Where to put default parameter value in C++? [duplicate]

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.

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.

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!).

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!

Where to put default parameter value in C++? [duplicate]

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