问题描述
What is the correct way of using C++11's range-based for?
What syntax should be used? for (auto elem : container),
or for (auto& elem : container) or for (const auto& elem : container)?
Or some other?
TL;DR: Consider the following guidelines:
For observing the elements, use the following syntax:
for (const auto& elem : container) // capture by const referenceIf the objects are cheap to copy (like
ints,doubles, etc.), it's possible to use a slightly simplified form:for (auto elem : container) // capture by value
For modifying the elements in place, use:
for (auto& elem : container) // capture by (non-const) referenceIf the container uses "proxy iterators" (like
std::vector<bool>), use:for (auto&& elem : container) // capture by &&
Of course, if there is a need to make a local copy of the element inside the loop body, capturing by value (for (auto elem : container)) is a good choice.
Detailed Discussion
Let's start differentiating between observing the elements in the container vs. modifying them in place.
Observing the elements
Let's consider a simple example:
vector<int> v = {1, 3, 5, 7, 9};
for (auto x : v)
cout << x << ' ';
The above code prints the elements (ints) in the vector:
1 3 5 7 9
Now consider another case, in which the vector elements are not just simple integers, but instances of a more complex class, with custom copy constructor, etc.
// A sample test class, with custom copy semantics.
class X
{
public:
X()
: m_data(0)
{}
X(int data)
: m_data(data)
{}
~X()
{}
X(const X& other)
: m_data(other.m_data)
{ cout << "X copy ctor.
"; }
X& operator=(const X& other)
{
m_data = other.m_data;
cout << "X copy assign.
";
return *this;
}
int Get() const
{
return m_data;
}
private:
int m_data;
};
ostream& operator<<(ostream& os, const X& x)
{
os << x.Get();
return os;
}
If we use the above for (auto x : v) {...} syntax with this new class:
vector<X> v = {1, 3, 5, 7, 9};
cout << "
Elements:
";
for (auto x : v)
{
cout << x << ' ';
}
the output is something like:
[... copy constructor calls for vector<X> initialization ...] Elements: X copy ctor. 1 X copy ctor. 3 X copy ctor. 5 X copy ctor. 7 X copy ctor. 9
As it can be read from the output, copy constructor calls are made during range-based for loop iterations.
This is because we are capturing the elements from the container by value
(the auto x part in for (auto x : v)).
This is inefficient code, e.g., if these elements are instances of std::string,
heap memory allocations can be done, with expensive trips to the memory manager, etc.
This is useless if we just want to observe the elements in a container.
So, a better syntax is available: capture by const reference, i.e. const auto&:
vector<X> v = {1, 3, 5, 7, 9};
cout << "
Elements:
";
for (const auto& x : v)
{
cout << x << ' ';
}
Now the output is:
[... copy constructor calls for vector<X> initialization ...] Elements: 1 3 5 7 9
Without any spurious (and potentially expensive) copy constructor call.
So, when observing elements in a container (i.e., for read-only access),
the following syntax is fine for simple cheap-to-copy types, like int, double, etc.:
for (auto elem : container)
Else, capturing by const reference is better in the general case,
to avoid useless (and potentially expensive) copy constructor calls:
for (const auto& elem : container)
Modifying the elements in the container
If we want to modify the elements in a container using range-based for,
the above for (auto elem : container) and for (const auto& elem : container)
syntaxes are wrong.
In fact, in the former case, elem stores a copy of the original
element, so modifications done to it are just lost and not stored persistently
in the container, e.g.:
vector<int> v = {1, 3, 5, 7, 9};
for (auto x : v) // <-- capture by value (copy)
x *= 10; // <-- a local temporary copy ("x") is modified,
// *not* the original vector element.
for (auto x : v)
cout << x << ' ';
The output is just the initial sequence:
1 3 5 7 9
Instead, an attempt of using for (const auto& x : v) just fails to compile.
g++ outputs an error message something like this:
TestRangeFor.cpp:138:11: error: assignment of read-only reference 'x' x *= 10; ^
The correct approach in this case is capturing by non-const reference:
vector<int> v = {1, 3, 5, 7, 9};
for (auto& x : v)
x *= 10;
for (auto x : v)
cout << x << ' ';
The output is (as expected):
10 30 50 70 90
This for (auto& elem : container) syntax works also for more complex types,
e.g. considering a vector<string>:
vector<string> v = {"Bob", "Jeff", "Connie"};
// Modify elements in place: use "auto &"
for (auto& x : v)
x = "Hi " + x + "!";
// Output elements (*observing* --> use "const auto&")
for (const auto& x : v)
cout << x << ' ';
the output is:
Hi Bob! Hi Jeff! Hi Connie!
The special case of proxy iterators
Suppose we have a vector<bool>, and we want to invert the logical boolean state
of its elements, using the above syntax:
vector<bool> v = {true, false, false, true};
for (auto& x : v)
x = !x;
The above code fails to compile.
g++ outputs an error message similar to this:
TestRangeFor.cpp:168:20: error: invalid initialization of non-const reference of type 'std::_Bit_reference&' from an rvalue of type 'std::_Bit_iterator::referen ce {aka std::_Bit_reference}' for (auto& x : v) ^
The problem is that std::vector template is specialized for bool, with an
implementation that packs the bools to optimize space (each boolean value is
stored in one bit, eight "boolean" bits in a byte).
Because of that (since it's not possible to return a reference to a single bit),
vector<bool> uses a so-called "proxy iterator" pattern.
A "proxy iterator" is an iterator that, when dereferenced, does not yield an
ordinary bool &, but instead returns (by value) a temporary object,
which is a proxy class convertible to bool.
(See also this question and related answers here on StackOverflow.)
To modify in place the elements of vector<bool>, a new kind of syntax (using auto&&)
must be used:
for (auto&& x : v)
x = !x;
The following code works fine:
vector<bool> v = {true, false, false, true};
// Invert boolean status
for (auto&& x : v) // <-- note use of "auto&&" for proxy iterators
x = !x;
// Print new element values
cout << boolalpha;
for (const auto& x : v)
cout << x << ' ';
and outputs:
false true true false
Note that the for (auto&& elem : container) syntax also works in the other cases
of ordinary (non-proxy) iterators (e.g. for a vector<int> or a vector<string>).
(As a side note, the aforementioned "observing" syntax of for (const auto& elem : container) works fine also for the proxy iterator case.)
Summary
The above discussion can be summarized in the following guidelines:
For observing the elements, use the following syntax:
for (const auto& elem : container) // capture by const referenceIf the objects are cheap to copy (like
ints,doubles, etc.), it's possible to use a slightly simplified form:for (auto elem : container) // capture by value
For modifying the elements in place, use:
for (auto& elem : container) // capture by (non-const) referenceIf the container uses "proxy iterators" (like
std::vector<bool>), use:for (auto&& elem : container) // capture by &&
Of course, if there is a need to make a local copy of the element inside the loop body, capturing by value (for (auto elem : container)) is a good choice.
Additional notes on generic code
In generic code, since we can't make assumptions about generic type T being cheap to copy, in observing mode it's safe to always use for (const auto& elem : container).
(This won't trigger potentially expensive useless copies, will work just fine also for cheap-to-copy types like int, and also for containers using proxy-iterators, like std::vector<bool>.)
Moreover, in modifying mode, if we want generic code to work also in case of proxy-iterators, the best option is for (auto&& elem : container).
(This will work just fine also for containers using ordinary non-proxy-iterators, like std::vector<int> or std::vector<string>.)
So, in generic code, the following guidelines can be provided:
For observing the elements, use:
for (const auto& elem : container)For modifying the elements in place, use:
for (auto&& elem : container)
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