C++ Data Type Sizes

Size matters! So let’s learn about the actual sizes of C++ data types. A Bit is the smallest unit of storage. It’s 0 or 1, on or off.

A Byte is 8 bits. I wish the world used the French version of the word which is Octet. The combination of on/off in 8 bits can give us our data values!

A char takes up just 1 byte. Below represents 8 bits with different on/off values which represent a char that represents 1. Note that it’s not a value of 1 but a character that reads as 1.

If you use std::bitset, you can see this representation with bitset<CHAR_BIT>('a').

A short takes up 2 bytes or 16 bits. The numeric range of a short would be from 0 to 2¹⁶ or 65536. This is assuming it’s unsigned. If we want to support negative values, the range shifts (imagine the number line shifting by half) to -32768 to -32767.

An int takes up (assuming you’re on a 32-bit system) 4 bytes or 32 bits and can store considerably bigger values. You’re looking at 2³² which is 4294967296.
Note: you can always use sizeof() to verify the number of bytes your type takes which is subject to your computer architecture.

A double is even greater at 8 bytes or 64 bits. At approximately 1.8 * 10¹⁹, you’re dealing with massive numerical capacity (and memory implications too). Needless to say, don’t use a double when an int would suffice.

A float takes up 4 bytes much like an int. This might seem odd because floats seem to be capable of holding much larger values. This is because a float is encoded to represent exponentials. The IEEE 754 standard dictates that 1 bit determines the sign, 23 bits determine the fraction and 8 bits determine the exponential. The value below represents PI.

So we’re still employing 32 bits but we can express larger numbers with exponents. The cost of this wider range is accuracy.

A bool is one byte. However, this too seems odd at first. A bool is much like a bit - on or off, 0 or 1, false or true. Why do we need a whole byte to express a bool? That’s because the CPU addresses memory in bytes not bits. However with bit fields, we can force members of classes or structures to use less space.