How can I conditionally instantiate an object?

问题

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);

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);
}

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

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

回答1:

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);

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

Here's a demo.

回答2:

Put it inside a function:

Type doStuffAndCreateType() {
    doSomeStuff();
    Type object = getObject();
    doMoreStuff();
    return object;
}

Type doOtherStuffAndCreateType() {
    doSomeOtherStuff();
    Type object = getObject();
    doEvenMoreStuff();
    return object;
}

Type object = cond ? doStuffAndCreateType() : doOtherStuffAndCreateType();
use( object );

回答3:

Have a look at std::optional:

#include <optional>

std::optional<Type> object;
if (cond) {
    doSomeStuff();
    object = getObject();
    doMoreStuff();
} else {
    doSomeOtherStuff();
    object = getDifferentObject();
    doEvenMoreStuff();
}
use(object.value());

来源：https://stackoverflow.com/questions/62647060/how-can-i-conditionally-instantiate-an-object

标签

c++

class

optimization

conditional-statements

instantiation