Where to put default parameter value in C++?

Question

What's the place for the default parameter value? Just in function definition, or declaration, or both places?

Answer 1

Default parameter values must appear on the declaration, since that is the only thing that the caller sees.

EDIT: As others point out, you can have the argument on the definition, but I would advise writing all code as if that wasn't true.

Answer 2

You can do either, but never both. Usually you do it at function declaration and then all callers can use that default value. However you can do that at function definition instead and then only those who see the definition will be able to use the default value.

Answer 3

C++ places the default parameter logic in the calling side, this means that if the default value expression cannot be computed from the calling place, then the default value cannot be used.

Other compilation units normally just include the declaration so default value expressions placed in the definition can be used only in the defining compilation unit itself (and after the definition, i.e. after the compiler sees the default value expressions).

The most useful place is in the declaration (.h) so that all users will see it.

Some people like to add the default value expressions in the implementation too (as a comment):

void foo(int x = 42,
         int y = 21);

void foo(int x /* = 42 */,
         int y /* = 21 */)
{
   ...
}

However, this means duplication and will add the possibility of having the comment out of sync with the code (what's worse than uncommented code? code with misleading comments!).

Answer 4

Although this is an "old" thread, I still would like to add the following to it:

I've experienced the next case:

• In the header file of a class, I had

int SetI2cSlaveAddress( UCHAR addr, bool force );

• In the source file of that class, I had

int CI2cHal::SetI2cSlaveAddress( UCHAR addr, bool force = false )
{
   ...
}

As one can see, I had put the default value of the parameter "force" in the class source file, not in the class header file.

Then I used that function in a derived class as follows (derived class inherited the base class in a public way):

SetI2cSlaveAddress( addr );

assuming it would take the "force" parameter as "false" 'for granted'.

However, the compiler (put in c++11 mode) complained and gave me the following compiler error:

/home/.../mystuff/domoproject/lib/i2cdevs/max6956io.cpp: In member function 'void CMax6956Io::Init(unsigned char, unsigned char, unsigned int)':
/home/.../mystuff/domoproject/lib/i2cdevs/max6956io.cpp:26:30: error: no matching function for call to 'CMax6956Io::SetI2cSlaveAddress(unsigned char&)'
/home/.../mystuff/domoproject/lib/i2cdevs/max6956io.cpp:26:30: note: candidate is:
In file included from /home/geertvc/mystuff/domoproject/lib/i2cdevs/../../include/i2cdevs/max6956io.h:35:0,
                 from /home/geertvc/mystuff/domoproject/lib/i2cdevs/max6956io.cpp:1:
/home/.../mystuff/domoproject/lib/i2cdevs/../../include/i2chal/i2chal.h:65:9: note: int CI2cHal::SetI2cSlaveAddress(unsigned char, bool)
/home/.../mystuff/domoproject/lib/i2cdevs/../../include/i2chal/i2chal.h:65:9: note:   candidate expects 2 arguments, 1 provided
make[2]: *** [lib/i2cdevs/CMakeFiles/i2cdevs.dir/max6956io.cpp.o] Error 1
make[1]: *** [lib/i2cdevs/CMakeFiles/i2cdevs.dir/all] Error 2
make: *** [all] Error 2

But when I added the default parameter in the header file of the base class:

int SetI2cSlaveAddress( UCHAR addr, bool force = false );

and removed it from the source file of the base class:

int CI2cHal::SetI2cSlaveAddress( UCHAR addr, bool force )

then the compiler was happy and all code worked as expected (I could give one or two parameters to the function SetI2cSlaveAddress())!

So, not only for the user of a class it's important to put the default value of a parameter in the header file, also compiling and functional wise it apparently seems to be a must!

Answer 5

One more point I haven't found anyone mentioned:

If you have virtual method, each declaration can have its own default value!

It depends on the interface you are calling which value will be used.

Example on ideone

struct iface
{
    virtual void test(int a = 0) { std::cout << a; }
};

struct impl : public iface
{
    virtual void test(int a = 5) override { std::cout << a; }
};

int main()
{
    impl d;
    d.test();
    iface* a = &d;
    a->test();
}

It prints 50

I strongly discourage you to use it like this

This is because it is impl's test function that executes in both cases even though the default argument value is picked from impl in d.test() and from iface in a->test(). Refer Gufino2's comment below, also. He is referring to Item 37 of Effective C++.

Answer 6

If the functions are exposed - non-member, public or protected - then the caller should know about them, and the default values must be in the header.

If the functions are private and out-of-line, then it does make sense to put the defaults in the implementation file because that allows changes that don't trigger client recompilation (a sometimes serious issue for low-level libraries shared in enterprise scale development). That said, it is definitely potentially confusing, and there is documentation value in presenting the API in a more intuitive way in the header, so pick your compromise - though consistency's the main thing when there's no compelling reason either way.

Answer 7

the declaration is generally the most 'useful', but that depends on how you want to use the class.

both is not valid.

Answer 8

Good question... I find that coders typically use the declaration to declare defaults. I've been held to one way (or warned) or the other too based on the compiler

void testFunct(int nVal1, int nVal2=500);
void testFunct(int nVal1, int nVal2)
{
    using namespace std;
    cout << nVal1 << << nVal2 << endl;
}

Answer 9

You may do in either (according to standard), but remember, if your code is seeing the declaration without default argument(s) before the definition that contains default argument, then compilation error can come.

For example, if you include header containing function declaration without default argument list, thus compiler will look for that prototype as it is unaware of your default argument values and hence prototype won't match.

If you are putting function with default argument in definition, then include that file but I won't suggest that.

Answer 10

Adding one more point. Function declarations with default argument should be ordered from right to left and from top to bottom.

For example in the below function declaration if you change the declaration order then the compiler gives you a missing default parameter error. Reason the compiler allows you to separate the function declaration with default argument within the same scope but it should be in order from RIGHT to LEFT (default arguments) and from TOP to BOTTOM(order of function declaration default argument).

//declaration
void function(char const *msg, bool three, bool two, bool one = false);
void function(char const *msg, bool three = true, bool two, bool one); // Error 
void function(char const *msg, bool three, bool two = true, bool one); // OK
//void function(char const *msg, bool three = true, bool two, bool one); // OK

int main() {
    function("Using only one Default Argument", false, true);
    function("Using Two Default Arguments", false);
    function("Using Three Default Arguments");
    return 0;
}

//definition
void function(char const *msg, bool three, bool two, bool one ) {
    std::cout<<msg<<" "<<three<<" "<<two<<" "<<one<<std::endl;
}