Advantage of using default function parameter

int add (int x, int y=1)
int main ()
{
  int result1 = add(5);
  int result2 = add(5, 3);
  result 0;
}

VS

int add (int x, int y)
int main ()
{
  int result1 = add(5, 1);
  int result2 = add(5, 3);
  result 0;
}

What is the advantage of using the default function parameter, in term of execution speed, memory usage and etc? For beginner like me, I sometimes got confused before I realized this usage of default function p开发者_StackOverflow社区arameter; isn't it coding without default function parameter made the codes easier to read?

Your add function is not a good example of how to use defaulted parameters, and you are correct that with one it is harder to read.

However, this not true for all functions. Consider std::vector::resize, which looks something like:

template<class T>
struct vector_imitation {
  void resize(int new_size, T new_values=T());
};

Here, resizing without providing a value uses T(). This is a very common case, and I believe almost everyone finds the one-parameter call of resize easy enough to understand:

vector_imitation<int> v;  // [] (v is empty)
v.resize(3);              // [0, 0, 0] (since int() == 0)
v.resize(5, 42);          // [0, 0, 0, 42, 42]

The new_value parameter is constructed even if it is never needed: when resizing to a smaller size. Thus for some functions, overloads are better than defaulted parameters. (I would include vector::resize in this category.) For example, std::getline works this way, though it has no other choice as the "default" value for the third parameter is computed from the first parameter. Something like:

template<class Stream, class String, class Delim>
Stream& getline_imitation(Stream &in, String &out, Delim delim);

template<class Stream, class String>
Stream& getline_imitation(Stream &in, String &out) {
  return getline_imitation(in, out, in.widen('\n'));
}

Defaulted parameters would be more useful if you could supply named parameters to functions, but C++ doesn't make this easy. If you have encountered defaulted parameters in other languages, you'll need to keep this C++ limitation in mind. For example, imagine a function:

void f(int a=1, int b=2);

You can only use the given default value for a parameter if you also use given defaults for all later parameters, instead of being able to call, for example:

f(b=42)  // hypothetical equivalent to f(a=1, b=42), but not valid C++

If there is a default value that will provide correct behavior a large amount of the time then it saves you writing code that constantly passes in the same value. It just makes things more simple than writing foo(SOME_DEFAULT) all over the place.

It has a wide variety of uses. I usually use them in class constructors:

class Container
{
    // ...

    public:

    Container(const unsigned int InitialSize = 0)
    {
        // ...
    }
};

This lets the user of the class do both this:

Container MyContainer; // For clarity.

And this:

Container MyContainer(10); // For functionality.

Like everything else it depends.

You can use it to make the code clearer.

void doSomething(int timeout=10)
{
    // do some task with a timeout, if not specified use a reasonable default
}

Is better than having lots of magic values doSomething(10) throughout your code

But be careful using it where you should really do function overloading.

int add(int a) 
{ 
    return a+1; 
}

int add(int a,int b) 
{ 
    return a+b; 
}

As Ed Swangren mentioned, some functions have such parameters that tend to have the same value in most calls. In these cases this value can be specified as default value. It also helps you see the "suggested" value for this parameter.

Other case when it's useful is refractoring, when you add some functionality and a parameter for it to a function, and don't want to break the old code. For example, strlen(const char* s) computes the distance to the first \0 character in a string. You could need to look for another characted, so that you'll write a more generic version: strlen(const char* s, char c='\0'). This will reuse the code of your old strlen without breaking compatibility with old code.

The main problem of default values is that when you review or use code written by others, you may not notice this hidden parameter, so you won't know that the function is more powerful than you can see from the code.

Also, google's coding style suggests avoiding them.

A default parameter is a function parameter that has a default value provided to it. If the user does not supply a value for this parameter, the default value will be used. If the user does supply a value for the default parameter, the user-supplied value is used. In computer programming, a default argument is an argument to a function that a programmer is not required to specify. In most programming languages, functions may take one or more arguments. Usually, each argument must be specified in full (this is the case in the C programming language)

Advantages of using default parameter, as others have pointed out, is indeed the "clarity" it brings in the code with respect to say function overloading.

But, it is important to keep in mind the major disadvantage of using this compile-time feature of the language: the binary compatibility and default function parameter does not go hand in hand. For this reason, it is always good to avoid using default params in your API/interfaces classes. Because, each time you change the default param to something else, your clients will need to be recompiled as well as relinked.

Symbian has some very good C++ design patterns to avoid such BC.

Default parameters are better to be avoided.

let's consider the below example

int DoThis(int a, int b = 5, int c = 6) {}

Now lets say you are using this in multiple places

Place 1: DoThis(1);
Place 2: DoThis(1,2);
Place 3: DoThis(1,2,3);

Now you wanted to add 1 more parameter to the function and it is a mandatory field (extended feature for that function).

int DoThis(int a, int x, int b =5, int c=6)

Your compiler throws error for only "Place 1". You fix that. What about other others?

Imagine what happens in a large project? It would become a nightmare to identify it's usages and updating it rightly.

Always overload:

int DoThis(int a) {}
int DoThis(int a, int b {}
int DoThis(int a, int b, int c) {}
int DoThis(int a, int b, int c, int x) {}