Monday, May 4, 2009

c++ array variable doesn't carry size information

Java arrays are objects and, unlike c, has extra storage to hold its size. C arrays have exactly { sizeof(element) x initialized elementCount} bytes, so about the only way to pass on the array size information is to pass another variable. Examples follow.

Q: in c++ standard, why must we pass in an arraySize param to main(int arraySize, char ** args)?

Answer is below, but let me clarify that array params are widely used in C and (consequently) c++ financial apps.

Q1: what's the difference between f(sometype*) and f(sometype[] )?
A: no diff. Either way f() receives a pointer.
A: [[Absolute c++]] section on array parameter explains nicely that an array variable has 3 parts
- starting address
- type
- size

....but the size is not passed to f(). f() could increment the pointer forever without knowing it trespasses on the real estate of another object or de-allocated wasteland in the free list.

Rule on array parameter -- Always add a size param to an array param.

To pass a std::vector<MyType> into a f(MyType* data, int size), do it this way

    f(&vec[0], vec.size(); //P74 [[effSTL]]

Q2: But why is there never array size in C string functions?
A: the exception to the Rule. c-string is null-terminated, so pointer won't trespass.

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