在 if 语句的条件部分定义变量?

Defining a variable in the condition part of an if-statement?(在 if 语句的条件部分定义变量?)

问题描述

我很震惊，这是允许的:

I was just shocked, that this is allowed:

if( int* x = new int( 20 ) )
{
    std::cout << *x << "!
";
    // delete x;
}
else
{
    std::cout << *x << "!!!
";
    // delete x;
}
// std:cout << *x; // error - x is not defined in this scope

那么，这是标准允许的还是只是编译器扩展?

So, is this allowed by the standard or it's just a compiler extension?

_{P.S.因为有几个关于这个的评论 - 请忽略这个例子是坏的"或危险的.我知道什么.举个例子，这只是我想到的第一件事.}

_{P.S. As there were several comments about this - please ignore that this example is "bad" or dangerous. I know what. This is just the first thing, that came to my mind, as an example.}

推荐答案

这是规范允许的，因为 C++98.

This is allowed by the specification, since C++98.

来自第 6.4 节选择声明":

From Section 6.4 "Selection statements":

由条件中的声明引入的名称(由类型说明符序列或条件的声明符引入)的范围从其声明点到由条件控制的子语句结束.

A name introduced by a declaration in a condition (either introduced by the type-specifier-seq or the declarator of the condition) is in scope from its point of declaration until the end of the substatements controlled by the condition.

以下示例来自同一部分:

The following example is from the same section:

if (int x = f()) {
    int x;    // ill-formed, redeclaration of x
}
else {
    int x;    // ill-formed, redeclaration of x
}

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