error: lvalue required as left operand of assignment

/*lvalue means an assignable value (variable), and in assignment the left value to the = has to be lvalue (pretty clear).

Both function results and constants are not assignable (rvalues), so they are rvalues. so the order doesn't matter and if you forget to use == you will get this error. (edit:)I consider it a good practice in comparison to put the constant in the left side, so if you write = instead of ==, you will get a compilation error. for example:*/

int a = 5;
if (a = 0) // Always evaluated as false, no error.
{
    //...
}
vs.

int a = 5;
if (0 = a) // Generates compilation error, you cannot assign a to 0 (rvalue)
{
    //...
}