C++ Data Structure and Union Tutorial

Related YoLinux tutorials:

°Linux and C++

°C++ enumerations

°C++ Templates

Simple example of a program using C++, a structure and a constructor:

// File: struct-test.cpp // // This example shows the use of a structure in C++ and how it behaves much // like a class including the use of a contructor yet maintains the useability // of a regular C structure. #include <iostream> #include <string> using namespace std; main() { struct DataElement { string SVal; int iVal; bool hasData; DataElement() // Example of a constructor used in a structure. { iVal=-1; hasData=0; } } *RealData; RealData = new DataElement [ 5 ]; // Assignment RealData[0].SVal = "Value loaded into first structure element."; RealData[0].hasData = 1; // True cout << "First element 0: " << RealData[0].SVal << endl; cout << " " << RealData[0].hasData << endl; cout << "Second element 1: " << RealData[1].SVal << endl; cout << " " << RealData[1].hasData << endl; // Show effect of contructor cout << " " << RealData[1].iVal << endl; // Show effect of contructor delete [] RealData; // Or: delete [5] RealData; }

Compile: g++ struct-test.cpp

[Potential Pitfall]: In Red Hat Linux versions 7.x one could omit the "using namespace std;" statement. Use of this statement is good programming practice and is required in Red Hat 8.0.

[Potential Pitfall]: Red Hat 8.0 requires the reference to "#include <fstream>". Red Hat versions 7.x used "#include <fstream.h>".

Output: ./a.out

First element  0: Value loaded into first structure element.
                  1
Second element 1: 
                  0
                  -1

The same example using typedef and a C++ initializer:

#include <iostream> #include <string> using namespace std; typedef struct dataElement { string SVal; int iVal; bool hasData; dataElement() // Example of a constructor used in a structure. : iVal(-1), hasData(0) {} } DataElement; main() { DataElement *RealData; RealData = new DataElement [ 5 ]; RealData[0].SVal = "Value loaded into first structure element."; RealData[0].hasData = 1; // True cout << "First element 0: " << RealData[0].SVal << endl; cout << " " << RealData[0].hasData << endl; cout << "Second element 1: " << RealData[1].SVal << endl; cout << " " << RealData[1].hasData << endl; // Show effect of contructor cout << " " << RealData[1].iVal << endl; // Show effect of contructor delete [] RealData; // Or: delete [5] RealData; }

Identical results as above example.

By contrast the typical "C" structure initialization is as follows:

#include <stdio.h> typedef struct dataElement { char *cVal; int iVal; } DataElement; main() { DataElement RealData = {"Text goes here", 5 }; // "C" style initialization. printf("%s \nInteger value=%d\n",RealData.cVal,RealData.iVal); }

Compile: gcc struct-simple.c -o gcc struct-simple
Run: ./struct-simple

Text goes here
Integer value=5

C++ structure notes:

• The struct can employ a constructor to initalize variables.
• The struct can employ a destructor.
• The structure constructor can not be declared virtual.
• Structure member variables are public by default.

The following example shows how a "C" union can be used with a constructor to initialize data.

#include <stdio.h> typedef union uAA { double dVal; int iVal[2]; uAA() : dVal(3.22) {} } UAA; main() { UAA rdata; printf("Array output: %d %d \nDouble output: %lf \n", rdata.iVal[0], rdata.iVal[1], rdata.dVal); }

Compile: g++ union-test.cpp -o union-test
Run: ./union-test

Array output: 1546188227 1074381455
Double output: 3.220000

[Potential Pitfall]: The C++ difference - Structures used with union. A struct can NOT be used with a constructor if it is to be used in a union. The C++ recognition of the constructor in a struct converts it to a C++ class which is not valid in a "C" union.

Typical use of a "C" union: #include <stdio.h> typedef struct { int iVal1; int iVal2; } DataElement; typedef union { DataElement de; int iVal[2]; } UAA; main() { UAA rdata; rdata.de.iVal1 = 0; rdata.de.iVal2 = 1; printf("Array output: %d %d \n", rdata.iVal[0], rdata.iVal[1]); } Compile: gcc -o union-test union-test.c Note: Compiles properly with the gcc and g++ compiler. Run: ./union-test Array output: 0 1

Use of C++ structure (with constructor) in a union: #include <stdio.h> typedef struct dataElement { int iVal1; int iVal2; dataElement() : iVal1(1), iVal2(2){}; } DataElement; typedef union { DataElement de; int iVal[2]; } UAA; main() { UAA rdata; printf("Array output: %d %d \n", rdata.iVal[0], rdata.iVal[1]); } Compile: g++ -o union-test union-test.cpp union-test.cpp:13: error: member `DataElement ::de' with constructor not allowed in union

C++ union notes:

• The union can employ a constructor to initalize variables.
• The union can employ a destructor.
• Union member functions can NOT be declared virtual.
• Union member variables are public by default.
• A union's data members can NOT be declared static.
• A union can not be used as a base class.

Why a tutorial on such a simple subject? While simple, it has been overlooked by every C++ book I have ever seen. Many programmers I have met didn't know that constructors could be used with a C structure. I didn't know until I was told by another programmer. It was not covered by my professor nor was it in our very thourough C++ text book. This is just an FYI.

Books:

C++ How to Program by Harvey M. Deitel, Paul J. Deitel ISBN #0131857576, Prentice Hall Fifth edition. The first edition of this book (and Professor Sheely at UTA) taught me to program C++. It is complete and covers all the nuances of the C++ language. It also has good code examples. Good for both learning and reference.