Default parameter and templates

Question

Why does this function give me an error:

template < typename T >
T foo( T s = 0, const vector < T > &v)
{
    ...
}

error: default argument missing for parameter 2 of ‘template summable sum(summable, const std::vector&)’

And why doesn't the following?:

template < typename T >
T foo( const vector < T > &v, T s = 0)
{
    ...
}

Possible duplicate of *[Can function default template parameter be put before non-default ones?](http://stackoverflow.com/questions/11684954/can-function-default-template-parameter-be-put-before-non-default-ones)*. — Peter Mortensen, Jun 23 '15 at 22:05

score 5 · Accepted Answer · answered Oct 20 '13 at 22:56

Optional arguments must be the last. I.e. non-optional arguments cannot follow an optional one.

How would you call

T foo( T s = 0, const vector < T > &v)

with just a v, and no s?

How would the compiler spot this if

s and v had the same type, or
there was an overload of foo taking just a const vector<T>&?

zch · Answer 2 · 2013-10-20T23:16:32.513

If argument has a default value, than all following arguments need to have default value as well.

Rationale is given in other answers, so I'll give you a quote from C++11 standard:

8.3.6 Default arguments [dcl.fct.default]

4 (...) In a given function declaration, each parameter subsequent to a parameter with a default argument shall have a default argument supplied in this or a previous declaration or shall be a function parameter pack.

score 0 · Answer 3 · answered Oct 20 '13 at 22:56

Arguments with defaults need to be the last arguments. In the first, you have s with a default of 0, then v with no default. You cannot have an argument with no default following an argument with a default.

How would you call such as argument using its defaults? foo(/*default*/,vector) ?

Default parameter and templates

3 Answers3