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Query: Would the compiler writers have to write compilers from scratch for C++20 or the features can be added without overwriting the entire system? I am aware

Query: Would the compiler writers have to write compilers from scratch for C++20 or the features can be added without overwriting the entire system? I am aware that new features means a few parts needs be rectified. But does the whole need to change? And I agree for C++ compiler writers to be gods. Looking at the source code for GCC/clang makes me doubt my programming skills. Same goes for the linux kernel source code.

lallysingh 8y ago
It's the parser (context dependent) that's hard. The rest is a regular compiler that you can learn to write out of a textbook. And the parser is hard in pain-in-the-ass terms, not CS theory terms. https://raw.githubusercontent.com/gcc-mirror/gcc/master/gcc/cp/parser.c https://raw.githubusercontent.com/gcc-mirror/gcc/master/gcc/...
- bsenftner 8y ago
  If the parser is hard to write, that means the syntax is conditional upon previous statements, which I've not seen in the spec. As long as the syntax is Lexical, no real issues parsing. How does the context throw curves?
  - demurgos 8y ago
    The example I know is the following line: foo * bar; This is either an expression statement computing the product of `foo` and `bar` or the declaration of a variable `bar` as a `foo` pointer. To solve this ambiguity, you need context: if `foo` is a type available in the current scope then it's a variable declaration, otherwise an expression.
  - the_why_of_y 8y ago
    See here for an example https://news.ycombinator.com/item?id=19017234 https://news.ycombinator.com/item?id=19017234
  - lallysingh 8y ago
    I did say context dependent, which is the opposite of context free.
    - waynecochran 8y ago
      Yes. May need to back track due to context sensitivity. A quote from the source comments: Some C++ constructs require arbitrary look ahead to disambiguate. For example, it is impossible, in the general case, to tell whether a statement is an expression or declaration without scanning the entire statement. I am sure there must be some devilish source code examples that require an exponential search for the correct parse. Also, has anyone constructed a modern C++ parser in bison which can backtrack?
      - lallysingh 8y ago
        Gcc 3.3 and before had one, but they decided to write one by hand going forward. I don't think it's reasonable to do so without keeping context.
gpderetta 8y ago
Making sure that features are implementable on existing compilers (without major changes to the AST or breaking the ABI for example), is a significant concern for the committee, and often features get dropped for this reason.

lallysingh 8y ago

It's the parser (context dependent) that's hard. The rest is a regular compiler that you can learn to write out of a textbook. And the parser is hard in pain-in-the-ass terms, not CS theory terms. https://raw.githubusercontent.com/gcc-mirror/gcc/master/gcc/cp/parser.c https://raw.githubusercontent.com/gcc-mirror/gcc/master/gcc/...

bsenftner 8y ago
If the parser is hard to write, that means the syntax is conditional upon previous statements, which I've not seen in the spec. As long as the syntax is Lexical, no real issues parsing. How does the context throw curves?
- demurgos 8y ago
  The example I know is the following line: foo * bar; This is either an expression statement computing the product of `foo` and `bar` or the declaration of a variable `bar` as a `foo` pointer. To solve this ambiguity, you need context: if `foo` is a type available in the current scope then it's a variable declaration, otherwise an expression.
- the_why_of_y 8y ago
  See here for an example https://news.ycombinator.com/item?id=19017234 https://news.ycombinator.com/item?id=19017234
- lallysingh 8y ago
  I did say context dependent, which is the opposite of context free.
  - waynecochran 8y ago
    Yes. May need to back track due to context sensitivity. A quote from the source comments: Some C++ constructs require arbitrary look ahead to disambiguate. For example, it is impossible, in the general case, to tell whether a statement is an expression or declaration without scanning the entire statement. I am sure there must be some devilish source code examples that require an exponential search for the correct parse. Also, has anyone constructed a modern C++ parser in bison which can backtrack?
    - lallysingh 8y ago
      Gcc 3.3 and before had one, but they decided to write one by hand going forward. I don't think it's reasonable to do so without keeping context.

gpderetta 8y ago

Making sure that features are implementable on existing compilers (without major changes to the AST or breaking the ABI for example), is a significant concern for the committee, and often features get dropped for this reason.