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[flang] Small edits to C++17 usage notes 

Original-commit: flang-compiler/f18@bdf62ac900
Reviewed-on: https://github.com/flang-compiler/f18/pull/306
This commit is contained in:
peter klausler 2019-02-27 16:00:37 -08:00
parent 340ecc8765
commit 9c45b0d369
1 changed files with 13 additions and 4 deletions
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flang/documentation/C++17.md
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@ -4,10 +4,10 @@ Copyright (c) 2018, NVIDIA CORPORATION. All rights reserved.
## C++14/17 features used in f18
The C++ dialect used in this project is a subset of the
The C++ dialect used in this project constitutes a subset of the
standard C++ programming language and library features.
We want it to be compatible with the LLVM C++ language
subset in use at the time that we integrate with that
We want our dialect to be compatible with the LLVM C++ language
subset that will be in use at the time that we integrate with that
project.
We also want to maximize portability, future-proofing,
compile-time error checking, and use of best practices.
@ -20,11 +20,19 @@ We have chosen to use some features of the recent C++17
language standard in f18.
The most important of these are:
* sum types (discriminated unions) in the form of `std::variant`
* `using` template parameter packs
* generic lambdas with `auto` argument types
* product types in the form of `std::tuple`
* `std::optional`
(`std::tuple` is actually a C++11 feature, but I include it
in this list because it's not particularly well known.)
### Sum types
First, some background information to explain the need for sum types
in f18.
Fortran is notoriously problematic to lex and parse, as tokenization
depends on the state of the partial parse;
the language has no reserved words in the sense that C++ does.
@ -85,7 +93,8 @@ of the possibilities, and f18 will fail to build if one is missing.
Were we unable to use `std::variant` directly, we would likely
have chosen to implement a local `SumType` replacement; in the
absence of C++17's ability of `using` a template parameter pack,
absence of C++17's abilities of `using` a template parameter pack
and allowing `auto` arguments in anonymous lambda functions,
it would be less convenient to use.
The other options for polymorphism in C++ at the level of C++11