The difference between += and =+

Question

I've misplaced += with =+ one too many times, and I think I keep forgetting because I don't know the difference between these two, only that one gives me the value I expect it to, and the other does not.

Why is this?

Solution 1

a += b is short-hand for a = a + b (though note that the expression a will only be evaluated once.)

a =+ b is a = (+b), i.e. assigning the unary + of b to a.

Examples:

int a = 15;
int b = -5;

a += b; // a is now 10
a =+ b; // a is now -5

Solution 2

+= is a compound assignment operator - it adds the RHS operand to the existing value of the LHS operand.

=+ is just the assignment operator followed by the unary + operator. It sets the value of the LHS operand to the value of the RHS operand:

int x = 10;

x += 10; // x = x + 10; i.e. x = 20

x =+ 5; // Equivalent to x = +5, so x = 5.

Solution 3

+= → Add the right side to the left

=+ → Don't use this. Set the left to the right side.

Solution 4

a += b equals a = a + b. a =+ b equals a = (+b).

Solution 5

x += y 

is the same as

x = x + y

and

x =+ y

is wrong but could be interpreted as

x = 0 + y

View more solutions

Comments

[Atreys]

+1 for "don't use this" If used intentionally, the intent is probably to confuse. If used unintentionally, it is a bug.

[deadcode]

Helpful and concise answer but, forgive me for being new, what is meant by unary + of b? Isn't a = b same as a =+ b according to this?

[dlev]

@deadcode Yes! The unary + isn't the "make operand positive" operator, it's actually the "identity" operator. It's... not that helpful :)

[SunnyAk]

The way a = +b works is as follows: int b = -5; Variable a is now set to a unary of b when we write a = +b; So, a = + (-5), resulting in a=-5