How to specify a default function argument with a template parameter type T

Question

Is it possible to define the default value for variables of a template function in C++?

Something like below:

template<class T> T sum(T a, T b, T c=????)
{
     return a + b + c;
}

Sagittarius

Answer 1

Try this:

template<class T> T sum(T a, T b, T c=T())
{
     return a + b + c;
}

You can also put in T(5) if you are expecting an integral type and want the default value to be 5.

Woodman

Answer 2

It all depends on the assumptions that you can do about the type.

template <typename T> T sum( T a, T b, T c = T() ) { return a+b+c; }
template <typename T> T sum2( T a, T b, T c = T(5) ) { return a+b+c; }

The first case, it only assumes that T is default constructible. For POD types that is value inititalization (IIRC) and is basically 0, so sum( 5, 7 ) will call sum( 5, 7, 0 ).

In the second case you require that the type can be constructed from an integer. For integral types, sum( 5, 7 ) will call sum( 5, 7, int(5) ) which is equivalent to sum( 5, 7, 5 ).

Claudclauddetta

The default arguments are instantiated only if they are used. Which means one can put any crazy thing into them that wouldn't be compatible with T at all, which would be fine as long as one passes an explicit argument. – Mystic

Answer 3

Yes you can define a default value.

template <class T> 
T constructThird()
{
    return T(1);
}

template <class T> 
T test(T a, 
       T b, 
       T c = constructThird<T>())
{
    return a + b + c;
}

Unfortunately constructThird cannot take a and b as arguments.

Dolt

Answer 4

Yes, there just needs to be a constructor for T from whatever value you put there. Given the code you show, I assume you'd probably want that argument to be 0. If you want more than one argument to the constructor, you could put T(arg1, arg2, arg3) as the default value.

Jubilant

Answer 5

Yes!

However you should at least have an idea about what T could be or it's useless.

You can't set the default value of template parameters for functions, i.e. this is forbidden:

template<typename T=int> void f(T a, T b);

Trump

Update - C++11 permits this. – Silvanus