Question

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What is the “<<” operator in C++?

In a piece of code I am looking at, the following takes place:

... (header[4] << 8) + header[5] ...

I’m fairly new to programming and have never seen the << operator before. Googling didn’t provide any results. Any quick pointers would be appreciated!

Answer 1

<< operator shifts the bits to left by N bits where N comes after the operator. In your example the bits at the address of header[4] are shifted to left by 8 bits.

What this is good for is that it effectively results in multiplication by 256, because 2^8 = 256. If it were a shift to right the value at header[4] would be divided by 2^8 = 256.

Some real bit-level examples:

• Value of 2 (0010 in binary) shifted to right by 2 bits becomes 1000 which results in value of 8.
• Value of 7 (0111 in binary) shifted to left by 2 bits becomes 0001) which results in value of 1. (7/4 is 1.75 which gets rounded down because there’s no way to represent fractionals in integer binary values)
• Value of 31 (11110 in binary) shifted to left by 3 bits becomes 00011 which results in value of 3. (really it becomes 3.785 but it gets rounded down)

So in the end, very often a bit shift means either multiplication(shift to left) or division(shift to right) because that’s what it results in. That is, you can actually replace multiplications and divisions by power-of-two values with bit shifts, or replace bitshifts with multiplications. Compilers often prefer replacing multiplications and divisions with number of bit shifts because for the computer shifting the bits is much faster than actually doing real multiplication or division of the values.