C + + : 临时参数的生命周期？

当创建 MyClass的新实例作为函数的参数时，如下所示:

class MyClass
{
MyClass(int a);
};


myFunction(MyClass(42));

该标准是否对析构函数的时间作出任何保证？

具体来说，我是否可以假设它将在调用 myFunction()之后的下一个语句之前被调用？

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最佳答案

Temporary objects are destroyed at the end of the full expression they're part of.

A full expression is an expression that isn't a sub-expression of some other expression. Usually this means it ends at the ; (or ) for if, while, switch etc.) denoting the end of the statement. In your example, it's the end of the function call.

Note that you can extend the lifetime of temporaries by binding them to a const reference. Doing so extends their lifetime to the reference's lifetime: 

MyClass getMyClass();


{
const MyClass& r = getMyClass(); // full expression ends here
...
} // object returned by getMyClass() is destroyed here

If you don't plan to change the returned object, then this is a nice trick to save a copy constructor call (compared to MyClass obj = getMyClass();), in case return value optimization was not being applied. Unfortunately it isn't very well known. (I suppose C++11's move semantics will render it less useful, though.)

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The standard does indeed offer guarantees - from section 12.2/5:

A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of the full expression containing the call

However, in your code it is not clear if the parameter is being passed by reference or by value, though at some point a copy constructor which does take a reference will be used.

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In section 12.2, Temporary Objects, clause 3, the ANSI/ISO C standard states: "... Temporary objects are destroyed as the last step in evaluating the full-expression that (lexically) contains the point where they were created."

This is closely related to the concept of Sequence Points. When a sequence point is reached, all side-effects of expressions are guaranteed to have taken place.

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Everyone has rightly cited 12.2/3 or similar, which answers your question:

Temporary objects are destroyed as the last step in evaluating the full-expression that (lexically) contains the point where they were created.

I find it amusing that over the next page in my printing of the standard, 12.2/4 says:

There are two contexts in which temporaries are destroyed at a different point than the end of the full-expression.

Neither of them applies to your example, they both relate to the use of temporaries in initializers. But it does go to show that you have to keep your wits about you when dealing with a tricky beast like the C++ standard.


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