What's the difference between pointer and value in struct?

Use the form which is most functionally useful for your program. Basically, this means if it's useful for the value to be nil, then use a pointer.

From a performance perspective, primitive numeric types are always more efficient to copy than to dereference a pointer. Even more complex data structures are still usually faster to copy if they are smaller than a cache line or two (under 128 bytes is a good rule of thumb for x86 CPUs).

When things get a little larger, you need to benchmark if performance concerns you. CPUs are very efficient at copying data, and there are so many variables involved which will determine the locality and cache friendliness of your data, it really depends on your program's behavior, and the hardware you're using.

This is an excellent series of articles if you want to better understand the how memory and software interact: "What every programmer should know about memory".

In short, I tell people to choose a pointer or not based on the logic of the program, and worry about performance later.

• Use a pointer if you need to pass something to be modified.
• Use a pointer if you need to determine if something was unset/nil.
• Use a pointer if you are using a type that has methods with pointer receivers.