C++ template function default value

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;
}

Solution 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.

Solution 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 ).

Solution 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.

Solution 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.

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Comments

[Johannes Schaub - litb]

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.