在'if'条件中定义fstream [英] Defining fstream inside a 'if' conditional

问题描述

在答案中，有以下代码：

if (std::ifstream input("input_file.txt"))
  ;

这似乎很方便，将'input'变量的范围限制在确认有效的范围内，然而VS2015和g ++似乎都没有编译它。它是某些编译器特定的东西还是需要一些额外的标志？

Which seems convenient, limiting the scope of the 'input' variable to where it's confirmed to be valid, however neither VS2015 nor g++ seems to compile it. Is it some compiler specific thing or does it require some extra flags?

在VS2015中，IDE突出显示std :: ifstream和input_file.txt以及最后的括号。 std :: ifstream标记为错误：此处不允许使用函数类型。

In VS2015 the IDE highlights "std::ifstream" and the "input_file.txt" as well as the last parentheses. "std::ifstream" is flagged with "Error: a function type is not allowed here".

VS2015 C ++编译器出现以下错误：

VS2015 C++ compiler gives following errors:

• C4430缺少类型说明符 - 假设为int。注意：C ++不支持default-int


• C2059语法错误：'（'

推荐答案

你所拥有的代码是不合法的..在C ++ 11之前，if语句可以是

The code you have is not legal.. yet. Prior to C++11 a if statement could be

if(condition)
if(type name = initializer)

并且 name 将被评估为 bool 以确定条件。在C ++ 11/14中，规则用于允许

and name would be evaluated as a bool to determine the condition. In C++11/14 the rules expended to allow

if(condition)
if(type name = initializer)
if(type name{initializer})

其中， name 是在初始化以确定条件后评估为 bool 。

Where, again, name is evaluted as a bool after it is initialized to determine the condition.

从C ++ 17开始，尽管你会能够将if语句中的变量声明为复合语句，如for循环，允许您使用括号初始化变量。

Starting in C++17 though you will be able to declare a variable in a if statement as a compound statement like a for loop which allows you to initialize the variable with parentheses.

if (std::ifstream input("input_file.txt"); input.is_open())
{
    // do stuff with input
}
else
{
    // do other stuff with input
}

应该注意的是，这只是语法糖，上面的代码实际上已翻译成

It should be noted though that this is just syntactic sugar and the above code is actually translated to

{
    std::ifstream input("input_file.txt")
    if (input.is_open())
    {
        // do stuff with input
    }
    else
    {
        // do other stuff with input
    }
}

这篇关于在'if'条件中定义fstream的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！