Strange extra pair of parentheses in std::vector constructor?

Question

Could someone elaborate on why the extra pair of parentheses in the code below is necessary for correct compilation? We are creating an std::ifstream and reading everything from it into an std::vector... I see these parentheses in the examples on stack overflow, but I can't figure out why they would change the way the compiler interprets the code:

#include <fstream>
#include <vector>

int main()
{
    std::ifstream curFile("file.txt", std::ios::binary);

    std::vector<char> fileContents(
        (std::istreambuf_iterator<char>(curFile)),      //why?
        std::istreambuf_iterator<char>()
    );
}

The parentheses I am talking about wrap the istreambuf_iterator constructor. Without them, the compiler seems to think that fileContents is a function declaration.

TL;DR of the dupes: In C++ `type(name)` in certain circumstances is treated as you just wrote `type name`. That means you are naming a function parameter, not creating an object. — NathanOliver, Oct 15 '19 at 20:31
https://en.wikipedia.org/wiki/Most_vexing_parse is a good reference too. — Lightness Races in Orbit, Oct 15 '19 at 20:32

Strange extra pair of parentheses in std::vector constructor?

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