C++ Programming/Code/Statements/Functions

Functions

A function (which can also be referred to as subroutine, procedure, subprogram or even method) carries out tasks defined by a sequence of statements called a statement block that need only be written once and called by a program as many times as needed to carry out the same task. Functions may depend on variables passed to them, called arguments, and may pass results of a task on to the caller of the function, this is called the return value.

In C++ is important to note that a function that exists in the global scope can also be called global function and a function that is defined inside a class is called a member function. (The term method is commonly used in other programming languages to refer to things like member functions, but this can lead to confusion in dealing with C++ which supports both virtual and non-virtual dispatch of member functions.)

Declarations

A Function must be declared before being used, with a name to identify it, what type of value the function returns and the types of any arguments that are to be passed to it. Parameters must be named and declare what type of value it takes. Parameters should always be passed as const if their arguments are not modified. Usually functions performs actions, so the name should make clear what it does. By using verbs in function names and following other naming conventions programs can be read more naturally.

Example:

int main()
{
    // code
    return 0;
}

defines a function named main that returns an integer value int and takes no parameters. The content of the function is called the body of the function. The word int is shown in bold because it is a keyword. C++ keywords have some special meaning and are also reserved words, i.e., cannot be used for any purpose other than what they are meant for. On the other hand main is not a keyword and you can use it in many places where a keyword cannot be used (though that is not recommended, as confusion could result).

main

The function main also happens to be the entry point of any (standard-compliant) C++ program and must be defined. The compiler arranges for the main function to be called when the program begins execution. main may call other functions which may call yet other functions.

The main function returns an integer value. In certain systems, this value is interpreted as a success/failure code. The return value of zero signifies a successful completion of the program. Any non-zero value is considered a failure. Unlike other functions, if control reaches the end of main(), an implicit return 0; for success is automatically added. To make return values from main more readable, the header file cstdlib defines the constants EXIT_SUCCESS and EXIT_FAILURE (to indicate successful/unsuccessful completion respectively).

The main function can also be declared like this:

int main(int argc, char **argv){
  // code
}

which defines the main function as returning an integer value int and taking two parameters. The first parameter of the main function, argc, is an integer value int that specifies the number of arguments passed to the program, while the second, argv, is an array of strings containing the actual arguments. There is almost always at least one argument passed to a program; the name of the program itself is the first argument, argv[0]. Other arguments may be passed from the system.

Example

#include <iostream>

int main(int argc, char **argv){
  std::cout << "Number of arguments: " << argc << std::endl;
  for(size_t i = 0; i < argc; i++)
    std::cout << "  Argument " << i << " = '" << argv[i] << "'" << std::endl;
}

If the program above is compiled into the executable arguments and executed from the command line like this in *nix:

$ ./arguments I love chocolate cake

Or in Command Prompt in Windows or MS-DOS:

C:\>arguments I love chocolate cake

It will output the following (but note that argument 0 may not be quite the same as this -- it might include a full path, or it might include the program name only, or it might include a relative path, or it might even be empty):

Number of arguments: 5
  Argument 0 = './arguments'
  Argument 1 = 'I'
  Argument 2 = 'love'
  Argument 3 = 'chocolate'
  Argument 4 = 'cake'

You can see that the command line arguments of the program are stored into the argv array, and that argc contains the length of that array. This allows you to change the behavior of a program based on the command line arguments passed to it.

Parameters

The syntax for declaring and invoking functions with multiple parameters is a common source of errors. First, remember that you have to declare the type of every parameter.

// Example - function using two int parameters by value
void printTime (int hour, int minute) { 
  std::cout << hour; 
  std::cout << ":"; 
  std::cout << minute; 
}

It might be tempting to write (int hour, minute), but that format is only legal for variable declarations, not for parameter declarations.

Another common source of confusion is that you do not have to declare the types of arguments when you call a function (indeed it is an error to attempt to do so).

Example

int hour = 11; 
int minute = 59; 
printTime( int hour, int minute ); // WRONG!

In this case, the compiler can tell the type of hour and minute by looking at their declarations. It is unnecessary and illegal to include the type when you pass them as arguments. The correct syntax is printTime( hour, minute ).

Passing by Pointer

Arrays are similar to pointers, remember?

Now might be a good time to reread the section on arrays. If you don't feel like flipping back that far, though, here's a brief recap: Arrays are blocks of memory space.

int my_array[5];

In the statement above, my_array is an area in memory big enough to hold five integers. To use an element of the array, it must be dereferenced. The third element in the array (remember they're zero-indexed) is my_array[2]. When you write my_array[2], you're actually saying "give me the third integer in the array my_array". Therefore, my_array is an array, but my_array[2] is an integer.

Passing a single array element

So let's say you want to pass one of the integers in your array into a function. How do you do it? Simply pass in the dereferenced element, and you'll be fine.

Example

#include <iostream>

void printInt(int printable){
  std::cout << "The int you passed in has value " << printable << std::endl;
}
int main(){
  int my_array[5];
  
  // Reminder: always initialize your array values!
  for(int i = 0; i < 5; i++)
    my_array[i] = i * 2;
  
  for(int i = 0; i < 5; i++)
    printInt(my_array[i]); // <-- We pass in a dereferenced array element
}

This program outputs the following:

The int you passed in has value 0
The int you passed in has value 2
The int you passed in has value 4
The int you passed in has value 6
The int you passed in has value 8

Look at that! We've passed in array elements just like normal integers! Because, of course, my_array[2] IS an integer.

Passing a whole array

Well, we can pass single array elements into a function. But what if we want to pass a whole array? We can do that too, of course.

Example

#include <iostream>

void printIntArr(int *array_arg, int array_len){
  std::cout << "The length of the array is " << array_len << std::endl;
  for(int i = 0; i < array_len; i++)
    std::cout << "Array[" << i << "] = " << array_arg[i] << std::endl;
}
 
int main(){
  int my_array[5];

  // Reminder: always initialize your array values!
  for(int i = 0; i < 5; i++)
    my_array[i] = i * 2;
  
  printIntArr(my_array, 5);
}

This will output the following:

The length of the array is 5
Array[0] = 0
Array[1] = 2
Array[2] = 4
Array[3] = 6
Array[4] = 8

Look at that, we've passed a whole array to a function! Now here's some important points to realize:

• Once you pass an array to a function, that function has no idea how to guess the length of the array. Unless you always use arrays that are the same size, you should always pass in the array length along with the array.
• You've passed in a POINTER. my_array is an array, not a pointer. If you change array_arg within the function, my_array doesn't change (i.e., if you set array_arg to point to a new array). But if you change any element of array_arg, you're changing the memory space pointed to by array_arg, which is the array my_array.

Passing by Reference

The same concept of references is used when passing variables.

Example

void foo( int &i )
{
  ++i;
}
 
int main()
{
  int bar = 5;   // bar == 5
  foo( bar );    // bar == 6
  foo( bar );    // bar == 7
 
  return 0;
}

Here we display one of the two common uses of references in function arguments -- they allow us to use the conventional syntax of passing an argument by value but manipulate the value in the caller.

However there is a more common use of references in function arguments -- they can also be used to pass a handle to a large data structure without making multiple copies of it in the process. Consider the following:

void foo( const std::string & s ) // const reference, explained below
{
  std::cout << s << std::endl;
}

void bar( std::string s )
{
  std::cout << s << std::endl;
}

int main()
{
  std::string const text = "This is a test.";

  foo( text ); // doesn't make a copy of "text"
  bar( text ); // makes a copy of "text"

  return 0;
}

In this simple example we're able to see the differences in pass by value and pass by reference. In this case pass by value just expends a few additional bytes, but imagine for instance if text contained the text of an entire book.

The reason why we use a constant reference instead of a reference is the user of this function can assure that the value of the variable passed does not change within the function. We technically call this "const-to-reference".

The ability to pass it by reference keeps us from needing to make a copy of the string and avoids the ugliness of using a pointer.

Passing by Value

When we want to write a function which the value of the argument is independent to the passed variable, we use pass-by-value approach.

Read the following code:

int add(int num1, int num2)
{
 num1 += num2; // change of value of "num1"
 return num1;
}

int main()
{
 int a = 10, b = 20, ans;
 ans = add(a, b);
 std::cout << a << " + " << b << " = " << ans << std::endl;
 return 0;
}

Output:

10 + 20 = 30

In function "add", we can see that the value of "num1" is changed in line 3. However, we can also see that the value of "a" keeps after passed to this function. This shows a property of pass-by-value, the arguments are copies of the passed variable and only in the scoop of the corresponding function.

Constant Parameters

The keyword const can also be used as a guarantee that a function will not modify a value that is passed in. This is really only useful for references and pointers (and not things passed by value), though there's nothing syntactically to prevent the use of const for arguments passed by value.

Take for example the following functions:

void foo( const std::string &s )
{
   s.append("blah"); // ERROR -- we can't modify the string

   std::cout << s.length() << std::endl; // fine
}

void bar( const Widget *w )
{
    w->rotate(); // ERROR - rotate wouldn't be const

    std::cout << w->name() << std::endl; // fine
}
       

In the first example we tried to call a non-const method -- append() -- on an argument passed as a const reference, thus breaking our agreement with the caller not to modify it and the compiler will give us an error.

The same is true with rotate(), but with a const pointer in the second example.

Default values

Parameters in C++ functions (including member functions and constructors) can be declared with default values, like this

int foo (int a, int b = 5, int c = 3);

Then if the function is called with fewer arguments (but enough to specify the arguments without default values), the compiler will assume the default values for the missing arguments at the end. For example, if I call

foo(6, 1)

that will be equivalent to calling

foo(6, 1, 3)

In many situations, this saves you from having to define two separate functions that take different numbers of parameters, which are almost identical except for a default value.

The "value" that is given as the default value is often a constant, but may be any valid expression, including a function call that performs arbitrary computation.

Default values can only be given for the last arguments; i.e. you cannot give a default value for a parameter that is followed by a parameter that doesn't have a default value, since it will never be used.

Once you define the default value for a parameter in a function declaration, you cannot re-define a default value for the same parameter in a later declaration, even if it is the same value.

Returning Values

When declaring a function, you must declare it in terms of the type that it will return, for example:

 int MyFunc(); // returns an int 
 SOMETYPE MyFunc(); // returns a SOMETYPE 
 
 int* MyFunc(); // returns a pointer to an int 
 SOMETYPE *MyFunc(); // returns a pointer to a SOMETYPE 
 SOMETYPE &MyFunc(); // returns a reference to a SOMETYPE 

If you have understood the syntax of pointer declarations, the declaration of a function that returns a pointer or a reference should seem logical. The above piece of code shows how to declare a function that will return a reference or a pointer; below are outlines of what the definitions (implementations) of such functions would look like:

SOMETYPE *MyFunc(int *p) 
{ 
  ... 
  ... 
  return p; 
} 

SOMETYPE &MyFunc(int &r) 
{ 
  ... 
  ... 
  return r; 
} 

Within the body of the function, the return statement should NOT return a pointer or a reference that has the address in memory of a local variable that was declared within the function, because as soon as the function exits, all local variables are destroyed and your pointer or reference will be pointing to some place in memory that you really do not care about. If the object to which a pointer refers is destroyed, the pointer is said to be a dangling pointer until it is given a new value; any use of the value of such a pointer is invalid. Having a dangling pointer like that is dangerous; pointers or references to local variables must not be allowed to escape the function in which those local (aka automatic) variables live.

However, within the body of your function, if your pointer or reference has the address in memory of a data type, struct, or class that you dynamically allocated the memory for, using the new operator, then returning said pointer or reference would be reasonable:

SOMETYPE *MyFunc()  //returning a pointer that has a dynamically 
{           //allocated memory address is valid code 
  int *p = new int[5]; 
  ... 
  ... 
  return p; 
}

(In most cases, a better approach in that case would be to return an object such as a smart pointer which could manage the memory; explicit memory management using widely distributed calls to new and delete (or malloc and free) is tedious, verbose and error prone. At the very least, functions which return dynamically allocated resources should be carefully documented. See this book's section on memory management for more details.)

const SOMETYPE *MyFunc(int *p) 
{
  ... 
  ... 
  return p; 
} 

in this case the SOMETYPE object pointed to by the returned pointer may not be modified, and if SOMETYPE is a class then only const member functions may be called on the SOMETYPE object.

If such a const return value is a pointer or a reference to a class then we cannot call non-const methods on that pointer or reference since that would break our agreement not to change it.

Functions with results

You might have noticed by now that some of the functions yield results. Other functions perform an action but don't return a value. That raises some questions:

• What happens if you call a function and you don't do anything with the result (i.e. you don't assign it to a variable or use it as part of a larger expression)?
• What happens if you use a function without a result as part of an expression, like newLine() + 7?
• Can we write functions that yield results, or are we stuck with things like newLine and printTwice?

The answer to the third question is "yes, you can write functions that return values," and we'll do it in a couple of chapters. I will leave it up to you to answer the other two questions by trying them out. Any time you have a question about what is legal or illegal in C++, a first step to find out is to ask the compiler. However you can not rely on the compiler for two reasons: First a compiler has bugs just like any other software, so it happens that not every source code which is forbidden in C++ is properly rejected by the compiler, and vice versa. The other reason is even more dangerous: You can write programs in C++ which a C++ implementation is not required to reject, but whose behavior is not defined by the language. Needless to say, running such a program can, and occasionally will, do harmful things to the system it is running or produce corrupt output!

Return Codes (best practices)

There are 2 kinds of behaviors :

Positive Means Success

This is the "logical" way to think, and as such the one used by almost all beginners. In C++, this takes the form of a boolean true/false test, where "true" (also 1 or any non-zero number) means success, and "false" (also 0) means failure.

The major problem of this construct is that all errors return the same value (false), so you must have some kind of externally visible error code in order to determine where the error occurred. For example:

 bool bOK;
 if (my_function1())
 {
     // block of instruction 1
     if (my_function2())
     {
         // block of instruction 2
         if (my_function3())
         {
              // block of instruction 3
              // Everything worked
              error_code = NO_ERROR;
              bOK = true;
         }
         else
         {
              //error handler for function 3 errors
              error_code = FUNCTION_3_FAILED;
              bOK = false;
         }
     }
     else
     {
         //error handler for function 2 errors
         error_code = FUNCTION_2_FAILED;
         bOK = false;
     }
 }
 else
 {
     //error handler for function 1 errors
     error_code = FUNCTION_1_FAILED;
     bOK = false;
 }
 return bOK;
 

As you can see, the else blocks (usually error handling) of my_function1 can be really far from the test itself; this is the first problem. When your function begins to grow, it's often difficult to see the test and the error handling at the same time.

This problem can be compensated by source code editor features such as folding, or by testing for a function returning "false" instead of true.

 if (!my_function1()) // or if (my_function1() == false) 
 {
     //error handler for function 1 errors
 ...   

This can also make the code look more like the "0 means success" paradigm, but a little less readable.

The second problem of this construct is that it tends to break up logical tests (my_function2 is one level more indented, my_function3 is 2 levels indented) which causes legibility problems.

One advantage here is that you follow the structured programming principle of a function having a single entry and a single exit.

The Microsoft Foundation Class Library (MFC) is an example of a standard library that uses this paradigm.

0 means success

This means that if a function returns 0, the function has completed successfully. Any other value means that an error occurred, and the value returned may be an indication of what error occurred.

The advantage of this paradigm is that the error handling is closer to the test itself. For example the previous code becomes:

 if (my_function1())
 {
     //error handler for function 1 errors
     return FUNCTION_1_FAILED;
 }
 // block of instruction 1
 if (my_function2())
 {
     //error handler for function 2 errors
     return FUNCTION_2_FAILED;
 }
 // block of instruction 2
 if (my_function3())
 {
     //error handler for function 3 errors
     return FUNCTION_3_FAILED;
 }
 // block of instruction 3
 // Everything worked
 return 0; // NO_ERROR

In this example, this code is more readable (this will not always be the case). However, this function now has multiple exit points, violating a principle of structured programming.

The C Standard Library (libc) is an example of a standard library that uses this paradigm.

Composition

Just as with mathematical functions, C++ functions can be composed, meaning that you use one expression as part of another. For example, you can use any expression as an argument to a function:

double x = cos (angle + pi/2); 

This statement takes the value of pi, divides it by two and adds the result to the value of angle. The sum is then passed as an argument to the cos function.

You can also take the result of one function and pass it as an argument to another:

double x = exp (log (10.0)); 

This statement finds the log base e of 10 and then raises e to that power. The result gets assigned to x; I hope you know what it is.

Recursion

In programming languages, recursion was first implemented in Lisp on the basis of a mathematical concept that existed earlier on, it is a concept that allows us to break down a problem into one or more subproblems that are similar in form to the original problem, in this case, of having a function call itself in some circumstances. It is generally distinguished from [[../Flow Control#Loops (iterations)|iterators or loops]].

A simple example of a recursive function is:

 void func(){
    func();
 }

It should be noted that non-terminating recursive functions as shown above are almost never used in programs (indeed, some definitions of recursion would exclude such non-terminating definitions). A terminating condition is used to prevent infinite recursion.

Example

 double power(double x, int n)
 {
  if(n < 0)
  {
     std::cout << std::endl
               << "Negative index, program terminated.";
     exit(1);
  }
  if(n)
     return x * power(x, n-1);
  else
     return 1.0;
 }

The above function can be called like this:

 x = power(x, static_cast<int>(power(2.0, 2)));

Why is recursion useful? Although, theoretically, anything possible by recursion is also possible by iteration (that is, while), it is sometimes much more convenient to use recursion. Recursive code happens to be much easier to follow as in the example below. The problem with recursive code is that it takes too much memory. Since the function is called many times, without the data from the calling function removed, memory requirements increase significantly. But often the simplicity and elegance of recursive code overrules the memory requirements.

The classic example of recursion is the factorial: $n!=(n-1)!n$, where $0!=1$ by convention. In recursion, this function can be succinctly defined as

unsigned factorial(unsigned n)
{
  if(n != 0) 
  {
    return n * factorial(n-1);
  } 
  else 
  {
    return 1;
  }
}

With iteration, the logic is harder to see:

unsigned factorial2(unsigned n)
{
  int a = 1;
  while(n > 0)
  {
    a = a*n;
    n = n-1;
  }
  return a;
}

Although recursion tends to be slightly slower than iteration, it should be used where using iteration would yield long, difficult-to-understand code. Also, keep in mind that recursive functions take up additional memory (on the stack) for each level. Thus they can run out of memory where an iterative approach may just use constant memory.

Each recursive function needs to have a Base Case. A base case is where the recursive function stops calling itself and returns a value. The value returned is (hopefully) the desired value.

For the previous example,

unsigned factorial(unsigned n)
{
  if(n != 0) 
  {
    return n * factorial(n-1);
  } 
  else 
  {
    return 1;
  }
}

the base case is reached when $n=0$. In this example, the base case is everything contained in the else statement (which happens to return the number 1). The overall value that is returned is every value from $n$ to $0$ multiplied together. So, suppose we call the function and pass it the value $3$. The function then does the math $3*2*1=6$ and returns 6 as the result of calling factorial(3).

Another classic example of recursion is the sequence of Fibonacci numbers:

0 1 1 2 3 5 8 13 21 34 ...

The zeroth element of the sequence is 0. The next element is 1. Any other number of this series is the sum of the two elements coming before it. As an exercise, write a function that returns the nth Fibonacci number using recursion.

Inline

Normally when calling a function, a program will evaluate and store the arguments, and then call (or branch to) the function's code, and then the function will later return back to the caller. While function calls are fast (typically taking much less than a microsecond on modern processors), the overhead can sometimes be significant, particularly if the function is simple and is called many times.

One approach which can be a performance optimization in some situations is to use so-called "inline" functions. Marking a function as inline is a request (sometimes called a hint) to the compiler to consider replacing a call to the function by a copy of the code of that function.

The result is in some ways similar to the use of the #define macro, but as mentioned before, macros can lead to problems since they aren't evaluated by the preprocessor. Inline functions do not suffer from the same problems.

If the inlined function is large, this replacement process (known for obvious reasons as "inlining") can lead to "code bloat", leading to bigger (and hence usually slower) code. However, for small functions it can even reduce code size, particularly once a compiler's optimizer runs.

Note that the inlining process requires that the function's definition (including the code) must be available to the compiler. In particular, inline headers that are used from more than one source file must be completely defined within a header file (whereas with regular functions that would be an error).

The most common way to designate that a function is inline is by the use of the inline keyword. One must keep in mind that compilers can be configured to ignore this keyword and use their own optimizations.

Further considerations are given when dealing with inline member function, this will be covered on the Object-Oriented Programming Chapter .

Pointers to functions

To declare a pointer to a function naively, the name of the pointer must be parenthesized, otherwise a function returning a pointer will be declared.

Example

 int (*ptof)(int arg);

The function to be referenced must obviously have the same return type and the same parameter type as that of the pointer to function. The address of the function can be assigned just by using its name, optionally prefixed with the address-of operator &. Calling the function can be done by using either ptof(<value>) or (*ptof)(<value>).

Example

 int (*ptof)(int arg);
 int func(int arg){
     //function body
 }
 ptof = &func; // get a pointer to func
 ptof = func;  // same effect as ptof = &func
 (*ptof)(5);   // calls func
 ptof(5);      // same thing.

It is often clearer to use a typedef for function pointer types; this also provides a place to give a meaningful name to the function pointer's type:

 typedef int (*int_to_int_function)(int);
 int_to_int_function ptof;

Overloading

In C++ you can define and use different functions with the same name. Multiple functions who share the same name must be differentiated by using another set of parameters for every such function. The functions can be different in the number of parameters they expect, or their parameters can differ in type. This way, the compiler can figure out the exact function to call by looking at the arguments the caller supplied. This is called overload resolution, and is quite complex.

Example:

// (1)
double geometric_mean(int, int);

// (2)
double geometric_mean(double, double);

// (3)
double geometric_mean(double, double, double);

...

// Will call (1):
geometric_mean(10, 25);
// Will call (2):
geometric_mean(22.1, 421.77);
// Will call (3):
geometric_mean(11.1, 0.4, 2.224);

Under some circumstances, a call can be ambiguous, because two or more functions match with the supplied arguments equally well.

Example, supposing the declaration of geometric_mean above:

// This is an error, because (1) could be called and the second
// argument casted to an int, and (2) could be called with the first
// argument casted to a double. None of the two functions is
// unambiguously a better match.
geometric_mean(7, 13.21);
// This will call (3) too, despite its last argument being an int,
// Because (3) is the only function which can be called with 3
// arguments
geometric_mean(1.1, 2.2, 3);

Templates and non-templates can be overloaded. A non-template function takes precedence over a template, if both forms of the function match the supplied arguments equally well.

Note that you can overload many operators in C++ too.

Overloading resolution

Please beware that overload resolution in C++ is one of the most complicated parts of the language. This is probably unavoidable in any case with automatic template instantiation, user defined implicit conversions, built-in implicit conversation and more as language features. So don't despair if you do not understand this at first go. It's really quite natural, once you have the ideas, but written down it seems extremely complicated.

The easiest way to understand overloading is to imagine that the compiler first finds every function which might possibly be called, using any legal conversions and template instantiations. The compiler then selects the best match, if any, from this set. Specifically, the set is constructed like this:

• All functions with matching name, including function templates, are put into the set. Return types and visibility are not considered. Templates are added with as closely matching parameterss as possible. Member functions are considered functions with the first parameter being a pointer-to-class-type.
• Conversion functions are added as so-called surrogate functions, with two parameters, the first being the class type and the second the return type.
• All functions that don't match the number of parameters, even after considering defaulted parameters and ellipses, are removed from the set.
• For each function, each argument is considered to see if a legal conversion sequence exists to convert the caller's argument to the function's parameters. If no such conversion sequence can be found, the function is removed from the set.

The legal conversions are detailed below, but in short a legal conversion is any number of built-in (like int to float) conversions combined with at most one user defined conversion. The last part is critical to understand if you are writing replacements to built-in types, such as smart pointers. User defined conversions are described above, but to summarize it is

1. implicit conversion operators like operator short toShort();
2. One argument constructors (If a constructor has all but one parameter defaulted, it is considered one-argument)

The overloading resolution works by attempting to establish the best matching function.

Easy conversions are preferred

Looking at one parameter, the preferred conversion is roughly based on scope of the conversion. Specifically, the conversions are preferred in this order, with most-preferred highest:

1. No conversion, adding one or more const, adding reference, convert array to pointer to first member
   1. const are preferred for rvalues (roughly constants) while non-const are preferred for lvalues (roughly assignables)
2. Conversion from short integral types (bool, char, short) to int, and float to double.
3. Built-in conversions, such as between int and double and pointer type conversion. Pointer conversion are ranked as
   1. Base to derived (pointers) or derived to base (for pointers-to-members), with most-derived preferred
   2. Conversion to void*
   3. Conversion to bool
4. User-defined conversions, see above.
5. Match with ellipses. (As an aside, this is rather useful knowledge for template meta programming)

The best match is now determined according to the following rules:

• A function is only a better match if all parameters match at least as well

In short, the function must be better in every respect --- if one parameter matches better and another worse, neither function is considered a better match. If no function in the set is a better match than both, the call is ambiguous (i.e, it fails) Example:

void foo(void*, bool);
void foo(int*, int);

int main() {
   int a;
   foo(&a, true); // ambiguous 
}

• Non-templates are preferred over templates

If all else is equal between two functions, but one is a template and the other not, the non-template is preferred. This seldom causes surprises.

• Most-specialized template is preferred

When all else is equal between two template function, but one is more specialized than the other, the most specialized version is preferred. Example:

template<typename T> void foo(T);  //1
template<typename T> void foo(T*); //2

int main() {
   int a;
   foo(&a); // Calls 2, since 2 is more specialized.
}

Which template is more specialized is an entire chapter unto itself.

• Return types are ignored

This rule is mentioned above, but it bears repeating: Return types are never part of overload resolutions, even if the function selected has a return type that will cause the compilation to fail. Example:

void foo(int);
int foo(float);

int main() { 
   // This will fail since foo(int) is best match, and void cannot be converted to int.
   return foo(5); 
}

• The selected function may not be visible

If the selected best function is not visible (e.g, private), the call fails.