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Update CppCoreGuidelines.md #2217

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Update CppCoreGuidelines.md #2217

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10 changes: 10 additions & 0 deletions CppCoreGuidelines.md
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Expand Up @@ -11165,6 +11165,16 @@ Use plain `{}`-initialization unless you specifically want to disable explicit c
// ...
}

##### Exception

For container-like types whose element type can be initialized from an instance of that very container
type iteself, e.g. `std::any` or a template parameter `T` of a template type having a constructor accepting `T`,
use `()` to avoid erroneously constructing a nested container:

vector<any> v{1,2,3};
vector<any> u{v}; // most likely unintended: u has 1 element of type any, which contains v
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I feel u{v}; // most likely unintended: u has 1 element contradicts v2{10}; // vector of 1 element with the value 10 a few lines above; constructing a 1-element vector holding v as its element is what curlies show the intent of.

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@Aster89 Aster89 Sep 11, 2024 •
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The "title" of the guidline is Prefer the {}-initializer syntax, which per se does not refer to initializer lists nor anything else that is discussed in the various examples.

The same holds for the Reason paragraph.

I think that's enough for the reader to interpret the guildeline as "Ok, I'll blindly use {} for initializing anything until I get some compile-time or run-time error".

Furthermore, the example you mention is constrasting list of elements vs size for containers, whereas the case I'm making is about list of elements vs copy, and the whole item does not make any mention as regards preferring {} vs () for copy constructing an entity, so in the absernce of an explicit mention of that, I think it's legit that the reader assumes that they should prefer {}, following the title of the guideline.

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Editors call: Thanks!

I think that's enough for the reader to interpret the guildeline as "Ok, I'll blindly use {} for initializing anything until I get some compile-time or run-time error".

The guideline starts with "Prefer" so we think that it doesn't say to blindly use braces. Also, any is a good example but a very special outlier type; vector<int> x(1, 2) is the stronger example and we think is covered in the first Exception. So we don't think a change is needed.

vector<any> w(v); // w is a copy of v

**See also**: [Discussion](#???)

##### Enforcement
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