Templated Functions

C++ templates can be used both for classes and for functions in C++. Templated functions are actually a bit easier to use than templated classes, as the compiler can often deduce the desired type from the function's argument list.

The syntax for declaring a templated function is similar to that for a templated class:

template <class type> type func_name(type arg1, ...);

For instance, to declare a templated function to add two values together, you could use the following syntax:

template <class type> type add(type a, type b)
{
    return a + b;
}

Now, when you actually use the add function, you can simply treat it like any other function because the desired type is also the type given for the arguments. This means that upon compiling the code, the compiler will know what type is desired:

int x = add(1, 2);

will correctly deduce that "type" should be int. This would be the equivalent of saying:

int x = add<int>(1, 2);

where the template is explicitly instantiated by giving the type as a template parameter.

On the other hand, type inference of this sort isn't always possible because it's not always feasible to guess the desired types from the arguments to the function. For instance, if you wanted a function that performed some kind of cast on the arguments, you might have a template with multiple parameters:

template <class type1, class type2> type2 cast(type1 x)
{
    return (type2)x;
}

Using this function without specifying the correct type for type2 would be impossible. On the other hand, it is possible to take advantage of some type inference if the template parameters are correctly ordered. In particular, if the first argument must be specified and the second deduced, it is only necessary to specify the first, and the second parameter can be deduced.

For instance, given the following declaration

template <class rettype, class argtype> rettype cast(argtype x)
{
    return (rettype)x;
}

this function call specifies everything that is necessary to allow the compiler deduce the correct type:

cast<double>(10);

which will cast an int to a double. Note that arguments to be deduced must always follow arguments to be specified. (This is similar to the way that default arguments to functions work.)

You might wonder why you cannot use type inference for classes in C++. The problem is that it would be a much more complex process with classes, especially as constructors may have multiple versions that take different numbers of parameters, and not all of the necessary template parameters may be used in any given constructor.

Templated Classes with Templated Functions

It is also possible to have a templated class that has a member function that is itself a template, separate from the class template. For instance,

template <class type> class TClass
{
    // constructors, etc
    
    template <class type2> type2 myFunc(type2 arg);
};

The function myFunc is a templated function inside of a templated class, and when you actually define the function, you must respect this by using the template keyword twice:

template <class type>  // For the class
    template <class type2>  // For the function
    type2 TClass<type>::myFunc(type2 arg)
    {
        // code
    }

The following attempt to combine the two is wrong and will not work:

// bad code!
template <class type, class type2> type2 TClass<type>::myFunc(type2 arg)
{
    // ...
}

because it suggests that the template is entirely the class template and not a function template at all.


Source

   

Templates and Templated Classes in C++

What's better than having several classes that do the same thing to different datatypes? One class that lets you choose which datatype it acts on.
Templates are a way of making your classes more abstract by letting you define the behavior of the class without actually knowing what datatype will be handled by the operations of the class. In essence, this is what is known as generic programming; this term is a useful way to think about templates because it helps remind the programmer that a templated class does not depend on the datatype (or types) it deals with. To a large degree, a templated class is more focused on the algorithmic thought rather than the specific nuances of a single datatype. Templates can be used in conjunction with abstract datatypes in order to allow them to handle any type of data. For example, you could make a templated stack class that can handle a stack of any datatype, rather than having to create a stack class for every different datatype for which you want the stack to function. The ability to have a single class that can handle several different datatypes means the code is easier to maintain, and it makes classes more reusable.
The basic syntax for declaring a templated class is as follows:

template <class a_type> class a_class {...};

The keyword 'class' above simply means that the identifier a_type will stand for a datatype. NB: a_type is not a keyword; it is an identifier that during the execution of the program will represent a single datatype. For example, you could, when defining variables in the class, use the following line:

a_type a_var;

and when the programmer defines which datatype 'a_type' is to be when the program instantiates a particular instance of a_class, a_var will be of that type.
When defining a function as a member of a templated class, it is necessary to define it as a templated function:

template<class a_type> void a_class<a_type>::a_function(){...}
               

When declaring an instance of a templated class, the syntax is as follows:

a_class<int> an_example_class;
              

An instantiated object of a templated class is called a specialization; the term specialization is useful to remember because it reminds us that the original class is a generic class, whereas a specific instantiation of a class is specialized for a single datatype (although it is possible to template multiple types).
Usually when writing code it is easiest to precede from concrete to abstract; therefore, it is easier to write a class for a specific datatype and then proceed to a templated - generic - class. For that brevity is the soul of wit, this example will be brief and therefore of little practical application.
We will define the first class to act only on integers.

class calc
{
  public:
    int multiply(int x, int y);
    int add(int x, int y);
 };
int calc::multiply(int x, int y)
{
  return x*y;
}
int calc::add(int x, int y)
{
  return x+y;
}

We now have a perfectly harmless little class that functions perfectly well for integers; but what if we decided we wanted a generic class that would work equally well for floating point numbers? We would use a template.

template <class A_Type> class calc
{
  public:
    A_Type multiply(A_Type x, A_Type y);
    A_Type add(A_Type x, A_Type y);
};
template <class A_Type> A_Type calc<A_Type>::multiply(A_Type x,A_Type y)
{
  return x*y;
}
template <class A_Type> A_Type calc<A_Type>::add(A_Type x, A_Type y)
{
  return x+y;
}

To understand the templated class, just think about replacing the identifier A_Type everywhere it appears, except as part of the template or class definition, with the keyword int. It would be the same as the above class; now when you instantiate an
object of class calc you can choose which datatype the class will handle.

calc <double> a_calc_class;

Templates are handy for making your programs more generic and allowing your code to be reused later.

Source

The C Preprocessor

The C preprocessor modifies a source code file before handing it over to the compiler. You're most likely used to using the preprocessor to include files directly into other files, or #define constants, but the preprocessor can also be used to create "inlined" code using macros expanded at compile time and to prevent code from being compiled twice.

There are essentially three uses of the preprocessor--directives, constants, and macros. Directives are commands that tell the preprocessor to skip part of a file, include another file, or define a constant or macro. Directives always begin with a sharp sign (#) and for readability should be placed flush to the left of the page. All other uses of the preprocessor involve processing #define'd constants or macros. Typically, constants and macros are written in ALL CAPS to indicate they are special (as we will see).

Header Files

The #include directive tells the preprocessor to grab the text of a file and place it directly into the current file. Typically, such statements are placed at the top of a program--hence the name "header file" for files thus included.

Constants

If we write

#define [identifier name] [value]

whenever [identifier name] shows up in the file, it will be replaced by [value].

If you are defining a constant in terms of a mathematical expression, it is wise to surround the entire value in parentheses:

#define PI_PLUS_ONE (3.14 + 1)

By doing so, you avoid the possibility that an order of operations issue will destroy the meaning of your constant:

x = PI_PLUS_ONE * 5;

Without parentheses, the above would be converted to

x = 3.14 + 1 * 5;

which would result in 1 * 5 being evaluated before the addition, not after. Oops!

It is also possible to write simply

#define [identifier name]

which defines [identifier name] without giving it a value. This can be useful in conjunction with another set of directives that allow conditional compilation.

Conditional Compilation

There are a whole set of options that can be used to determine whether the preprocessor will remove lines of code before handing the file to the compiler. They include #if, #elif, #else, #ifdef, and #ifndef. An #if or #if/#elif/#else block or a #ifdef or #ifndef block must be terminated with a closing #endif.

The #if directive takes a numerical argument that evaluates to true if it's non-zero. If its argument is false, then code until the closing #else, #elif, of #endif will be excluded.

Commenting out Code

Conditional compilation is a particularly useful way to comment out a block of code that contains multi-line comments (which cannot be nested).

#if 0
/* comment ...
*/

// code

/* comment */
#endif

Avoiding Including Files Multiple Times (idempotency)

Another common problem is that a header file is required in multiple other header files that are later included into a source code file, with the result often being that variables, structs, classes or functions appear to be defined multiple times (once for each time the header file is included). This can result in a lot of compile-time headaches. Fortunately, the preprocessor provides an easy technique for ensuring that any given file is included once and only once.

By using the #ifndef directive, you can include a block of text only if a particular expression is undefined; then, within the header file, you can define the expression. This ensures that the code in the #ifndef is included only the first time the file is loaded.

#ifndef _FILE_NAME_H_
#define _FILE_NAME_H_

/* code */

#endif // #ifndef _FILE_NAME_H_

Notice that it's not necessary to actually give a value to the expression _FILE_NAME_H_. It's sufficient to include the line "#define _FILE_NAME_H_" to make it "defined". (Note that there is an n in #ifndef--it stands for "if not defined").

A similar tactic can be used for defining specific constants, such as NULL:

#ifndef NULL
#define NULL (void *)0
#endif // #ifndef NULL

Notice that it's useful to comment which conditional statement a particular #endif terminates. This is particularly true because preprocessor directives are rarely indented, so it can be hard to follow the flow of execution.

Macros

The other major use of the preprocessor is to define macros. The advantage of a macro is that it can be type-neutral (this can also be a disadvantage, of course), and it's inlined directly into the code, so there isn't any function call overhead. (Note that in C++, it's possible to get around both of these issues with templated functions and the inline keyword.)

A macro definition is usually of the following form:

#define MACRO_NAME(arg1, arg2, ...) [code to expand to]

For instance, a simple increment macro might look like this:

#define INCREMENT(x) x++

They look a lot like function calls, but they're not so simple. There are actually a couple of tricky points when it comes to working with macros. First, remember that the exact text of the macro argument is "pasted in" to the macro. For instance, if you wrote something like this:

#define MULT(x, y) x * y

and then wrote

int z = MULT(3 + 2, 4 + 2);

what value do you expect z to end up with? The obvious answer, 30, is wrong! That's because what happens when the macro MULT expands is that it looks like this:

int z = 3 + 2 * 4 + 2;    // 2 * 4 will be evaluated first!

So z would end up with the value 13! This is almost certainly not what you want to happen. The way to avoid it is to force the arguments themselves to be evaluated before the rest of the macro body. You can do this by surrounding them by parentheses in the macro definition:

#define MULT(x, y) (x) * (y)
// now MULT(3 + 2, 4 + 2) will expand to (3 + 2) * (4 + 2)

But this isn't the only gotcha! It is also generally a good idea to surround the macro's code in parentheses if you expect it to return a value. Otherwise, you can get similar problems as when you define a constant. For instance, the following macro, which adds 5 to a given argument, has problems when embedded within a larger statement:

#define ADD_FIVE(a) (a) + 5

int x = ADD_FIVE(3) * 3;
// this expands to (3) + 5 * 3, so 5 * 3 is evaluated first
// Now x is 18, not 24!

To fix this, you generally want to surround the whole macro body with parentheses to prevent the surrounding context from affecting the macro body.

#define ADD_FIVE(a) ((a) + 5)

int x = ADD_FIVE(3) * 3;

On the other hand, if you have a multiline macro that you are using for its side effects, rather than to compute a value, you probably want to wrap it within curly braces so you don't have problems when using it following an if statement.

// We use a trick involving exclusive-or to swap two variables
#define SWAP(a, b)  a ^= b; b ^= a; a ^= b; 

int x = 10;
int y = 5;

// works OK
SWAP(x, y);

// What happens now?
if(x < 0)
    SWAP(x, y);

When SWAP is expanded in the second example, only the first statement, a ^= b, is governed by the conditional; the other two statements will always execute. What we really meant was that all of the statements should be grouped together, which we can enforce using curly braces:

#define SWAP(a, b)  {a ^= b; b ^= a; a ^= b;} 

Now, there is still a bit more to our story! What if you write code like so:

#define SWAP(a, b)  { a ^= b; b ^= a; a ^= b; }

int x = 10;
int y = 5;
int z = 4;

// What happens now?
if(x < 0)
    SWAP(x, y);
else
    SWAP(x, z); 

Then it will not compile because semicolon after the closing curly brace will break the flow between if and else. The solution? Use a do-while loop:

#define SWAP(a, b)  do { a ^= b; b ^= a; a ^= b; } while ( 0 )

int x = 10;
int y = 5;
int z = 4;

// What happens now?
if(x < 0)
    SWAP(x, y);
else
    SWAP(x, z); 

Now the semi-colon doesn't break anything because it is part of the expression. (By the way, note that we didn't surround the arguments in parentheses because we don't expect anyone to pass an expression into swap!)

More Gotchas

By now, you've probably realized why people don't really like using macros. They're dangerous, they're picky, and they're just not that safe. Perhaps the most irritating problem with macros is that you don't want to pass arguments with "side effects" to macros. By side effects, I mean any expression that does something besides evaluate to a value. For instance, ++x evaluates to x+1, but it also increments x. This increment operation is a side effect.

The problem with side effects is that macros don't evaluate their arguments; they just paste them into the macro text when performing the substitution. So something like

#define MAX(a, b) ((a) < (b) ? (b) : (a))
int x = 5, y = 10;
int z = MAX(x++, y++);

will end up looking like this:

int x = (x++ < y++ ? y++ : x++)

The problem here is that y++ ends up being evaluated twice! The nasty consequence is that after this expression, y will have a value of 12 rather than the expected 11. This can be a real pain to debug!

Multiline macros

Until now, we've seen only short, one line macros (possibly taking advantage of the semicolon to put multiple statements on one line.) It turns out that by using a the "\" to indicate a line continuation, we can write our macros across multiple lines to make them a bit more readable.

For instance, we could rewrite swap as

#define SWAP(a, b)  {                   \
                        a ^= b;         \
                        b ^= a;         \ 
                        a ^= b;         \
                    } 

Notice that you do not need a slash at the end of the last line! The slash tells the preprocessor that the macro continues to the next line, not that the line is a continuation from a previous line.

Aside from readability, writing multi-line macros may make it more obvious that you need to use curly braces to surround the body because it's more clear that multiple effects are happening at once.

Advanced Macro Tricks

In addition to simple substitution, the preprocessor can also perform a bit of extra work on macro arguments, such as turning them into strings or pasting them together.

Pasting Tokens

Each argument passed to a macro is a token, and sometimes it might be expedient to paste arguments together to form a new token. This could come in handy if you have a complicated structure and you'd like to debug your program by printing out different fields. Instead of writing out the whole structure each time, you might use a macro to pass in the field of the structure to print.

To paste tokens in a macro, use ## between the two things to paste together.

For instance

#define BUILD_FIELD(field) my_struct.inner_struct.union_a.##field

Now, when used with a particular field name, it will expand to something like

my_struct.inner_struct.union_a.field1

The tokens are literally pasted together.

String-izing Tokens

Another potentially useful macro option is to turn a token into a string containing the literal text of the token. This might be useful for printing out the token. The syntax is simple--simply prefix the token with a pound sign (#).

#define PRINT_TOKEN(token) printf(#token " is %d", token)

For instance, PRINT_TOKEN(foo) would expand to

printf("<foo>" " is %d" <foo>)

(Note that in C, string literals next to each other are concatenated, so something like "token" " is " " this " will effectively become "token is this". This can be useful for formatting printf statements.)

For instance, you might use it to print the value of an expression as well as the expression itself (for debugging purposes).

PRINT_TOKEN(x + y);

Avoiding Macros in C++

In C++, you should generally avoid macros when possible. You won't be able to avoid them entirely if you need the ability to paste tokens together, but with templated classes and type inference for templated functions, you shouldn't need to use macros to create type-neutral code. Inline functions should also get rid of the need for macros for efficiency reasons. (Though you aren't guaranteed that the compiler will inline your code.)

Moreover, you should use const to declare typed constants rather than #define to create untyped (and therefore less safe) constants. Const should work in pretty much all contexts where you would want to use a #define, including declaring static sized arrays or as template parameters.


Source