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The reason C++ acts as though it builds an isolating fence around each
function's parameters is that C++ reserves a chunk of memory for each
parameter every time the corresponding function is called. In the
box_car_volume example, a new chunk of memory is reserved for the
parameters, h, w, and l, and the arguments' values are
placed in those chunks, as shown here for l:
Memory reserved for l, Memory reserved for l, a variable in main a parameter in box_car_volume *------* *------* | | ---------------------> | | *------* *------*
Thus, the reassignment of the parameter, l, inside the function has
no effect on the value of the variable, l, outside, even though the
names, l and l, happen to be the same.
Because C++ generally reserves new chunks of memory for parameters and variables, into which values are copied, C++ is said to be a call-by-value language.