Question

When does memory get allocated to a data type, class and function? At the time of declaration or at the time of definition?

Also is there a way to know the sizeof function?

Answer 1

Size of different program elements in C++

When does memory get allocated to a data type, class and function? At the time of declaration or at the time of definition?

A data type is just an abstract notion understood by the compiler. When a variable of that type is needed, memory must be allocated. The:

• amount of memory – which is available at compile time using the sizeof operator (it’s not a function) – is determined by the compiler: for inbuilt types it’s fixed based on their binary representation, for user-defined types it’s basically a "recursive" sum of the data member’s individual sizes adjusted for alignment/packing)

• An actual address may be determined at:

  • compile time (typically global and static variables) [further details in comments]
  • at run-time (for heap-allocated variables),
  • in a hybrid run-time-stack-frame-relative way for local variables
  • at load-time or in a base-address-relative way (for global/extern/static symbols in dynamically loaded libraries).

Classes are just user-defined data types, and follow these same rules, though for classes it’s possible for the size to be unknown if a declaration has been seen by the compiler, but the definition is further into the source code: until the compiler sees the definition it can’t calculate the size of the object, and you can make very limited use of the class identifier – creating pointers and references that will point to an instance, but not actual variables of that type.

Also is there any way to know the sizeof function?

Again – it’s an operator which basically injects the compile-time constant size value that the compiler’s calculated. As such, there is no sizeof function in any machine-code object the compiler produces.