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Improved F.41

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Added potential C++17 example based on EWG direction in Kona
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hsutter 2015-11-18 20:28:38 -08:00
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@ -1,6 +1,6 @@
# <a name="main"></a> C++ Core Guidelines # <a name="main"></a> C++ Core Guidelines
November 3, 2015 November 18, 2015
Editors: Editors:
@ -1797,7 +1797,7 @@ Argument passing rules:
Value return rules: Value return rules:
* [F.40: Prefer return values to out-parameters](#Rf-T-return) * [F.40: Prefer return values to out-parameters](#Rf-T-return)
* [F.41: Prefer to return tuples to multiple out-parameters](#Rf-T-multi) * [F.41: Prefer to return tuples or structs instead of multiple out-parameters](#Rf-T-multi)
* [F.42: Return a `T*` to indicate a position (only)](#Rf-return-ptr) * [F.42: Return a `T*` to indicate a position (only)](#Rf-return-ptr)
* [F.43: Never (directly or indirectly) return a pointer to a local object](#Rf-dangle) * [F.43: Never (directly or indirectly) return a pointer to a local object](#Rf-dangle)
* [F.44: Return a `T&` when "returning no object" isn't an option](#Rf-return-ref) * [F.44: Return a `T&` when "returning no object" isn't an option](#Rf-return-ref)
@ -2625,7 +2625,7 @@ It's self-documenting. A `&` parameter could be either in/out or out-only.
Flag non-`const` reference parameters that are not read before being written to and are a type that could be cheaply returned. Flag non-`const` reference parameters that are not read before being written to and are a type that could be cheaply returned.
### <a name="Rf-T-multi"></a> F.41: Prefer to return tuples or structs to multiple out-parameters ### <a name="Rf-T-multi"></a> F.41: Prefer to return tuples or structs instead of multiple out-parameters
##### Reason ##### Reason
@ -2662,6 +2662,11 @@ With C++11 we can write this, putting the results directly in existing local var
tie(iter, success) = myset.insert("Hello"); // normal return value tie(iter, success) = myset.insert("Hello"); // normal return value
if (success) do_something_with(iter); if (success) do_something_with(iter);
With C++17 we may be able to write something like this, also declaring the variables:
auto { iter, success } = myset.insert("Hello");
if (success) do_something_with(iter);
**Exception**: For types like `string` and `vector` that carry additional capacity, it can sometimes be useful to treat it as in/out instead by using the "caller-allocated out" pattern, which is to pass an output-only object by reference to non-`const` so that when the callee writes to it the object can reuse any capacity or other resources that it already contains. This technique can dramatically reduce the number of allocations in a loop that repeatedly calls other functions to get string values, by using a single string object for the entire loop. **Exception**: For types like `string` and `vector` that carry additional capacity, it can sometimes be useful to treat it as in/out instead by using the "caller-allocated out" pattern, which is to pass an output-only object by reference to non-`const` so that when the callee writes to it the object can reuse any capacity or other resources that it already contains. This technique can dramatically reduce the number of allocations in a loop that repeatedly calls other functions to get string values, by using a single string object for the entire loop.
##### Note ##### Note