跳过变量初始化格式不正确还是会导致未定义的行为? [英] Is jumping over a variable initialization ill-formed or does it cause undefined behaviour?

问题描述

考虑以下代码:

void foo()
{
    goto bar;
    int x = 0;
    bar: ;
}

GCC和Clang 拒绝它，因为跳转到bar:会绕过变量初始化. MSVC根本不抱怨(除非在bar:引起警告后使用x).

GCC and Clang reject it, because the jump to bar: bypasses variable initialization. MSVC doesn't complain at all (except using x after bar: causes a warning).

我们可以使用switch做类似的事情:

We can do a similar thing with a switch:

void foo()
{
    switch (0)
    {
        int x = 0;
        case 0: ;
    }
}

现在所有三个编译器发出错误.

这些代码片段格式不正确吗?还是会导致UB?

Are those snippets ill-formed? Or do they cause UB?

我曾经以为两者都是不正确的形式，但是我找不到该标准的启示性部分. [stmt.goto] 对此没有说什么， [stmt.select] .

I used to think that both were ill-formed, but I can't find the revelant parts of the standard. [stmt.goto] doesn't say anything about this, and neither does [stmt.select].

推荐答案

初始化非空时，它的格式不正确.

It's ill-formed when the initialization is non-vacuous.

[stmt.dcl]

3 可以转移到块中，但不以这种方式 绕过初始化声明(包括中的声明) 条件和初始化声明).从一个点跳下来的程序 具有自动存储期限的变量不在范围内 除非变量具有 空泡初始化([basic.life]).在这种情况下，变量 空虚初始化按其顺序构造 声明.

3 It is possible to transfer into a block, but not in a way that bypasses declarations with initialization (including ones in conditions and init-statements). A program that jumps from a point where a variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has vacuous initialization ([basic.life]). In such a case, the variables with vacuous initialization are constructed in the order of their declaration.

初始化程序使初始化不为空.相比之下，

The initializer makes the initialization non-vacuous. To contrast, this

void foo()
{
    goto bar;
    int x; // no initializer
    bar: ;
}

将格式正确.尽管通常会出现关于将x使用不确定值的警告.

would be well-formed. Though the usual caveats about using x with an indeterminate value would apply.

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