C++: Default values in class member

Question

I have a problem with specifying the default values for my C++ class members. My code is:

From Someclass.h:

class SomeClass
{
public:
    SomeClass();
    ~SomeClass();
    void printOut (bool);
}

...from SomeClass.cpp:

void SomeClass::printOut(bool foobar=true)
{
    if (foobar) { std::cout << foobar << std::endl; }
}

...and finally, from main.cpp:

int main()
{
    SomeClass s;
    s.printOut();
    return 0;
}

This however gives error message (gcc):

../main.cpp: In function `int main()':
../main.cpp:8: error: no matching function for call to `SomeClass::printOut()'
../SomeClass.h:18: note: candidates are: void SomeClass::printOut(bool)
subdir.mk:21: recipe for target `main.o' failed
make: *** [main.o] Error 1

I have tried specifying the default value directly into the class declaration in the header file, etc. I also tried searching both Stack Overflow and Google in general, but cannot find any solution anywhere. What am I doing wrong?

Answer 1

You haven't specified the default value for the parameter in the header as such, the compiler is looking for a function of signature void printOut(void) for your statement s.printOut(); but correctly not finding it. What you need is:

class SomeClass
{
public:
    SomeClass();
    ~SomeClass();
    void printOut( bool fValue = true );  // Note change in param in definition
}

And in your cpp :

void SomeClass::printOut(bool foobar /*=true*/ )
{
    if (foobar) { std::cout << foobar << std::endl; }
}

As a side note, bear in mind you don't have to put the commented out default value for the parameter in the implementation file but it is a good idea for readability.

Answer 2

You need to declare the default value inside the class definition, not in the implementation.

class SomeClass
{
public:
    SomeClass();
    ~SomeClass();
    void printOut (bool foobar = true);   //move default here
}

void SomeClass::printOut(bool foobar)     //remove from here
{
    if (foobar) { std::cout << foobar << std::endl; }
}

Also, note that:

SomeClass s();

doesn't do what you expect it to do. It doesn't create an object s of type SomeClass, but declares a function s with return type SomeClass. s.printOut(); shouldn't compile.

You probably want:

SomeClass s;

Answer 3

Default value must be specified in the declaration of the method, not in the implementation.

Answer 4

Try to specify default value in header file:

class SomeClass
{
public:
    SomeClass();
    ~SomeClass();
    void printOut (bool foobar=true);
}

Answer 5

Default parameters must be defined in the header like this:

class SomeClass
{
public:
    SomeClass();
    ~SomeClass();
    void printOut (bool value = true);
}

Answer 6

rewrite as follow .. note bool b=false

class SomeClass
{
public:
    SomeClass();
    ~SomeClass();
    void printOut (bool b=false);
}