MFC Programmer's SourceBook : Thinking in C++

Composite type creation

The fundamental data types and their variations are essential, but rather primitive. C and C++ provide tools that allow you to compose more sophisticated data types from the fundamental data types. As you’ll see, the most important of these is struct, which is the foundation for class in C++. However, the simplest way to create more sophisticated types is simply to alias a name to another name via typedef.

Aliasing names with typedef

This keyword promises more than it delivers: typedef suggests “type definition” when “alias” would probably have been a more accurate description, since that’s all it really does. The syntax is:

typedef existing-type-description alias-name

People often use typedef when data types get slightly complicated, just to prevent extra keystrokes. Here is a commonly-used typedef:

typedef unsigned long ulong;

Now if you say ulong the compiler knows that you mean unsigned long . You might think that this could as easily be accomplished using preprocessor substitution, but there are key situations where the compiler must be aware that you’re treating a name as if it were a type, so typedef is essential.

You can argue that it’s more explicit and therefore more readable to avoid typedefs for primitive types, and indeed programs rapidly become difficult to read when many typedefs are used. However, typedefs become especially important in C when used with struct.

Combining variables with struct

A struct is a way to collect a group of variables into a structure. Once you create a struct, then you can make many instances of this “new” type of variable you’ve invented. For example:

//: C03:SimpleStruct.cpp

struct Structure1 {
  char c;
  int i;
  float f;
  double d;
};

int main() {
  struct Structure1 s1, s2;
  s1.c = 'a'; // Select an element using a '.'
  s1.i = 1;
  s1.f = 3.14;
  s1.d = 0.00093;
  s2.c = 'a';
  s2.i = 1;
  s2.f = 3.14;
  s2.d = 0.00093;
} ///:~

The struct declaration must end with a semicolon. In main( ) , two instances of Structure1 are created: s1 and s2. Each of these have their own separate versions of c, i, f and d. So s1 and s2 represent clumps of completely independent variables. To select one of the elements within s1 or s2, you use a ‘ .’, syntax you’ve seen in the previous chapter when using C++ class objects – since classes evolved from structs, this is where that syntax arose.

One thing you’ll notice is the awkwardness of the use of Structure1. You can’t just say Structure1 when you’re defininge variables, you must say struct Structure1 . This is where typedef becomes especially handy:

//: C03:SimpleStruct2.cpp
// Using typedef with struct
typedef struct {
  char c;
  int i;
  float f;
  double d;
} Structure2;

int main() {
  Structure2 s1, s2;
  s1.c = 'a';
  s1.i = 1;
  s1.f = 3.14;
  s1.d = 0.00093;
  s2.c = 'a';
  s2.i = 1;
  s2.f = 3.14;
  s2.d = 0.00093;
} ///:~

By using typedef in this way, you can pretend that Structure2 is a built-in type, like int or float, when you define s1 and s2 (but notice it only has data – characteristics – and does not include behavior, which is what we get with real objects in C++). You’ll notice that the struct name has been left off at the beginning, because the goal is to create the typedef. However, there are times when you might need to refer to the struct during its definition. In those cases, you can actually repeat the name of the struct as the struct name and as the typedef:

//: C03:SelfReferential.cpp
// Allowing a struct to refer to itself

typedef struct SelfReferential {
  int i;
  SelfReferential* sr; // Head spinning yet?
} SelfReferential;

int main() {
  SelfReferential sr1, sr2;
  sr1.sr = &sr2;
  sr2.sr = &sr1;
  sr1.i = 47;
  sr2.i = 1024;
} ///:~

If you look at this for awhile, you’ll see that sr1 and sr2 point to each other, as well as each holding a piece of data.

Actually, the struct name does not have to be the same as the typedef name, but it is usually done this way as it tends to keep things simpler.

Pointers and structs

In the above examples, all the structs are manipulated as objects. However, like any piece of storage you can take the address of a struct object (as seen in SelfReferential.cpp above). To select the elements of a particular struct object, you use a ‘ .’, as seen above. However, if you have a pointer to a struct object, you must select an element of that object using a different operator: the ‘ ->’. Here’s an example:

//: C03:SimpleStruct3.cpp
// Using pointers to structs
typedef struct Structure3 {
  char c;
  int i;
  float f;
  double d;
} Structure3;

int main() {
  Structure3 s1, s2;
  Structure3* sp = &s1;
  sp->c = 'a';
  sp->i = 1;
  sp->f = 3.14;
  sp->d = 0.00093;
  sp = &s2; // Point to a different struct object
  sp->c = 'a';
  sp->i = 1;
  sp->f = 3.14;
  sp->d = 0.00093;
} ///:~

In main( ), the struct pointer sp is initially pointing to s1, and the members of s1 are initialized by selecting them with the ‘ ->’ (and you use this same operator in order to read those members). But then sp is pointed to s2, and those variables are initialized the same way. So you can see that another benefit of pointers is that they can be dynamically redirected to point to different objects; this provides more flexibility in your programming, as you shall learn.

For now, that’s all you need to know about structs, but you’ll become much more comfortable with structs (and especially their more potent successors, classes) as the book progresses.

Clarifying programs with enum

An enumerated data type is a way of attaching names to numbers, thereby giving more meaning to anyone reading the code. The enum keyword (from C) automatically enumerates any list of words you give it by assigning them values of 0, 1, 2, etc. You can declare enum variables (which are always ints). The declaration of an enum looks similar to a struct declaration.

An enumerated data type is very useful when you want to keep track of some sort of feature:

//: C03:Enum.cpp
// Keeping track of shapes.

enum ShapeType {
  circle,
  square,
  rectangle
};  // Must end with a semicolon like a struct

int main() {
  ShapeType shape = circle;
  // Activities here....
  // Now do something based on what the shape is:
  switch(shape) {
    case circle:  /* circle stuff */ break;
    case square:  /* square stuff */ break;
    case rectangle:  /* rectangle stuff */ break;
  }
} ///:~

shape is a variable of the ShapeType enumerated data type, and its value is compared with the value in the enumeration. Since shape is really just an int, however, it can be any value an int can hold (including a negative number). You can also compare an int variable with a value in the enumeration.

If you don’t like the way the compiler assigns values, you can do it yourself, like this:

enum ShapeType { 
  circle = 10, square = 20, rectangle = 50
};

If you give values to some names and not to others, the compiler will use the next integral value. For example,

enum snap { crackle = 25, pop };

The compiler gives pop the value 26.

You can see how much more readable the code is when you use enumerated data types. However, to some degree this is still an attempt (in C) to accomplish the things that we can do with a class in C++, so you’ll see enum used less in C++.

Saving memory with union

Sometimes a program will handle different types of data using the same variable. In this situation, you have two choices: you can create a struct containing all the possible different types you might need to store, or you can use a union. A union piles all the data into a single space; it figures out the amount of space necessary for the largest item you’ve put in the union, and makes that the size of the union. Use a union to save memory.

Anytime you place a value in a union, the value always starts in the same place at the beginning of the union, but only uses as much space as is necessary. Thus, you create a “super-variable,” capable of holding any of the union variables. All the addresses of the union variables are the same (in a class or struct, the addresses are different).

Here’s a simple use of a union. Try removing various elements and see what effect it has on the size of the union. Notice that it makes no sense to declare more than one instance of a single data type in a union (unless you’re just doing it to use a different name).

//: C03:Union.cpp
// The size and simple use of a union
#include <iostream>
using namespace std;

union packed { // Declaration similar to a class
  char i;
  short j;
  int k;
  long l;
  float f;
  double d;  
  // The union will be the size of a 
  // double, since that's the largest element
};  // Semicolon ends a union, like a struct

int main() {
  cout << "sizeof(packed) = " 
    << sizeof(packed) << endl;
  packed x;
  x.i = 'c';
  cout << x.i << endl;
  x.d = 3.14159;
  cout << x.d << endl;
} ///:~

The compiler performs the proper assignment according to the union member you select.

Once you perform an assignment, the compiler doesn’t care what you do with the union. In the above example, you could assign a floating-point value to x:

x.f = 2.222;

and then send it to the output as if it were an int:

cout << x.i;

This would produce garbage.

Arrays

Arrays are a kind of composite type because they allow you to clump a lot of variables together, one right after the other, under a single identifier name. If you say:

int a[10];

You create storage for 10 int variables stacked on top of each other, but without unique identifier names for each variable. Instead, they are all lumped under the name a.

To access one of these array elements , you use the same square-bracket syntax that you use to define an array:

a[5] = 47;

However, you must remember that even though the size of a is 10, you select array elements starting at zero (this is sometimes called zero indexing ), so you can only select the array elements 0-9, like this:

//: C03:Arrays.cpp
#include <iostream>
using namespace std;

int main() {
  int a[10];
  for(int i = 0; i < 10; i++) {
    a[i] = i * 10;
    cout << "a[" << i << "] = " << a[i] << endl;
  }
} ///:~

Array access is extremely fast. However, if you index past the end of the array, there is no safety net – you’ll step on other variables. The other drawback is the fact that you must define the size of the array at compile time; if you want to change the size at run-time you can’t do it with the above syntax (C does have a way to create an array dynamically, but it’s significantly messier). The C++ vector, introduced in the previous chapter, provides an array-like object that automatic resizes itself, so it is usually a much better solution if your array size cannot be constant.

You can make an array of any type, even of structs:

//: C03:StructArray.cpp
// An array of struct

typedef struct {
  int i, j, k;
} ThreeDpoint;

int main() {
  ThreeDpoint p[10];
  for(int i = 0; i < 10; i++) {
    p[i].i = i + 1;
    p[i].j = i + 2;
    p[i].k = i + 3;
  }
} ///:~

Notice how the struct identifier i is independent of the for loop’s i.

To see that each element of an array is contiguous with the next, you can print out the addresses like this:

//: C03:ArrayAddresses.cpp
#include <iostream>
using namespace std;

int main() {
  int a[10];
  cout << "sizeof(int) = " << sizeof(int) <<endl;
  for(int i = 0; i < 10; i++)
    cout << "&a[" << i << "] = " 
      << (long)&a[i] << endl;
} ///:~

When you run this program, you’ll see that each element is one int size away from the previous one. That is, they are stacked one on top of the other.

Pointers and arrays

The identifier of an array is unlike the identifiers for ordinary variables. For one thing, an array identifier is not an lvalue – you cannot assign to it. It’s really just a hook into the square-bracket syntax, and when you give the name of an array, without square brackets, what you get is the starting address of the array:

//: C03:ArrayIdentifier.cpp
#include <iostream>
using namespace std;

int main() {
  int a[10];
  cout << "a = " << a << endl;
  cout << "&a[0] =" << &a[0] << endl;
} ///:~

When you run this program you’ll see that the two addresses (which will be printed in hexadecimal, since there is no cast to long) are the same.

So one way to look at the array identifier is as a read-only pointer to the beginning of an array. And although we can’t change the array identifier to point somewhere else, we can create another pointer and use that to move around in the array. In fact, the square-bracket syntax works with regular pointers, as well:

//: C03:PointersAndBrackets.cpp

int main() {
  int a[10];
  int* ip = a;
  for(int i = 0; i < 10; i++)
    ip[i] = i * 10;
} ///:~

The fact that naming an array produces its starting address turns out to be quite important when you want to pass an array to a function. If you declare an array as a function argument, what you’re really declaring is a pointer. So in the following example, func1( ) and func2( ) effectively have the same argument lists:

//: C03:ArrayArguments.cpp
#include <iostream>
#include <string>
using namespace std;

void func1(int a[], int size) {
  for(int i = 0; i < size; i++)
    a[i] = i * i - i;
}

void func2(int* a, int size) {
  for(int i = 0; i < size; i++)
    a[i] = i * i + i;
}

void print(int a[], string name, int size) {
  for(int i = 0; i < size; i++)
    cout << name << "[" << i << "] = " 
      << a[i] << endl;
}

int main() {
  int a[5], b[5];
  // Probably garbage values:
  print(a, "a", 5);
  print(b, "b", 5);
  // Initialize the arrays:
  func1(a, 5);
  func1(b, 5);
  print(a, "a", 5);
  print(b, "b", 5);
  // Notice the arrays are always modified:
  func2(a, 5);
  func2(b, 5);
  print(a, "a", 5);
  print(b, "b", 5);
} ///:~

Even though func1( ) and func2( ) declare their arguments differently, the usage is the same inside the function. There are some other issues that this example reveals: arrays cannot be passed by value [21], that is, you never automatically get a local copy of the array that you pass into a function. Thus, when you modify an array, you’re always modifying the outside object. This can be a bit confusing at first, if you’re expecting the pass-by-value provided with ordinary arguments.

You’ll notice that print( ) uses the square-bracket syntax for array arguments. Even though the pointer syntax and the square-bracket syntax are effectively the same when passing arrays as arguments, the square-bracket syntax makes it clearer to the reader that you mean for this argument to be an array.

Also note that the size argument is passed in each case. Just passing the address of an array isn’t enough information; you must always be able to know how big the array is inside your function, so you don’t run off the end of that array.

Arrays can be of any type, including arrays of pointers. In fact, when you want to pass command-line arguments into your program, C & C++ have a special argument list for main( ) which looks like this:

int main(int argc, char* argv[]) { // ...

The first argument is the number of elements in the array which is the second argument. The second argument is always an array of char*, because the arguments are passed from the command line as character arrays (and remember, an array can only be passed as a pointer). Each whitespace-delimited cluster of characters on the command line is turned into a separate array argument. The following program prints out all its command-line arguments by stepping through the array:

//: C03:CommandLineArgs.cpp
#include <iostream>
using namespace std;

int main(int argc, char* argv[]) {
  cout << "argc = " << argc << endl;
  for(int i = 0; i < argc; i++)
    cout << "argv[" << i << "] = " 
      << argv[i] << endl;
} ///:~

You’ll notice that argv[0] is the path and name of the program itself. This allows the program to discover information about itself. It also adds one more to the array of program arguments, so a common error when fetching command-line arguments is to grab argv[0] when you want argv[1].

You are not forced to use argc and argv as identifiers in main( ); those identifiers are only conventions (but it will confuse people if you don’t use them). Also, there are alternate ways to declare argv:

int main(int argc, char** argv) { // ...
int main(int argc, char argv[][]) { // ...

All three forms are equivalent, but I find the form used in this book to be the most intuitive when reading the code, since it says, directly, “this is an array of character pointers.”

All you get from the command-line is character arrays; if you want to treat an argument as some other type, you are responsible for converting it, inside your program. To facilitate the conversion to numbers, there are some helper functions in the Standard C library, declared in <cstdlib>. The simplest ones to use are atoi( ), atol( ) and atof( ), to convert an ASCII character array to an int, long and double floating-point value, respectively. Here’s an example using atoi( ) (the other two functions are called the same way):

//: C03:ArgsToInts.cpp
// Converting command-line arguments to ints
#include <iostream>
#include <cstdlib>
using namespace std;

int main(int argc, char* argv[]) {
  for(int i = 1; i < argc; i++)
    cout << atoi(argv[i]) << endl;
} ///:~

In this program, you can put any number of arguments on the command line. You’ll notice that the for loop starts at the value 1 to skip over the program name at argv[0]. Also, if you put a floating-point number containing a decimal point on the command line, atoi( ) only takes the digits up to the decimal point. If you put non-numbers on the command line, these come back from atoi( ) as zero

Pointer arithmetic

If all you could do with a pointer that points at an array is treat it as if it were an alias for that array, pointers into arrays wouldn’t be very interesting. However, pointers are more flexible than this, since they can be moved around (and remember, the array identifier cannot be moved).

Pointer arithmetic refers to the application of some of the arithmetic operators to pointers. The reason pointer arithmetic is a separate subject from ordinary arithmetic is that pointers must conform to special constraints in order to make them behave properly. For example, a common operator to use with pointers is ++, which “adds one to the pointer.” What this actually means is that the pointer is changed to move to “the next value,” whatever that means. Here’s an example:

//: C03:PointerIncrement.cpp
#include <iostream>
using namespace std;

int main() {
  int i[10];
  double d[10];
  int* ip = i;
  double* dp = d;
  cout << "ip = " << (long)ip << endl;
  ip++;
  cout << "ip = " << (long)ip << endl;
  cout << "dp = " << (long)dp << endl;
  dp++;
  cout << "dp = " << (long)dp << endl;
} ///:~

For one run on my machine, the output is:

ip = 6684124
ip = 6684128
dp = 6684044
dp = 6684052

What’s interesting here is that even though the operation ++ appears to be the same operation for both the int* and the double*, you can see that the pointer has been changed only 4 bytes for the int* but 8 bytes for the double*. Not coincidently, these are the sizes of int and double on my machine. And that’s the trick of pointer arithmetic: the compiler figures out the right amount to change the pointer so that it’s pointing to the next element in the array (pointer arithmetic is only meaningful within arrays). This even works with arrays of structs:

//: C03:PointerIncrement2.cpp
#include <iostream>
using namespace std;

typedef struct {
  char c;
  short s;
  int i;
  long l;
  float f;
  double d;
  long double ld;
} Primitives;

int main() {
  Primitives p[10];
  Primitives* pp = p;
  cout << "sizeof(Primitives) = " 
    << sizeof(Primitives) << endl;
  cout << "pp = " << (long)pp << endl;
  pp++;
  cout << "pp = " << (long)pp << endl;
} ///:~

The output for one run on my machine was:

sizeof(Primitives) = 40
pp = 6683764
pp = 6683804

So you can see the compiler also does the right thing for pointers to structs (and classes and unions).

Pointer arithmetic also works with the operators --, + and -, but the latter two operators are limited: you cannot add or subtract two pointers. Instead, you must add or subtract an integral value. Here’s an example demonstrating the use of pointer arithmetic:

//: C03:PointerArithmetic.cpp
#include <iostream>
using namespace std;

#define P(EXP) \
  cout << #EXP << ": " << EXP << endl;

int main() {
  int a[10];
  for(int i = 0; i < 10; i++)
    a[i] = i; // Give it index values
  int* ip = a;
  P(*ip);
  P(*++ip);
  P(*(ip + 5));
  int* ip2 = ip + 5;
  P(*ip2);
  P(*(ip2 - 4));
  P(*--ip2);  
} ///:~

It begins with another macro, but this one uses a preprocessor feature called stringizing (implemented with the ‘ #’ sign before an expression) which takes any expression and turns it into a character array. This is quite convenient, since it allows the expression to be printed, followed by a colon, followed by the value of the expression. In main( ) you can see the useful shorthand that is produced.

Although pre- and postfix versions of ++ and -- are valid with pointers, only the prefix versions are used in this example because they are applied before the pointers are dereferenced in the above expressions, so they allow us to see the effects of the operations. Note that only integral values are being added and subtracted; if two pointers were combined this way the compiler would not allow it.

Here is the output of the above program:

*ip: 0
*++ip: 1
*(ip + 5): 6
*ip2: 6
*(ip2 - 4): 2
*--ip2: 5

In all cases, the pointer arithmetic results in the pointer being adjusted to point to the “right place,” based on the size of the elements being pointed to.

If pointer arithmetic seems a bit overwhelming at first, don’t worry. Most of the time you’ll only need to create arrays and index into them with [ ], and the most sophisticated pointer arithmetic you’ll usually need is ++ and --. Pointer arithmetic is generally reserved for more clever and complex programs, and many of the containers in the Standard C++ library hide most of these clever details so you don’t have to worry about them.

[21] Unless you take the very strict approach that “all argument passing in C/C++ is by value, and the ‘value’ of an array is what is produced by the array identifier: it’s address.” This can be seen as true from the assembly-language standpoint, but I don’t think it helps when trying to work with higher-level concepts. The addition of references in C++ makes the “all passing is by value” argument more confusing, to the point where I feel it’s more helpful to think in terms of “passing by value” vs. “passing addresses.”

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