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class Student{
    int rolll;

int main(){
    Student x, y;
    x.roll = 33;
    y.roll = 3;

    if(x != y) {
    // do something

I am looking for a solution to why I am getting the “Invalid operands to binary expression c++” error?

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15,250 points

2 Answers

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Best answer


An expression  x!= y is like a function call to this operator, in fact, it is the same as  x.operator!=(y).

class Student{
    int roll;

    bool operator!=(const Student &other)
        return roll != other.roll;



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You've abject victim to the most vexing parse, which refers this isanumber is being employed as a function. Take out the parentheses and you must be fine:

int thisisanumber;

Further consider making it a bit more readable, like thisIsANumber. In case you ever require to know it, thisIsANumber uses the camel-case naming convention.

Declare your variable without brackets, see the below statements 

int thisisanumber;

Now, you know with brackets, it is interpreted as a function, and a function can't be passed as a parameter to the >> operator.

Your issue is the reputed most vexing parse. Originally everything, which could be parsed as a function declaration will be parsed as such. Hence the compiler will interpret int thisisanumber(); as a declaration of a function thisisanumber taking zero arguments and returning an int. In case you consider this behaviour the issues with cin>>thisisanumber; must be somewhat selfevident.

In case you remove the parantheses, altering the variable declaration to int thisisanumber;, your program must behave like you'd expect it to with thisisanumber being a variable of type int.

You might but reconsider your naming usage, thisisanumber isn't accurately readable. I would point out going with this_is_a_numberthisIsANumber or ThisIsANumber.

The surplus operator (otherwise known as the modulo operator) % is a binary operator (for example takes accurately 2 operands) and operates just on integer types (for example shortintlonglong long, etc).

It looks from the error message that the variable number1 is of type double. Further, the function pow from the math library returns a value of type double. These two values are the main reason for the error.

There are numerous solutions to this each of which rely on what you want to do. I'll unravel a couple of them:

  1. throwing the operands to ints
foo = (int) bar % (int) quux; // WARNING: you'll be risking to lose data
  1. Declaring number1 to be of an int type and writing your own version of pow that perhaps takes and returns ints
// works only for non-negative integer exponents
int myPow(int n, int exp)
    int result = 1;
    for (; exp > 0; exp--)
        result *= n;

    return result;

// some code

foo = number1 % pow(bar, quux);
  1. Declaring number1 to be of an int type and employing a library function like round, to round the value returned from pow then throwing it to an int

foo = number1 % (int) round(pow(bar, quux));
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31,120 points

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