A ParameterPassing mode where the actual parameter is copied into the formal parameter at the point of function invocation (for example, implemented by copying into an agreed-upon register or location on the program stack).
Has the following attributes and consequences:
- Does not introduce aliasing; caller and callee get separate copies.
- Does not allow callee to influence state of caller (modifications to callee's formal parameter have no effect on caller; though there is a similar mode called CallByValueResult where the callee's formal parameter is passed back when the function returns).
- Can be expensive to implement for large mutable objects, especially those which cannot fit in registers.
- Forces expression evaluation at each passing point, i.e. all SideEffects are pre-call.
CallByValue refers to ParameterPassing modes with the above semantics, even if implemented differently under the hood.
See ParameterPassing, CallByReference, ExplicitLazyProgramming, CallByThunk