C++ Data Structure and Union Tutorial

The C++ compiler treats the C data structure like a C++ class. This allows one to use C++ constructs with a "struct" when using the C++ compiler. This is also true for a "C" union with a constructor. Sample code examples with explanations and tips are discussed.

Tutorial Contents:

# C++ and a struct
# C++ and a union

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°YoLinux Tutorials Index

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C++ Structure example:

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.

C++ Union example:

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.
	Dr. Dobb's Journal Free subscription to the premier resource for professional programmers and software developers. Multi-language and multi-platform with program listings, coding tips, design issue discussions and algorithms. Subscribe here!	Free Subscription