如何有条件地实例化对象？

问题描述

我正在尝试像这样做一些有条件的工作：

I'm trying to do some conditional work like so:

Type object;
if (cond) {
    doSomeStuff();
    object = getObject();
    doMoreStuff();
} else {
    doSomeOtherStuff();
    object = getDifferentObject();
    doEvenMoreStuff();
}
use(object);

我认为解决此问题的唯一方法是重复使用 代码（实际上是我的应用程序中的内联代码），并在 if 块的每个分支中声明 object 。如果我想避免重复的代码，则必须像上面一样将其包装在一些use函数中。在实际情况下，此 use 函数可能会使用5个以上的参数来实质上继承上下文。

The only way I can think of solving this is the duplicate the use code (which is actually inline code in my application) and declare object in each branch of the if block. If I wanted to avoid duplicate code I'd have to wrap it in some use function, as I have above. In a real situation, this use function will probably take 5+ parameters to essentially carry over the context. This all seems messy, and impossible to maintain.

if (cond) {
    doSomeStuff();
    Type object = getObject();
    doMoreStuff();
    use(object);
} else {
    doSomeOtherStuff();
    Type object = getDifferentObject();
    doEvenMoreStuff();
    use(object);
}

解决此问题的最佳方法是什么？ Type 没有默认构造函数，因此代码段1无法编译。

What's the best approach to tackling this? Type has no default constructor, thus snippet 1 doesn't compile.

某些其他语言支持代码段1-相关问题：

Some other languages support snippet 1 - Related question: Forcing uninitialised declaration of member with a default constructor

推荐答案

您可以使用IIILE（立即调用初始化lambda表达式）：

You can use an IIILE (immediately invoked initializing lambda expression):

auto object = [&] {
  if (cond) {
    doSomeStuff();
    auto object = getObject();
    doMoreStuff();
    return object;
  } else {
    doSomeOtherStuff();
    auto object = getDifferentObject();
    doEvenMoreStuff();
    return object;
  }
}();  // note that the lambda must be called

use(object);

即使 Type 不是默认可构造的，这也将起作用。

This will work even if Type is not default-constructible.

这是一个。

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