C++ error C2106: '=' : left operand must be l-value - c++

Question

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?

Answer 1

Here is the error:
year % 4 = 0
you probably meant to write ==

Answer 2

= operator as in
year % 4 = 0
means assignment, not comparison. Hence your error.
Fix it to
year % 4 == 0

Answer 3

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();
}

Answer 4

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.

Answer 5

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)