begin main body
int main ()
{
    int count, sum, ex, num;
    sum = count + PRIME; 
    num = count * 1 + 2;
    (sum + count = ex);
    ex = sum * count;
    count = 25.67;
    cout << " Count = " << count << ", Sum = " << sum << ", Prime = " << PRIME << endl;
    return 0;

}

Debug errors received:

Error   1   error C2106: '=' : left operand must be l-value (error on line in bold)
Warning 2   warning C4244: '=' : conversion from 'double' to 'int', possible loss of data

Let me know what you all think.