Is it possible to know if the parameter was defaulted

Question

Caution: This problem is limited to MSVS

I have this function signature:

void do_somthing(std::vector<foo>&  bar={});

Is it possible to differ between those two calls for the function:

First:

do_something()

Second:

std::vector<foo> v;
do_something(v);

In other words, I want something like:

void do_somthing(std::vector<foo>&  bar={}){
    if(/* bar was defaulted*/){

    }
    else{

    }
}

EDIT: The actual code:

template<class Tinput_iterator>
            Tmodel perform_fitting(Tinput_iterator begin_data, Tinput_iterator end_data, std::vector<Tpoint>& inliers = {});

Answer 1

I have this function signature:

void do_somthing(std::vector<foo>&  bar=std::vector<foo>{});

This cannot compile, except with dangerous non-standard compiler settings you should stay away from.

In particular, Visual C++ allows this if /Za is not specified, but with /W4 still produces a warning like this:

stackoverflow.cpp(6): warning C4239: nonstandard extension used: 'default argument': conversion from 'std::vector<foo,std::allocator<_Ty>>' to 'std::vector<foo,
std::allocator<_Ty>> &'
        with
        [
            _Ty=foo
        ]
stackoverflow.cpp(6): note: A non-const reference may only be bound to an lvalue

void do_somthing(std::vector<foo>&  bar=std::vector<foo>{}){
    if(/* bar was defaulted*/){

    }
    else{

    }
}

Even if we assume that you actually included the missing const to make the code compile, the answer would be: no, it is not possible to know if bar was defaulted.

Whatever you plan to do here, you have to find a completely different solution.