在if语句的条件部分定义一个变量？ [英] Defining a variable in the condition part of an if-statement?

问题描述

我只是震惊，这是允许的：

I was just shocked, that this is allowed:

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

因此，这是标准允许的还是只是一个编译器扩展？

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

_{PS由于对此有几个意见 - 请忽略此示例是坏或危险的。我知道什么。}

_{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

这是允许的

From Section 6.4 "Selection statements":

由条件中的声明引入的名称（由类型说明符seq或条件的声明者引入）在其声明点到由条件控制的子句结束之间的范围内。 / p>

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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