为什么文件读取函数中有额外的括号？ [英] Why are extra parenthesis in file read function?

问题描述

我了解以下代码（从此处） ）用于读取文件的字符串内容：

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

但是，我不明白为什么需要这种看似冗余的括号。例如，以下代码不编译：

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

为什么需要这么多括号来编译？

<因为没有圆括号，编译器将其视为一个函数声明，声明一个名为 content 的函数，返回 std :: istreambuf_iterator< char> 命名为 a std :: string ifs 和一个无参参数，它是一个不带返回 std :: istreambuf_iterator< char> 的参数的函数。

你可以使用括号，或者作为Alexandre在注释中注释，你可以使用C ++的统一初始化特性，没有这样的模糊性：

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

或如Loki提及：

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

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?

解决方案

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>());

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