C struct, typedef, enum & union - Tenouk C & C++

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STRUCT, TYPEDEF, ENUM & UNION• We have declared the structure, inventory, at the top of the sourcefile.• This is called an external declaration and the scope is the entire fileafter the declaration.• Since all the functions in the file use this structure tag, the structuremust be visible to each of them.• The main() calls readpart() to read data into a structure and returnit to be assigned to the variable, items.• Next, it calls printpart() passing items which merely prints thevalues of the structure fields, one at a time.• External declarations of structure templates and prototypes facilitateconsistent usage of tags and functions.• Sometimes, external structure tag declarations will be placed in aseparate header file, which is then made part of the source file by aninclude directive (#include).• From this example, we can see that using structures with functions isno different than using any scalar data type like int.www.tenouk.com, © 34/93
STRUCT, TYPEDEF, ENUM & UNION• When the function readpart() is called, memory is allocated for all of its localvariables, including the struct inventory variable, part.• As each data item is read, it is placed in the corresponding field of part,accessed with the dot operator.• The value of part is then returned to main() where it is assigned to the variableitems.• As would be the case for a scalar data type, the value of the return expression iscopied back to the calling function.• Since this is a structure, the entire structure (each of the fields) is copied.• For our inventory structure, this isn't too bad because only two floats and aninteger.• If structure having many members or nested, many values would need to becopied.• For call to printpart(), an inventory structure is passed to the function.• Recall that in C, all parameters are passed by value, hence the value of eachargument expression is copied from the calling function into the cell allocated forthe parameter of the called function.• Again, for large structures, this may not be a very efficient way to pass data tofunctions.www.tenouk.com, © 35/93
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C struct, typedef, enum & union - Tenouk C & C++

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