Iterating through an rvalue container

问题

Is the following code causing undefined behavior?

std::map<int, vector<int>> foo()
{
return ...
}

BOOST_FOREACH(const int& i, foo()[42])
{
std::cout << i << std::endl;
}

If undefined, What is the good way to fix it? What if I use c++11 range-for loop instead of BOOST_FOREACH?

回答1:

This is, unfortunately, most probably undefined behavior.

The problem is that you have two levels here:

std::map<...> is an r-value, its lifetime will be expanded until the end of the full-expression
std::vector<int>& is an l-value reference (into an object), its lifetime is that of the object.

The problem arises because the code (roughly) expands to something like:

// from
for (<init>: <expr>) {
    <body>
}

// to
auto&& __container = <expr>;
for (auto __it = begin(container), __e = end(container); __it != __e; ++__it)
{
    <init> = *__it;
    <body>
}

The issue here is in the initialization of __container:

auto&& __container = foo()[42];

If it where just foo(), this would work, because the lifetime of std::map<...> would be extended to match that of __container, however in this case we get:

// non-standard gcc extension, very handy to model temporaries:
std::vector<int>& __container = { std::map<...> m = foo(); m[42] };

And thus __container ends up pointing into the nether.

回答2:

The return value exists until the end of the full expression which creates it. So it all depends on how BOOST_FOREACH expands; if it creates a scope outside of the for loop, and copies the return value to a variable in it (or uses it to initialize a reference), then you're safe. If it doesn't, you're not.

The C++11 range-for loop basically has the semantics of binding to a reference in a scope outside of a classic for-loop, so it should be safe.

EDIT:

This would apply if you were capturing the return value of foo. As Benjamin Lindley points out, you aren't. You're capturing the return value of operator[] on a map. And this is not a temporary; it is a reference. So no extension of lifetime occurs, neither in BOOST_FOREACH nor in range-for. Which means that the map itself will be destructed at the end of the full expression which contains the function call, and that undefined behavior occurs. (Boost could, I suppose, make a deep copy of the map, so you'd be safe. But somehow, I doubt that it does.)

END OF EDIT:

Never the less, I would question the wisdom of returning an std::map when all you want is a single entry in it. If the map actually exists outside the function (is not on the heap), then I'd return a reference to it. Otherwise, I'd find some what that it did.

回答3:

From: http://www.boost.org/doc/libs/1_55_0/doc/html/foreach.html

Iterate over an expression that returns a sequence by value (i.e. an rvalue):

extern std::vector<float> get_vector_float();
BOOST_FOREACH( float f, get_vector_float() )
{
    // Note: get_vector_float() will be called exactly once
}

So it is well defined and works.

As well, it is well defined in C++11 (and works):

for (const int& i : get_vector()) // get_vector() computed only once
{
    std::cout << i << std::endl;
}

The problem here is that foo()[42] returns a reference from a temporary (via a method).

auto& v = foo()[42];

life of foo() temporary is not extended...

You may solve that by extending foo temporary lifetime

auto&& m = foo();

for (const int& i : m[42]) {
    std::cout << i << std::endl;
}

来源：https://stackoverflow.com/questions/21680125/iterating-through-an-rvalue-container

标签

c++

loops

c++11

rvalue

boost-foreach