Question

I am trying to understand the concept of currying and calling a function which concats three strings but by passing only two strings and using the second argument twice.

However when I do this, the second argument is not getting sent to the function at all and it prints out an empty string. Is it some really obvious mistake?

string concatthreestrings(string a,string b,string c){
    cout<<"Value of A: "<<a<<endl;
    cout<<"Value of B: "<<b<<endl;
    cout<<"Value of C: "<<c<<endl;
    return a+b+c;
}


int main()
{
    typedef std::function< string( string,string) > fun_t ;
    using namespace std::placeholders;
    fun_t fn = std::bind( concatthreestrings, _1, _2, _2);
    cout<<endl<<fn( "First","Second")<<endl;

}

This is giving the below output. Doesnt using the _2 twice mean that second argument be passed for both second and third. If a use a string in its place its working fine.

Answer 1

Copying strings is expensive. Since std::bind thinks that the values of the placeholders are only used once, it performs a std::move on the strings. This is done for each Parameter and as a consequence, either b or c is a moved, that means empty string.

You can change that behavior by explicitly saying what you mean, by passing the arguments by const-reference:

string concatthreestrings(string const& a,string const& b,string const& c)

Now, it should work.