Vector constructor with two parameters is parsed as a function declaration

Question

Consider this example:

#include 
#include 
#include 
#include 

int main()
{
    std::string sen          


        

Answer 1

It's still the most vexing parse.

std::vector<std::string>                     // return type
vec(                                         // function name
    std::istream_iterator<std::string>(iss), // param 1: an iterator called (iss), or just iss
    std::istream_iterator<std::string>()     // param 2: unnamed function 
);                                           //          returning iterator

geordi says:

<tomalak> << ETYPE_DESC(vec); std::vector<std::string> vec(std::istream_iterator<std::string>(iss), std::istream_iterator<std::string>());
<geordi> lvalue function taking a istream_iterator<string, char, char_traits<char>, long> , a pointer to a nullary function returning a istream_iterator<string, char, char_traits<char>, long> , and returning a vector of strings

The crux of it, really, is that your parameter names can have parentheses around them (i.e. iss → (iss)) without altering the semantics of the declaration. Sometimes.

Use another set of parentheses that also surround the type, as you showed, to force that first parameter (and, consequently, the second) to be parsed as an expression rather than a declaration.

---

If it helps, also consider:

void foo(int (x)) {
   cout << x;
}

int main() {
   foo(42);
}

Output is 42.