Why are extra parenthesis in file read function?

问题:

I understand that the following code (from here) is used to read the contents of a file to string:

#include <fstream>
#include <string>

  std::ifstream ifs("myfile.txt");
  std::string content( (std::istreambuf_iterator<char>(ifs) ),
                       (std::istreambuf_iterator<char>()    ) );

However, I don't understand why such seemingly redundant parentheticals are required. For example, the following code does not compile:

#include <fstream>
#include <string>

  std::ifstream ifs("myfile.txt");
  std::string content(std::istreambuf_iterator<char>(ifs),
                      std::istreambuf_iterator<char>()    );

Why are so many parentheses are needed for this to compile?

回答1:

Because without the parentheses, the compiler treats it as a function declaration, declaring a function named content that returns a std::string and takes as arguments a std::istreambuf_iterator<char> named ifs and a nameless parameter that is a function taking no arguments that returns a std::istreambuf_iterator<char>.

You can either live with the parens, or as Alexandre notes in the comments, you can use the uniform initialisation feature of C++ which has no such ambiguities:

std::string content { std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>() };

Or as Loki mentions:

std::string content = std::string(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());