为什么不能用参数构造条件中定义的变量? [英] Why can't variables defined in a conditional be constructed with arguments?

问题描述

问题很简单.为什么编译:

The question is simple. Why does this compile:

bool b(true);
if (b) { /* */ }

然后编译:

if (bool b = true) { /* */ }

但不是这样:

if (bool b(true)) { /* */ }

在我的真实代码中，我需要构造一个对象并对其进行测试，同时还要在if块结束时销毁它.基本上，我正在寻找这样的东西:

In my real code, I need to construct an object and test it, while also having it destroyed when the if-block ends. Basically, I'm looking for something like this:

{
    Dingus dingus(another_dingus);
    if (dingus) {
        // ...
    }
}

当然可以，

if (Dingus dingus = another_dingus) { /* */ }

但是接下来我要构造一个 Dingus 并在其上调用 operator = .在我看来，我可以使用任何我喜欢的构造函数来构造对象.

But then I'm constructing a Dingus and calling operator= on it. It seems logical to me that I would be able to construct the object using whatever constructor I please.

但是我困惑为什么这在语法上是不正确的.我已经用G ++和MSVC ++进行了测试，他们都抱怨这种构造，所以我确定它是规范的一部分，但是我对此的原因以及可能存在的不难看的解决方法感到好奇.

But I'm baffled why this isn't grammatically correct. I've tested with G++ and MSVC++ and they both complain about this construct, so I'm sure it's part of the spec but I'm curious as to the reasoning for this and what non-ugly workarounds there may be.

推荐答案

有点技术性.没有理由不能允许您想要的东西，事实并非如此.这是语法.

It's a bit technical. There's no reason why what you want couldn't be allowed, it just isn't. It's the grammar.

if 语句是一个选择语句，它采用语法形式:

An if statement is a selection statement, and it takes the grammatical form:

if (condition) statement

在这里，条件可以是:

• 表达式或
• type-specifier-seq声明符=赋值表达式

就在那里.允许在条件中进行声明是一种特殊情况，它必须遵循 that 格式，否则您的程序格式不正确.他们可能允许直接初始化而不是复制初始化，但是现在并没有任何动机.正如约翰内斯·绍布(Johannes Schaub)指出的那样，此更改将破坏现有代码，因此几乎永远不会发生.

And there you have it. Allowing a declaration in a condition is a special case, and it must follow that form or your program is ill-formed. They could have probably allow direct-initialization instead of copy-initialization, but there isn't really any motivation to do so now. As Johannes Schaub points out, this change would break existing code, so it's pretty much never going to happen.

Let_Me_Be注意，C ++ 11添加了第三种形式(我在这里忽略了属性):

Let_Me_Be notes that C++11 added a third form (I'm ignoring attributes here):

decl-specifier-seq declarator braced-init-list

所以 if(bool b {true})很好.(这不可能破坏任何有效的现有代码.)

So if (bool b{true}) is fine. (This can't possibly break any valid existing code.)

请注意，您的问题似乎与效率有关:请放心.编译器将忽略该临时值，而直接直接构造左侧.但是，这要求您的类型是可复制的(或在C ++ 11中可移动).

Note your question seems to do with efficiency: don't worry. The compiler will elide the temporary value and just construct the left-hand side directly. This, however, requires your type be copyable (or movable in C++11).

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