- to determine the correct memory allocation required initially.
- to interpret the bit patterns found in the object during subsequent accesses.
- in many type-checking situations, to ensure that illegal assignments are trapped.
The C++Builder standard libraries and your own program and header files must provide unambiguous identifiers (or expressions derived from them) and types so that C++Builder can consistently access, interpret, and (possibly) change the bit patterns in memory corresponding to each active object in your program.
Objects and declarations
Declarations establish the necessary mapping between identifiers and objects. Each declaration associates an identifier with a data type. Most declarations, known as defining declarations, also establish the creation (where and when) of the object; that is, the allocation of physical memory and its possible initialization. Other declarations, known as referencing declarations, simply make their identifiers and types known to the compiler. There can be many referencing declarations for the same identifier, especially in a multifile program, but only one defining declaration for that identifier is allowed.
Generally speaking, an identifier cannot be legally used in a program before its declaration point in the source code. Legal exceptions to this rule (known as forward references) are labels, calls to undeclared functions, and class, struct, or union tags.