trouble trying out vector iteration

[jgarvin]

I was curious to see if cpp2 improvements included iterator invalidation safety so I tried the following:

#include <vector>

main: () -> int = {
    v : std::vector = (1, 2, 3, 4, 5);

    i : int = 0;
    for (auto& item : v) {
        if i > 3 {
            v.resize(100);
        }
        i += 1;
    }
}

I got the for loop syntax from other regression tests but cppfront doesn't accept it. It appears to be complaining about imbalanced parens but AFAICT they are balanced. Errors:

~/cppfront/regression-tests$ ../cppfront resize-invalidation-check.cpp2  && g++-10 -fconcepts -std=c++2a -I../include resize-invalidation-check.cpp
resize-invalidation-check.cpp2...
resize-invalidation-check.cpp2(7,21): error: unexpected text - expression-list is not terminated by ) (at ':')
resize-invalidation-check.cpp2(7,23): error: expected valid range expression after 'for' (at 'v')
resize-invalidation-check.cpp2(7,24): error: expected ; at end of statement (at ')')
resize-invalidation-check.cpp2(7,24): error: invalid statement in compound-statement (at ')')
resize-invalidation-check.cpp2(7,24): error: ill-formed initializer (at ')')
resize-invalidation-check.cpp2(3,1): error: unexpected text at end of Cpp2 code section (at 'main')
resize-invalidation-check.cpp2(2,0): error: parse failed for section starting here

resize-invalidation-check.cpp2: In function ‘int main()’:
resize-invalidation-check.cpp2:12:15: error: request for member ‘resize’ in ‘v’, which is of non-class type ‘int’
   12 |     }
      |               ^     

Also not sure what prevents call to resize... in generated code v is shadowed, not sure if this is incidental or some deliberate attempt to prevent what I'm doing?

[filipsajdak]

The syntax for for loops in cpp2 is different. It should be rewritten to:

#include <vector>

main: () -> int = {
    v : std::vector = (1, 2, 3, 4, 5);

    i : int = 0;
    for v do :(inout item : _) = {
        if i > 3 {
            v.resize(100);
        }
        i += 1;
    }
}

(https://godbolt.org/z/d8o1bzMWc)

or using next syntax

#include <vector>

main: () -> int = {
    v : std::vector = (1, 2, 3, 4, 5);

    i : int = 0;
    for v next i++ do :(inout item : _) = {
        if i > 3 {
            v.resize(100);
        }
    }
}

When cppfront complain about braces you should check regression-tests directory for some examples of syntax. For loops please check:

• https://github.com/hsutter/cppfront/blob/main/regression-tests/pure2-intro-example-three-loops.cpp2

Please remember that in cpp2 sections you can use only cpp2 syntax.

[jgarvin]

I got the syntax from a cpp2 regression test file already, I copied the last loop here: https://github.com/hsutter/cppfront/blob/main/regression-tests/mixed-intro-example-three-loops.cpp2

Why does it work there but not here?

[JohelEGP]

Seems like a lie. The whole file is Cpp1 syntax. If the function signature is not Cpp2 syntax, the body won't be parsed as Cpp2 syntax, AFAIK.

[filipsajdak]

Please learn the cpp2 syntax from files starting with pure2-. These are the files that have only cpp2 syntax.

[hsutter]

@jgarvin wrote:

I was curious to see if cpp2 improvements included iterator invalidation safety

Not yet implemented, but planned. See this project's home page README.md > 2015: Lifetime safety section, which has links to the static analysis specification (part of the C++ Core Guidelines and videos showing a couple of Cpp1 demo implementations.

[hsutter]

@JohelEGP wrote:

The whole file is Cpp1 syntax.

Yes, a .cpp2 file can be all today's syntax. That particular file was one of a series of demos that shows gradual migration from Cpp1 syntax to Cpp2 syntax so it started with Cpp2 syntax, so I checked it in when I checked in all my other test/demo files.